The How and Why Wonder Book of Air and Water

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•

THE

A

I

HOW

AND WHY

WONDER

BOOK

OF

R A

N

W

D

A

T

E

R

W r i t t e n b y M A R T I N L. K E E N and

CLAIRE

Illustrated Editorial

COOPER

by T O N Y

Production:

CUNNIFF TALLARICO

DONALD

D.

WOLF

E d i t e d u n d e r t h e s u p e r v i s i o n o f D r . P a u l E. B l a c k w o o d Washington, Text and

illustrations

D.C. approved

by

O a k e s A. W h i t e , B r o o k l y n C h i l d r e n s M u s e u m , B r o o k l y n ,

^W**M;

WONDER BOOKS • NEW YORK A Division of GROSSET & DUNLAP, Inc.

N.Y.

Introduction I like to t h i n k of air a n d w a t e r as bridges, for across t h e m m o v e the airplanes a n d ships t h a t bring closer together the world's people. B u t air a n d w a t e r are bridges in a n o t h e r sense. W h a t scientists h a v e learned a b o u t m a n y of the physical characteristics of these fluids, they h a v e been able t o use as bridges t o u n d e r s t a n d i n g o t h e r fluids — gases a n d liquids. W i t h this How and Why Wonder Book of Air and Water you c a n cross these bridges of discovery with the scientists. Y o u will see just how m a n y of the big ideas a b o u t air a n d w a t e r are t r u e of other gases a n d liquids. P e r h a p s , as you study, you will cross a n o t h e r bridge, the o n e t h a t will b r i n g to you a greater a p p r e c i a t i o n of air a n d water. T h o u g h they are all a r o u n d you, you m a y never h a v e noticed h o w surprising a n d interesting these substances are. Because you m a y w a n t to b e the scientist a n d cross t h a t first bridge yourself, this b o o k suggests experiments for you to try. So r e a d a n d think, experiment, observe, a n d check y o u r findings. H a v e a good passage as you cross n e w bridges with The How and Why Wonder Book of Air and Water. Paul E.

Blackwood

Library of Congress Catalog Card Number: 69-20500 Copyright © 1969 by Grosset & Dunlap, Inc. All rights reserved under International and Pan-American Copyright Conventions. Published simultaneously in Canada. Printed in the United States of America.

C O N T E N T S Page CHARACTERISTICS OF AIR AND WATER W h a t is air?

4 5

W h a t is air m a d e of? H o w can you show t h a t air takes u p space?

6

W h a t is a diving bell?

8

H o w c a n w e p r o v e t h a t air h a s weight? W h y does a p e n n y fall faster t h a n a

8

piece of p a p e r ?

6

11

H o w c a n y o u show t h a t air is elastic? W h a t is w a t e r ?

11 12

W h a t is t h e scientific n a m e for w a t e r ? W h e n is w a t e r n o t wet? W h y is ice lighter t h a n w a t e r ? W h y aren't fish frozen i n t o the ice of

12 12 13

lakes a n d rivers? H o w c a n y o u float a piece of steel?

14 14

Page H o w c a n you d e m o n s t r a t e capillary a c t i o n in plants? W h a t is a t m o s p h e r i c p r e s s u r e ? H o w c a n y o u show t h a t air exerts pressure? W h a t was von Guericke's e x p e r i m e n t ? H o w d o we m e a s u r e a t m o s p h e r i c pressure?

29 31

AIR AND WATER AT WORK H o w did ancient m a n use w a t e r

32

to m e a s u r e time? H o w d o windmills w o r k ? H o w d o w a t e r wheels w o r k ?

32 33 35

H o w c a n y o u m a k e a water wheel?

36

H o w does w a t e r p r o d u c e electricity?

37 38 38 39

AIR AND WATER IN NATURE

16

W h a t a r e the layers of the a t m o s p h e r e ? W h a t are t h e a u r o r a s ? H o w c a n y o u show t h a t cool air takes

16 17

will lift an airplane? H o w does a baseball pitcher t h r o w

up less space t h a n w a r m air? H o w c a n y o u p r o v e t h a t cool air is

17

heavier t h a n w a r m air? W h a t m a k e s the wind blow? W h y is t h e s e a s h o r e cool in

18 18

the s u m m e r ? H o w does water get i n t o t h e air? W h a t is h u m i d i t y ?

19 20 21

How c a n you m e a s u r e h u m i d i t y ? H o w does m o v i n g air h e l p cool your b o d y ?

22 23

H o w do desert plants store w a t e r ?

23

W h a t a r e clouds? How can scientists m a k e r a i n ?

24 24

W h a t is the w a t e r cycle? How does w a t e r get to the

25 26

28 28

H o w does air get into our lungs?

W h a t is Bernoulli's Principle? W h a t holds a n airplane u p ? H o w c a n y o u show why its wings

tops of trees?

27 27

a c u r v e ball? 40 H o w c a n you m a k e a siphon? 40 H o w does a siphon w o r k ? 41 H o w did A r c h i m e d e s solve the p r o b l e m of K i n g Hiero's c r o w n ?

41

W h a t is A r c h i m e d e s ' Principle? H o w c a n you p r o v e A r c h i m e d e s ' Principle?

43

Why How in How

44

d o battleships float? does a fish m o v e u p a n d d o w n water? does a s u b m a r i n e m o v e u p a n d

d o w n in w a t e r ?

43

45 46

W h a t is Pascal's Principle?

46

H o w does a hydraulic j a c k w o r k ? AIR, WATER, AND YOU H o w c a n you help k e e p air a n d

47 48

w a t e r clean?

48

Characteristics

of

A i r a n d w a t e r a r e f a m i l i a r t o all of

Air

a n d

W a t e r

N e x t t o a i r , w a t e r is p r o b a b l y

u s . T h e e a r t h w e live o n is s u r r o u n d e d

substance

b y a g r e a t o c e a n of a i r c a l l e d t h e a t -

living things c o n t a i n water. M o r e t h a n

m o s p h e r e . T h r e e q u a r t e r s of t h e e a r t h ' s

t w o t h i r d s of y o u r b o d y is m a d e

surface

of w a t e r . T h e w a t e r i n y o u r b o d y h e l p s

is c o v e r e d

by

the waters

of

oceans, lakes, rivers, a n d streams.

most

important

the

t o us.

All up

d i s s o l v e t h e f o o d y o u e a t , w h i c h is t h e n

A i r fills a l l t h e o p e n h o l l o w s o n t h e

c a r r i e d i n t h e b l o o d t o y o u r l i v i n g cells.

s u r f a c e of t h e e a r t h . T h e a i r s u r r o u n d -

W a t e r a l s o h e l p s c a r r y a w a y t h e dis-

i n g u s is n e c e s s a r y f o r life. A l l a n i m a l s

s o l v e d w a s t e m a t e r i a l s f r o m t h e cells.

b r e a t h e a i r , a n d if d e p r i v e d of it, t h e y

A n d water helps to keep your body at

die. B e c a u s e air s u r r o u n d s o u r b o d i e s

the right temperature.

all t h e time, w e usually a r e n o t a w a r e of h o w i m p o r t a n t it is. B u t

W a t e r is i m p o r t a n t n o t o n l y i n s i d e

whenever

y o u r b o d y , b u t in y o u r d a i l y life a s w e l l .

m a n travels into space or under water,

I t is u s e d f o r c o o k i n g , b a t h i n g , w a s h -

he realizes air's i m p o r t a n c e because he

ing clothes a n d dishes, a n d

m u s t t a k e a s u p p l y of i t w i t h h i m .

A n d water provides recreation such as

cleaning.

s w i m m i n g , sailing, or water-skiing. Plants, Whenever man travels under water he has to take his supply of air with him. Scuba divers carry theirs in tanks strapped to their backs.

plant

can

too, must grow

have

without

m e a n s t h a t all o u r f o o d

water. water. depends

No This on

has shaped

the coastlines

of

all

the

continents. A i r , too, causes c h a n g e s in t h e e a r t h ' s surface. A

strong wind may pick

p a r t i c l e s of soil a n d m o v e t h e m

up long

distances. W h e n the wind quiets down, t h e p a r t i c l e s of soil fall t o e a r t h . I n t h i s w a y , t h e w i n d m o v e s p o r t i o n s of earth's

surface

About

200,000

Mississippi

from

River

place

square basin

to

place.

miles are

the

of

the

covered

w i t h r i c h soil t h a t w a s c a r r i e d t h e r e b y the wind. M u c h of o u r w e a t h e r is d u e t o t h e a m o u n t of w a t e r i n t h e a i r . C l o u d s , f o g , d e w , r a i n , s n o w , a n d s l e e t a r e f o r m s of w a t e r in t h e air. M o v i n g air c a u s e s m o s t of t h e c h a n g e s i n w e a t h e r .

As

wind

blows w a r m or cold, d a m p or dry, the weather changes. W h e n p l a y i n g o u t d o o r s , y o u feel

the

w i n d o n y o u r face.

On

a very windy day,

you

What is air?

have to lean forward a n d p u s h h a r d in Solid rock is carved into beautiful and strange shapes when sand, carried by the force of the wind, scrapes against it. The rock spires in Colorado's Garden of the Gods are examples of such formations.

order to walk against the wind. W i n d is m o v i n g a i r , s o it is m o v i n g a i r t h a t y o u feel, a n d it is m o v i n g a i r a g a i n s t w h i c h y o u h a v e to p u s h so h a r d to w a l k on a windy day.

w a t e r b e c a u s e all f o o d —

even meat,

w h i c h is t h e flesh of p l a n t - e a t i n g

ani-

mals — comes from plants. Water

is

an

important

Y o u c a n ' t see air, b u t y o u h a v e often seen w h a t m o v i n g air can do. It m o v e s l e a v e s o n t r e e s , a n d it b l o w s p i e c e s of

source

of

paper along the ground. Y o u know that

p o w e r . M a n u s e s t h e f o r c e of r u n n i n g

in s t o r m s t h e w i n d b l o w s t r e e s

w a t e r t o t u r n t h e w h e e l s of m i l l s a n d t o

a n d m a y e v e n b l o w t h e r o o f s off h o u s e s .

m a k e electricity.

A

Water

shapes

the

surface

of

the

strong wind blowing from

over,

the sea

t o w a r d shore c a n p u s h so m u c h w a t e r

e a r t h . T i m e a n d a g a i n , t h e f o r c e of t h e

before

r u n n i n g w a t e r s of s t r e a m s a n d

rivers

m a y b e f l o o d e d . B e c a u s e a i r c a n d o all

has w o r n d o w n whole m o u n t a i n ranges

t h e s e t h i n g s , w e k n o w t h a t it m u s t b e a

u n t i l t h e y b e c a m e flat p l a i n s . A n d t h e

real material, like w o o d or p a p e r .

p o u n d i n g of o c e a n w a v e s o n t h e s h o r e s

it t h a t t o w n s a l o n g t h e

shore

A i r is a k i n d of m a t t e r , t h e s u b s t a n c e

t h i n g s a r e m a d e of. M a t t e r is a n y t h i n g

t o d o t h i s . P l a n t s p r o v i d e f o o d f o r all

that takes u p space and has weight. A

o t h e r living things.

s t o n e is m a t t e r . Y o u c a n s e e t h a t a s t o n e

O n e p a r t i n e v e r y h u n d r e d p a r t s of

t a k e s u p s p a c e , a n d if y o u p i c k it u p ,

a i r is a m i x t u r e of h a l f a d o z e n o t h e r

y o u c a n feel t h a t it h a s w e i g h t . A p i e c e

g a s e s . T w o of t h e s e , h e l i u m a n d h y d r o -

of i r o n , y o u r h a t , a b a l l , a n d a g l a s s a n d

g e n , a r e v e r y l i g h t g a s e s . B e c a u s e of

t h e m i l k it c o n t a i n s a l l a r e e x a m p l e s of

t h e i r l i g h t n e s s , o n e u s e of t h e s e g a s e s is

m a t t e r b e c a u s e t h e y all t a k e u p s p a c e

f o r filling b a l l o o n s ; s m a l l t o y o n e s a n d

and

up

large weather balloons. T h e other four

s p a c e a n d h a s w e i g h t . T h a t is w h y w e

gases in air are neon, argon, krypton,

s a y t h a t a i r is a k i n d of m a t t e r .

a n d x e n o n . N e o n gives the bright red-

have weight. Air, too, takes

o r a n g e g l o w t o t h e g l a s s t u b e s of elecA i r is a m i x t u r e of s e v e r a l g a s e s . T h e s e

t r i c a d v e r t i s i n g s i g n s . A r g o n in

these

gases do not have any What is air color, odor, or taste. Y o u made of? k n o w t h a t t h i s is t r u e b e -

g l a s s t u b e s g i v e s off a p u r p l e l i g h t , a n d

c a u s e y o u c a n n o t see or smell or taste

fill t h e e l e c t r i c l i g h t b u l b s i n y o u r h o m e .

air.

T h e argon makes the bulbs glow more

A l i t t l e m o r e t h a n o n e fifth of t h e a i r

k r y p t o n a n d x e n o n g i v e off a b l u e l i g h t . A r g o n , a l o n g w i t h n i t r o g e n , is u s e d t o

brightly a n d last longer.

is o x y g e n . T h i s g a s is v e r y i m p o r t a n t t o

T h e p r o p o r t i o n s of t h e m i x t u r e of

all l i v i n g t h i n g s . If p l a n t s o r a n i m a l s a r e

gases in t h e a t m o s p h e r e a r e n o t a l w a y s

d e p r i v e d of o x y g e n , t h e y d i e .

t h e s a m e . If y o u c o u l d t a k e a p a r t

A l i t t l e less t h a n f o u r fifths of t h e a i r

a

s a m p l e of a i r f r o m n e a r t h e g r o u n d a n d

is n i t r o g e n . T h i s g a s is a l s o i m p o r t a n t

another

t o l i v i n g t h i n g s . N i t r o g e n is a p a r t of all

a b o v e t h e e a r t h , y o u w o u l d find

living tissues. H o w e v e r ,

t h e a m o u n t s of t h e g a s e s w o u l d b e dif-

living

things

cannot use nitrogen directly from

the

one

from

a

hundred

miles that

ferent in the t w o samples. T h e sample

air, as they d o oxygen. P l a n t s get nitro-

taken near the ground would have

g e n f r o m b a c t e r i a i n t h e soil, a n d a n i -

h i g h e r p r o p o r t i o n of t h e h e a v i e r g a s e s :

m a l s g e t it b y e a t i n g p l a n t s .

carbon

Humans

obtain their nitrogen supply from

ani-

dioxide,

argon,

krypton

a

and

xenon. T h e upper sample would have

m a l s i n t h e f o r m of m e a t a n d fish a n d ,

a h i g h e r p r o p o r t i o n of h y d r o g e n ,

m o r e directly, f r o m p l a n t s in fruits a n d

lium, nitrogen, oxygen, a n d neon.

he-

vegetables. T h e t h i r d i m p o r t a n t g a s i n t h e a i r is

W e h a v e s a i d t h a t a i r is a k i n d of m a t t e r

c a r b o n dioxide. A l t h o u g h air contains

only living things t h a t c a n m a k e their

a n d t h a t m a t t e r is H o w can you s o m e t h i n g t h a t t a k e s show that air T, , . 0 u p space. It m a y seem J takes up space? r r h a r d to believe that

own food and they need carbon dioxide

air t a k e s u p space, b u t y o u c a n easily

very little c a r b o n d i o x i d e c o m p a r e d t o oxygen a n d nitrogen, the c a r b o n dioxi d e is v e r y i m p o r t a n t . P l a n t s a r e

the

s h o w t h a t t h i s is t r u e . A l l y o u n e e d is

Air takes up space. a n o r d i n a r y d r i n k i n g g l a s s a n d a p a i l of

will s e e t h a t o n l y a s m a l l a m o u n t

w a t e r . T h e g l a s s l o o k s e m p t y . If y o u

w a t e r h a s risen i n t o t h e glass. T h e rest

were to show t h e glass to a friend a n d

of t h e g l a s s is filled w i t h a i r . T h e a i r is

a s k h i m w h a t is i n t h e g l a s s , h e w o u l d

t a k i n g u p t h e s p a c e inside t h e glass, a n d

probably

t h e w a t e r c a n n o t fill it.

say,

"Nothing,"

of

or,

"It's

e m p t y . " P u t y o u r h a n d in the

glass.

N o w s l o w l y tilt t h e g l a s s t o o n e s i d e

There seems to be nothing there; the

u n t i l s o m e of t h e a i r e s c a p e s . Y o u w i l l

g l a s s c e r t a i n l y feels e m p t y . B u t w e h a v e

s e e a l a r g e b u b b l e of a i r r i s e t o t h e s u r -

s a i d t h a t a i r fills all t h e h o l l o w p l a c e s

f a c e of t h e w a t e r w i t h a p o p p i n g s o u n d .

o n t h e s u r f a c e of t h e e a r t h . S u r e l y , t h e r e

L o o k at t h e glass again. T h i s time, y o u

m u s t b e air inside t h e glass.

will see t h a t t h e w a t e r h a s risen h i g h e r

P o u r w a t e r i n t o t h e g l a s s u n t i l it is filled

i n t h e g l a s s . N o w t h a t t h e b u b b l e of a i r

t o t h e b r i m . T h e w a t e r t h a t fills

is n o l o n g e r t a k i n g u p s p a c e i n t h e g l a s s ,

t h e g l a s s h a s p u s h e d a l l t h e a i r o u t of

s o m e w a t e r c a n get in. E a c h t i m e y o u

t h e glass. B u t

tip t h e glass a n d let air escape,

suppose

the

air

were

t r a p p e d inside the glass a n d the w a t e r c o u l d n o t p u s h it o u t . If a i r r e a l l y t a k e s

more

w a t e r w i l l r i s e i n t o it. Using this s a m e equipment, you c a n

u p s p a c e , h o w c a n y o u fill t h e g l a s s w i t h

perform

water?

Bet them that you can put a handker-

P o u r a l l t h e w a t e r o u t of t h e g l a s s . N o w t u r n t h e glass upside d o w n p u s h it d o w n i n t o t h e p a i l of B e sure y o u h o l d t h e glass

and

water.

perfectly

a trick to fool y o u r

friends.

chief u n d e r w a t e r w i t h o u t g e t t i n g it w e t . C r u m p l e t h e h a n d k e r c h i e f a n d p a c k it firmly

i n t o t h e b o t t o m of t h e g l a s s . N o w

t u r n t h e glass u p s i d e d o w n a n d l o w e r it

s t r a i g h t . D o n o t tilt it t o o n e s i d e . P u s h

into t h e w a t e r as you did before.

t h e g l a s s d o w n i n t o t h e w a t e r u n t i l all of

h a n d k e r c h i e f will stay d r y as l o n g as

The

it is u n d e r w a t e r . N o w l o o k c l o s e l y . Y o u

you h o l d the glass straight.

AIR HOSE

must work under water. Perhaps

the

m e n m u s t dig holes for the f o u n d a t i o n of a b r i d g e o r a d a m , o r p e r h a p s t h e y m u s t repair a s u n k e n ship. T h e y can d o t h i s a n d still r e m a i n d r y b y w o r k i n g inside a diving bell. A diving bell looks l i k e a l a r g e i r o n b e l l . I t is l o w e r e d beneath the water — open end down — u n t i l it r e s t s o n t h e b o t t o m . B e c a u s e t h e d i v i n g b e l l is filled w i t h a i r , w a t e r d o e s n o t r i s e i n t o it, j u s t a s w a t e r d i d n o t r i s e i n t o t h e g l a s s . A i r is t a k i n g u p s p a c e inside the diving bell a n d w a t e r

cannot

take u p the same space. Y o u remember that when you pushed t h e g l a s s all t h e w a y d o w n i n t o t h e p a i l , a s m a l l a m o u n t of w a t e r d i d r i s e i n t o t h e g l a s s . If y o u h a d p u s h e d t h e g l a s s m a n y feet u n d e r w a t e r , t h e w a t e r w o u l d h a v e r i s e n i n t o it e v e n f u r t h e r . T o p r e v e n t w a t e r f r o m rising i n t o a diving bell, a i r is p u m p e d i n t o it f r o m t h e s u r f a c e .

T h e p r o b l e m of s h o w i n g t h a t a i r h a s How can we prove that air has weight? Very little water will enter the diving bell, or underwater caisson, because the air in it takes up space and prevents the water from rising. But if the men working at the bottom of the bell are to stay dry, no water at all should enter. To accomplish this compressed air is forced into the diving bell. Weighted in this way, it sinks. The workmen go in and out through airlocks which allow them gradually to get used to the high pressure.

w e i g h t is a v e r y

o n e . I t w a s first s o l v e d i n t h e s e c o n d h a l f of the sixteenth

by a young

old

Italian

century

scientist,

Galileo

It would b e nice to repeat

exactly

Galilei. w h a t Galileo did, b u t we c a n n o t cause

we

need

equipment

be-

especially

m a d e f o r t h i s e x p e r i m e n t . I n s t e a d , let u s r e a d w h a t G a l i l e o d i d , a n d w e will l e a r n t h e interesting w a y in w h i c h h e

The

experiment

»A#L » • W h a t is a diving bell?

you

just

performed

p r o v e d that air h a s weight. Galileo obtained a sturdy bottle with

with the glass a n d the ° ail of w a t e r P showed you

a narrow neck. T h e n he had a craftsman

t h a t w a t e r could not get

m a k e a leather sack both airtight and

into the glass as long as there was air

watertight,

i n s i d e it. T h i s f a c t is u s e d b y m e n w h o

T h e s a c k w a s s h a p e d p r e t t y m u c h like

8

and with

a narrow

neck.

the bottle but h a d only three quarters

s q u e e z e d i n t o a smaller space, h a d be-

as m u c h s p a c e i n s i d e .

c o m e compressed

G a l i l e o filled t h e s a c k w i t h w a t e r . H e then wet

the neck

of

the

sack

and

air.

U s i n g these facts, G a l i l e o

reasoned

t h i s w a y : a s p a c e filled w i t h c o m p r e s s e d

s l i p p e d it o v e r t h e n e c k of t h e b o t t l e .

air contains m o r e air t h a n the

T h e wet leather shrank and

s p a c e filled w i t h o r d i n a r y a i r — a i r t h a t

fitted

t i g h t l y a r o u n d t h e n e c k of t h e

so

bottle

is n o t c o m p r e s s e d . If a i r h a s

same

weight,

that n o t even air c o u l d m o v e in o r out-of

t h e n a v o l u m e of c o m p r e s s e d a i r s h o u l d

where the sack and bottle joined.

w e i g h m o r e t h a n t h e s a m e v o l u m e of

Squeezing

and

twisting

the

sack,

o r d i n a r y air.

Galileo forced water from the sack into

T o p r o v e his reasoning, G a l i l e o care-

the bottle. T h e n he w r a p p e d the twisted

fully w e i g h e d t h e b o t t l e w i t h t h e s a c k

e m p t y s a c k a r o u n d t h e n e c k of t h e b o t -

w r a p p e d a r o u n d it a n d t h e w a t e r a n d

tle a n d t i e d it i n p l a c e .

compressed

Galileo n o w h a d these facts

before

h i m : t h e b o t t o m t h r e e f o u r t h s of bottle

contained

water;

the

the

upper

f o u r t h c o n t a i n e d all t h e air t h a t

had

air

inside.

Having

made

this weighing, Galileo untied the sack a n d p u l l e d it f r o m t h e b o t t l e . T h e c o m pressed

air

expanded

causing

three

q u a r t e r s of a b o t t l e f u l t o e s c a p e . T h i s

c o m p l e t e l y filled t h e b o t t l e b e f o r e t h e

left o n l y o n e - q u a r t e r b o t t l e f u l

of

or-

water was forced

d i n a r y air in t h e bottle. K n o t t i n g

the

in. T h a t

air,

now

To prove that air has weight you can perform an experiment much like Galileo's. For the step by step description see page 10.

e m p t y s a c k a r o u n d t h e n e c k of t h e b o t -

Using the pliers, cut a n d b e n d

the

tle, G a l i l e o a g a i n w e i g h e d b o t t l e , s a c k ,

coat hanger to m a k e a wire cradle as

water, a n d air. H e c o m p a r e d the results

s h o w n in t h e illustration o n p a g e 9.

of t h e t w o w e i g h i n g s a n d f o u n d

that

T i e t h e c a r p e n t e r ' s level to t h e yardstick as shown. T h e center hairline on

t h e first w a s h e a v i e r . In b o t h weighings, nothing h a p p e n e d

t h e g l a s s t u b e of t h e l e v e l s h o u l d b e a t

to the bottle, sack, and water to m a k e

the 18-inch m a r k on the yardstick. At-

them change weight. T h e only

thing

t a c h t h e D - c l a m p t o t h e t o p of a d o o r -

t h a t c h a n g e d w a s t h e a m o u n t of a i r i n

w a y . P l a c e t h e y a r d s t i c k a n d level in

t h e bottle. T h e r e w a s m o r e air in t h e

the wire cradle and h a n g them from the

b o t t l e d u r i n g t h e first w e i g h i n g , a n d t h i s

c l a m p . B e s u r e t h a t t h e d o o r w a y is n o t

weighing was heavier. T h e extra

air

drafty, because wind blowing on your

that

e x p e r i m e n t will m a k e y o u r results c o m e

m a d e it h e a v i e r , a n d t h i s p r o v e d

out wrong.

air has weight. did

H a n g the two shopping bags on the

n o t c o m p r e s s a i r s i m p l y b y p u m p i n g it

y a r d s t i c k b y p l a c i n g t h e h a n d l e s of a

into a bottle, s o m e w h a t as w e compress

b a g i n o n e of t h e n o t c h e s .

air in a bicycle or a u t o m o b i l e tire. T h e

L e t t h e a i r o u t of t h e

Y o u m a y wonder why Galileo

basketball.

a n s w e r is t h a t t h e first a i r p u m p w a s n o t

T h e n p u m p air i n t o it just until

the

i n v e n t e d until m o r e t h a n half a c e n t u r y

creases have smoothed out. T h e

ball

after G a l i l e o p e r f o r m e d his e x p e r i m e n t

n o w s h o u l d b e soft w h e n y o u p o k e it

to show that air has weight.

w i t h y o u r finger.

Although

you

cannot

repeat

Gali-

C a r e f u l l y p u t t h e b a l l i n t o o n e of t h e

l e o ' s e x p e r i m e n t e x a c t l y a s h e d i d it, y o u

shopping

bags.

Put

weights

such

as

c a n p e r f o r m a n e x p e r i m e n t t h a t is m u c h

pebbles, nails, or s a n d into the

like the o n e h e devised.

b a g u n t i l t h e b u b b l e i n t h e level s h o w s

other

F o r G a l i l e o ' s b o t t l e , y o u will u s e a

t h e y a r d s t i c k t o b e h a n g i n g level. T h i s

basketball, for t h e leather sack, substi-

s h o w s t h a t t h e t w o b a g s a r e in b a l a n c e ,

t u t e t h e k i n d of p u m p u s e d t o i n f l a t e

that they weigh the same. R e m o v e the ball from the bag, being

basketballs or footballs. Y o u have to m a k e a weighing appa-

careful

that

the

bag

containing

the

r a t u s . F o r this y o u will n e e d a y a r d s t i c k ,

weights does n o t lower so fast a n d h a r d

s e v e r a l f e e t of t h i n , y e t s t r o n g , s t r i n g

as to t h r o w y o u r w h o l e weighing appa-

( f i s h i n g l i n e is g o o d ) , a c o a t h a n g e r , a

r a t u s o u t of k i l t e r .

c a r p e n t e r ' s level, a D - c l a m p , a n d

two

N o w p u m p a i r i n t o t h e b a l l u n t i l it is a s h a r d a s w h e n it is r e a d y t o b e u s e d

shopping bags. T h e tools y o u will n e e d a r e a penk n i f e , a s m a l l s a w , a n d a p a i r of p l i e r s .

in a b a s k e t b a l l g a m e . P u m p i n g the additional air into the ball

compresses

one

all t h e air in t h e ball. M o r e air t a k e s u p

i n c h f r o m a n e n d of t h e y a r d s t i c k . E a c h

t h e s a m e s p a c e as t h e air t h a t w a s in

notch

t h e b a l l w h e n y o u w e i g h e d it.

C u t two square notches, each

wide.

10

should be about

half

an

inch

Gently put the ball back into

the

shopping bag and examine the bubble

the same instant. W h y ? Something must

in t h e level. Y o u will see t h a t t h e b u b -

h a v e s l o w e d t h e f a l l of t h e p a p e r s t r i p

ble

on

t h e first t i m e w e m a d e it fall; a n d w h a t -

hanging.

e v e r s l o w e d t h e p a p e r t h e first t i m e d i d

has

which

moved

the

toward

weighted

the

bag

side

is

This means that the ball n o w more

than

weights) ball

the

that

before

pebbles

weighs

(or

additional

c h a n g e w e m a d e in t h e glass t u b e a n d

the

its c o n t e n t s w a s t o r e m o v e t h e a i r . S o ,

was

it m u s t h a v e b e e n t h e a i r i n t h e t u b e

air

p u m p e d in. T h e additional weight m u s t

t h a t s l o w e d t h e fall of t h e p a p e r . A i r is a k i n d of m a t t e r a n d t h e r e f o r e

h a v e c o m e from the a d d e d air.

takes u p H o l d a p e n n y in one h a n d a n d a small s t r i p of p a p e r i n Why does a penny fall faster than a _ piece of paper?

Q t h e r

tance above the

floor.

h a n d

, lt , , , H o l d both hands

TT

the

only

other

h a d just balanced the

n o t d o so the second time. T h e

same

filled

space. W h e n

with

air,

the

the tube

penny,

was

which

is

h e a v i e r t h a n t h e s m a l l s t r i p of p a p e r , c o u l d m o r e e a s i l y p u s h its w a y

down

t h r o u g h t h e air.

dis-

L e t g o of

the

W e have demonstrated that air

u p space; air also has

penny and the paper at the same instant. W h i c h s t r i k e s t h e f l o o r first? T h e p e n n y d o e s . N o w let u s s e e w h y . L e t us suppose that we h a v e a wide

takes

0

" ^ . T . / " weight. These are charshow that air a c t e r i s t i c s of a l l m a t is elastic? ter. A i r h a s a n o t h e r

g l a s s t u b e t h a t is five f e e t l o n g . W e p l a c e

c h a r a c t e r i s t i c t h a t is s h a r e d b y all g a s e s :

a p e n n y a n d a s m a l l s t r i p of p a p e r in-

I t is elastic.

side t h e t u b e , c o v e r b o t h e n d s tightly,

s p r i n g y . I t w i l l s p r i n g b a c k t o its o r i g -

and hold the tube upright. T h e penny

inal v o l u m e after being compressed or

and the p a p e r are n o w resting at the bot-

squeezed.

tom

of t h e t u b e .

We

turn

the

tube

This means

that

air

is

Y o u c a n p r o v e t h a t a i r is e l a s t i c b y

a r o u n d , s o t h a t t h e b o t t o m is n o w t h e

m e a n s of a b i c y c l e p u m p o r a

top. W e do this so quickly t h a t

the

u s e d t o inflate footballs o r b a s k e t b a l l s .

p e n n y a n d t h e p i e c e of p a p e r b e g i n t o

T o d o t h i s , p u l l t h e p u m p h a n d l e all t h e

fall f r o m t h e t o p a t t h e s a m e i n s t a n t .

w a y o u t so t h a t air will e n t e r t h e p u m p .

T h e p e n n y falls r a p i d l y t o t h e b o t t o m ,

Now

w h i l e t h e s t r i p of p a p e r g l i d e s s l o w l y

n o z z l e of t h e p u m p a n d p u s h t h e h a n d l e

downward.

into t h e p u m p as far as y o u can. Sud-

L e t us n o w connect the glass t u b e to a n e x h a u s t p u m p t h a t p u m p s a l m o s t all

hold

a

finger

tightly

pump

over

the

d e n l y l e t g o of t h e h a n d l e . I t w i l l s p r i n g outward.

t h e a i r o u t of t h e t u b e . ( W e w o u l d l i k e

W h e n y o u p u s h e d t h e h a n d l e in, y o u

t o t a k e a l l t h e a i r o u t of t h e t u b e , b u t

c o m p r e s s e d the air in the p u m p . W h e n

t h e r e is n o p u m p t h a t c a n d o

this.)

y o u l e t g o of t h e h a n d l e , it s p r a n g o u t -

Again we quickly turn the tube around.

w a r d b e c a u s e t h e a i r b e h i n d it i n t h e

T h i s t i m e t h e s t r i p of p a p e r falls a s f a s t

pump

as t h e p e n n y a n d s t r i k e s t h e b o t t o m a t

springiness proves that the air inside the

was

springing

outward.

This

11

p u m p is e l a s t i c . S i n c e t h e a i r i n s i d e t h e

hydrogen

p u m p w a s d r a w n in f r o m t h e o u t s i d e ,

f o r m e d c o n t a i n s t w o v o l u m e s of h y d r o -

it w a s n o t d i f f e r e n t f r o m t h e a i r o u t s i d e .

g e n f o r e v e r y o n e v o l u m e of o x y g e n . I n

T h u s , the outside air m u s t be springy,

f a c t , all w a t e r e v e r y w h e r e c o n t a i n s t w o

o r e l a s t i c , t o o . A l l a i r is e l a s t i c . T h i s is

v o l u m e s of h y d r o g e n f o r e v e r y o n e vol-

the reason water rose a short way into

u m e of o x y g e n . S a m p l e s of w a t e r t a k e n

t h e g l a s s w h e n y o u d i d t h e first e x p e r i -

from a raindrop, from a faucet, or from

m e n t in this b o o k . T h e air w a s

t h e b o t t o m of t h e o c e a n will all h a v e t h e

com-

p r e s s e d b y t h e w a t e r p r e s s i n g a g a i n s t it.

or

oxygen

left.

The

water

s a m e a m o u n t s of t h e s e t w o g a s e s . B e c a u s e w a t e r is a l w a y s t h e s a m e , scient i s t s c a n w r i t e a forWhat is the m u l a f o r it T o t h e scientific s c i e n t i s t , w a t e r is name for water? H . O . This simply m e a n s t h a t w a t e r h a s t w o v o l u m e s of

You can easily prove that air is elastic — springy — by compressing it within a bicycletire-pump.

h y d r o g e n ( H ) f o r e v e r y o n e of o x y g e n ( O ) . T h e n e x t t i m e y o u r f r i e n d s visit y o u , offer t h e m a g l a s s of H 2 0 t o d r i n k . They'll probably be surprised when you c o m e b a c k from the kitchen with a glass of p l a i n w a t e r .

W e t h i n k of w a t e r a s a l i q u i d , b u t w a t e r W a t e r is m a d e of h y d r o g e n a n d o x y g e n , What is water?

t w o of t h e g a s e s t h a t , , ° , a r e f o u n d in t h e air.

I t is a w o n d e r f u l f a c t of n a t u r e t h a t t w o gases can c o m b i n e to m a k e a

liquid,

a n d t h i s is w h a t h a p p e n s w h e n o x y g e n and

hydrogen

are

combined

in

the

proper way. T o combine hydrogen and oxygen, a scientist p u t s t w o v o l u m e s ( a v o l u m e of a g a s , l i q u i d , o r s o l i d is t h e a m o u n t of s p a c e it t a k e s u p ) of h y d r o g e n a n d o n e v o l u m e of o x y g e n i n t o a s t u r d y c o n t a i n e r , p e r h a p s a flask of v e r y t h i c k g l a s s . T h e n h e s e t s off a n e l e c t r i c spark within the container to produce an explosion. After the explosion, drops of w a t e r will a p p e a r o n t h e i n s i d e of t h e c o n t a i n e r a n d t h e r e will b e n o 12

more

c a n a l s o b e a solid o r When is water a gas. Solid, liquid, not wet? and gas are the three states

of matter.

W a t e r in t h e solid state

is c a l l e d ice. I c e is n o t w e t . T h i s m a y s u r p r i s e y o u , b u t t h e r e a s o n ice feels wet

when

is t h a t

the

melts the

ice,

c h a n g i n g s o m e of it t o w a t e r . I n

the

warmth gaseous

you

touch

it

of y o u r h a n d state

water

is

called

water

v a p o r . Y o u c a n n o t see w a t e r v a p o r bec a u s e it is a n i n v i s i b l e g a s , b u t

when

w a t e r v a p o r cools y o u c a n see a c l o u d of s t e a m . S t e a m is m a d e of t i n y d r o p l e t s of w a t e r i n t h e a i r . Y o u c a n easily c h a n g e ice to water, water to water vapor, and water vapor t o s t e a m . T a k e f o u r o r five i c e c u b e s

a n d p u t t h e m i n t h e b o t t o m of a t e a kettle. N o w p u t the tea kettle on the s t o v e . L e a v e t h e c o v e r of t h e t e a k e t t l e off s o y o u c a n s e e w h a t h a p p e n s . S o o n all t h e i c e h a s m e l t e d a n d t u r n e d i n t o water. N o w p u t the cover on the tea

When you melt ice, it turns into water.

k e t t l e . I n a s h o r t t i m e t h e w a t e r will begin t o boil a n d t h e r e will b e a c l o u d of s t e a m

near the

spout.

Now

look

c l o s e l y a t t h e e n d of t h e s p o u t . B e t w e e n t h e c l o u d of s t e a m a n d t h e s p o u t is a small space that seems to b e e m p t y . B u t it is n o t e m p t y . I t is filled w i t h

water

vapor. Y o u c a n p r o v e t h a t this invisible

When you boil water, it turns into water vapor and steam.

s u b s t a n c e is a f o r m of w a t e r b y t u r n i n g s o m e of it b a c k i n t o a l i q u i d . W r a p a towel or pot holder a r o u n d the handle of a m e t a l s p o o n a n d h o l d t h e b o w l of the s p o o n in front

of t h e t e a

kettle

s p o u t . D r o p s of w a t e r w i l l f o r m o n t h e s p o o n . Y o u c a n c h a n g e t h e d r o p s of water b a c k into ice b y p u t t i n g the s p o o n

When you cool water vapor, you can turn it back into water.

in t h e f r e e z e r of a r e f r i g e r a t o r . L a t e r w e will s e e t h a t a p r o c e s s v e r y m u c h l i k e this is g o i n g o n a l l t h e t i m e i n n a t u r e .

the ice y o u w o u l d b e w r o n g . A b u c k e t of i c e is l i g h t e r t h a n a b u c k e t of w a t e r .

If s o m e o n e w e r e t o a s k y o u w h i c h is h e a v i e r , a b u c k e t of i c e W h y is ice Q r fl ^uc]^Qt Q f w a t e r > y o u lighter than . , ^ „ , , m i g h t very well c h o o s e t h e b u c k e t of i c e b e c a u s e y o u p r o b a b l y t h i n k of s o l i d s a s b e i n g h e a v i e r t h a n l i q u i d s . B u t if y o u c h o s e

Water

is

different

from

most

other

l i q u i d s b e c a u s e w h e n it f r e e z e s a n d b e comes

s o l i d , it b e c o m e s

lighter

than

w h e n it is a l i q u i d . Y o u c a n d e m o n s t r a t e this very easily. T a k e t w o m e t a l m e a s u r i n g cups like the ones your m o t h e r uses to m e a s u r e

flour.

A cup of ice, when melted, does not make a cup of water.

The water reaches the "1 cup" mark.

Water expands when it freezes.

13

Fill t h e m with water to the m a r k that

mals—fish,

s a y s " o n e c u p . " N o w p u t o n e c u p of

snakes,

w a t e r i n t h e f r e e z e r of y o u r r e f r i g e r a t o r

living t h e r e w o u l d freeze a n d die. F u r -

a n d l e a v e it t h e r e u n t i l t h e w a t e r h a s

t h e r m o r e , in t h e spring, t h e w a r m sun-

t u r n e d t o s o l i d ice. W h e n a l l t h e w a t e r

light w o u l d n o t b e a b l e t o m e l t all t h e

h a s frozen, y o u will see t h a t t h e

i c e a s i t c a n b e c a u s e t h e i c e is o n l y a t

sticks u p a b o v e the " o n e c u p " T h i s is b e c a u s e w a t e r

expands

ice

mark. as

the "one cup" mark. N o w you have one c u p of i c e . L e t it s t a n d u n t i l a l l t h e i c e When

t h e ice h a s

salamanders,

others—and

water

the

plants

the surface.

it

f r e e z e s . N o w c h i p a w a y all t h e i c e a b o v e

has melted.

and

frogs,

com-

Y o u k n o w t h a t s t e e l is m u c h

t h a n water, yet you can How can you float a piece of steel?

p l e t e l y m e l t e d , l o o k a t t h e l e v e l of t h e water. It n o longer reaches to the "one c u p " m a r k . A p a r t l y filled c u p of w a t e r w e i g h s less t h a n a full c u p . T h e r e f o r e a c u p of i c e m u s t w e i g h l e s s t h a n a c u p of w a t e r . If y o u h a v e a s c a l e y o u c a n

heavier

float

water!

iece of s t e d

o n

It

im-

sounds

p o s s i b l e , y e t it is r e a l l y

very easy t o m a k e a steel needle

float.

A l l y o u n e e d is a g l a s s of w a t e r , a f o r k , a n d a needle from your mother's sewing basket. W a s h the fork and needle with soap. B e s u r e t o r i n s e off all t h e s o a p . P l a c e

m e a s u r e the difference.

the needle on the fork and lower the it

f o r k s l o w l y i n t o t h e g l a s s of w a t e r . T h e

When you put

n e e d l e w i l l float. W h y d o e s t h i s h a p p e n ?

Why aren't fish ice cubes in a glass frozen into the of w a t e r , t h e i c e b o b s ice of lakes u p a n d d o w n for a and rivers? second or two, and

W a t e r is m a d e u p of v e r y s m a l l p a r -

B e c a u s e i c e is l i g h t e r t h a n floats.

water,

t h e n floats a t t h e t o p of t h e w a t e r . I c e o n f r o z e n l a k e s a n d r i v e r s a l s o floats a t t h e t o p of w a t e r . T h i s f a c t is v e r y i m p o r t a n t t o p l a n t s a n d a n i m a l s t h a t live i n t h e w a t e r . B e l o w t h e ice, t h e l i v i n g t h i n g s c a n c o n t i n u e t o live. T h e i c e a c t s as a shield, protecting t h e w a t e r

be-

n e a t h it f r o m t h e c o l d a i r a b o v e . If t h e ice w e r e heavier t h a n w a t e r , t h e b o t t o m of t h e l a k e w o u l d fill w i t h i c e . E v e n t u ally, t h e w h o l e l a k e w o u l d b e

frozen

from b o t t o m t o surface a n d all t h e ani-

As ice is lighter than water, it floats. This permits life to go on in frozen lakes and rivers. 14

ticles called molecules. E a c h m o l e c u l e is m a d e u p of t w o a t o m s of h y d r o g e n a n d o n e a t o m of o x y g e n . A m o l e c u l e of w a t e r is s o s m a l l t h a t it is i m p o s s i b l e t o

Surface tension allows the steel needle to float (illustrations at left). Dropping a few grains of soap powder into the glass lessens the surface tension and the needle sinks (illustration below).

see it e v e n

with

the

most

powerful

microscope. M o l e c u l e s cling

together.

T h o s e o n t h e surface cling t o g e t h e r so t i g h t l y t h a t t h e y f o r m a film, o r s k i n . I m a g i n e a l a r g e g r o u p of c h i l d r e n i n a schoolyard. E a c h child h a s his a r m s a b o u t t h e w a i s t s of t h e c h i l d r e n n e x t t o h i m . T h e c h i l d r e n o n t h e o u t s i d e of t h e group form an unbroken chain. These c h i l d r e n a r e a l s o h e l d b y s o m e of t h e c h i l d r e n n e x t t o t h e m o n t h e i n s i d e of the g r o u p . A s a result, t h e children o n t h e o u t s i d e will f o r m a s t r o n g r i n g a b o u t t h e o t h e r s . If s o m e o n e t r i e d t o w a l k i n t o the g r o u p , h e w o u l d h a v e a h a r d t i m e p u s h i n g t h r o u g h t h e r i n g of c h i l d r e n o n the outside. T h e y w o u l d p r o b a b l y

be

able to hold h i m out. T h e m o l e c u l e s a t t h e s u r f a c e of t h e water a r e like t h e c h i l d r e n in t h e ring

If t h e s u r f a c e film is b r o k e n , t h e n e e d l e

a r o u n d t h e r e s t of t h e g r o u p . T h e s u r -

w i l l i m m e d i a t e l y s i n k t o t h e b o t t o m of

face m o l e c u l e s c l i n g t o g e t h e r s o t i g h t l y

the

that

t h r o u g h t h e s u r f a c e of t h e w a t e r , a n d i t

they

can

hold

the

steel

needle

afloat. S c i e n t i s t s h a v e a n a m e f o r s t r o n g c l i n g i n g t o g e t h e r of t h e

the

mole-

glass.

Push

the

floating

needle

will s i n k t o t h e b o t t o m . If y o u l e s s e n t h e s u r f a c e t e n s i o n , y o u

cules a t t h e s u r f a c e of a l i q u i d , s u c h a s

will n o t b e a b l e t o

w a t e r . T h e y c a l l it surface

w a t e r . L e t us try a n e x p e r i m e n t t h a t will

tension.

If y o u l o o k c l o s e l y a t t h e needle, y o u will see t h e surface

floating film

b e n d i n g u n d e r t h e w e i g h t of t h e n e e d l e .

float

a needle

on

p r o v e t h i s . T a k e t h e n e e d l e o u t of t h e w a t e r a n d d r y it. U s e t h e f o r k t o

float

the needle

few

again.

Now,

drop

a

15

g r a i n s of s o a p p o w d e r i n t o t h e g l a s s . T h e needle sinks. W h a t h a p p e n e d ? T h e s o a p l e s s e n e d t h e s u r f a c e t e n s i o n so t h a t the molecules at the surface no longer clung together strongly enough to hold t h e n e e d l e a f l o a t . T h i n k of t h e c h i l d r e n in t h e

schoolyard.

Suppose

someone

t o l d m o s t of t h e c h i l d r e n m a k i n g u p t h e outside ring to remove their arms from t h e w a i s t s of t h e c h i l d r e n n e x t t o t h e m . T h e outside ring would no longer

be

strong, a n d a n y o n e wishing to walk into t h e g r o u p c o u l d e a s i l y d o so. A l t h o u g h molecules

have

no

arms,

you

might

t h i n k of t h e a c t i o n of t h e s o a p a s o n e t h a t c a u s e d e a c h m o l e c u l e at t h e surface t o r e m o v e its " a r m s " f r o m t h e m o l e c u l e n e x t t o it. W h e n your mother puts soap into the w a s h i n g m a c h i n e , s h e is r e d u c i n g

the

s u r f a c e t e n s i o n of t h e w a t e r so t h a t t h e water can soak into the clothes and

float

a w a y dirt m o r e easily.

Air

and in

The

W a t e r

Nature

earth's ocean

of a i r , t h e

atmos-

sphere, extends W h a t a r e the layers of the , _ atmosphere?

6QQ

mile§

about

a b o y e

t h e

. , , . r e a r t h s s u r f a c e a n d is divided into layers. Let

us p r e t e n d that we are a b o a r d a rocket t h a t is r i s i n g f r o m t h e e a r t h ' s

surface

a n d p a s s i n g t h r o u g h all t h e l a y e r s of the atmosphere. W e begin our journey i n t h e l o w e s t l a y e r of a i r . T h i s l a y e r , The earth's ocean of air is divided into layers.

16

c a l l e d t h e troposphere

(trop'-uh-sfeer),

is a b o u t s e v e n m i l e s t h i c k . N o t e v e n t h e

highest m o u n t a i n s rise a b o v e the tropo-

O b t a i n a toy balloon at least ten inches

sphere. W h e n we leave the troposphere, we enter the layer called the

in d i a m e t e r w h e n H o w can you s h o w that cool air takes . ,. u p less s p a c e t h a n w « r m «,r-> w a r m airr

stratosphere

(strat'-uh-sfeer). This layer extends to about 53 miles above the earth.

Here

t h e a i r is v e r y t h i n , c l e a r , a n d c o l d . O u r

b ] o w n

. the

u p

B J o w

, ,, balloon

up , rub-

a n d tie t h e

ber at the mouth-

rocket leaves t h e s t r a t o s p h e r e a n d ent e r s t h e n e x t l a y e r , t h e ionosphere

(eye-

p i e c e of t h e b a l l o o n i n t o a double

knot.

on'-uh-sfeer). This layer extends

from

T h i s is t o m a k e s u r e t h a t n o a i r e s c a p e s .

60 miles to 2 5 0 miles above the earth's

Simply tying the balloon closed

s u r f a c e . I t is c a l l e d t h e i o n o s p h e r e b e -

s t r i n g will n o t s e a l it t i g h t l y e n o u g h . Place a ruler or yardstick on a table

cause here the sun's rays break u p the a t o m s of a i r i n t o e l e c t r i c a l l y particles called

charged

ions. H e r e the air

t h i n n e r t h a n in t h e

stratosphere.

is

The

a n d p u t t h e b a l l o o n u p o n it, m e a s u r i n g t h e d i a m e t e r of t h e b a l l o o n . W r i t e d o w n the measurement. T h e n , place the balloon into a freezer

u p p e r m o s t l a y e r of t h e a t m o s p h e r e is c a l l e d t h e exosphere

(eks'-o-sfeer).The

o r i n t o t h e f r e e z i n g c o m p a r t m e n t of a

air c o n t i n u e s t o b e c o m e t h i n n e r until,

refrigerator.

at a b o u t 6 0 0 miles a b o v e the

overnight.

earth's

there

through

l o o n f r o m t h e f r e e z e r , m e a s u r e it a g a i n .

space.

Y o u w i l l find t h a t it is s m a l l e r . ( A s s o o n

the e x o s p h e r e , w e a r e in o u t e r T h e entire atmosphere

Leave the balloon

I m m e d i a t e l y u p o n r e m o v i n g the bal-

surface, there are n o m o r e air particles. When our rocket has passed

with

has been

left

a s y o u h a v e m e a s u r e d it, p u t t h e b a l l o o n back into the freezer.)

behind us.

T h e air in t h e b a l l o o n c o n t r a c t e d , o r

M o r e t h a n 9 9 p e r c e n t of t h e a i r is i n

shrank. W h e n you blew up the balloon,

the lowest 2 0 miles.

t h e a i r in it w a s a t t h e t e m p e r a t u r e of your lungs, from where the air came. W e learned that the sun's rays break u p the _ the a u r o r a s ?

air

in

the

iono-

This 98

temperature F.

After

you

is took

approximately it f r o m

the

s p h e r e i n t o e l e c t r i c a l lJy \ charged particles

f r e e z e r , t h e a i r in t h e b a l l o o n w a s s o m e -

c a l l e d e l e c t r o n s a n d i o n s . If y o u live i n

l o o n is s m a l l e r , t h e a i r filling t h e c o o l

the far n o r t h e r n or far s o u t h e r n

parts

b a l l o o n t a k e s u p less s p a c e t h a n

of t h e e a r t h y o u h a v e p r o b a b l y

seen

s a m e air w h e n w a r m . Therefore, w h e n

g r e a t c u r t a i n s a n d s t r e a m e r s of

light

a n y a i r is c o o l , it t a k e s u p less s p a c e

moving

across

the

night

shifting

lights a r e m a d e

sky. of

These

w h e r e n e a r 0° F . S i n c e t h e c o o l e r b a l the

than when warm.

electrons

a n d ions m o v i n g in t h e i o n o s p h e r e . W e call t h e s e t h e N o r t h e r n L i g h t s a n d t h e Southern

Lights,

but

n a m e s f o r t h e m a r e aurora aurora

the

scientific

borealis

and

australis. 17

«$*•

These are the six great wind belts of the world. Within each belt, local winds may temporarily blow backward or forward in any direction.

i it is a s s m a l l a s t h e c o o l b a l l o o n . S i n c e s o m e a i r w a s l e t o u t of t h e w a r m b a l l o o n , it n o w c o n t a i n s less a i r t h a n t h e cool balloon. T h i n k a g a i n . If y o u w e i g h b o t h b a l loons, w h i c h will w e i g h less? T h e w a r m b a l l o o n , of c o u r s e , s i n c e y o u let

out

s o m e of its a i r . B u t b o t h b a l l o o n s a r e n o w t h e s a m e size ( e v e n t h o u g h o n e h a s m o r e air in i t ) , therefore, t h e h e a v i e r balloon must contain heavier air—the cool air. O b t a i n a n o t h e r b a l l o o n t h e s a m e size

W e h a v e l e a r n e d t h a t w i n d is m o v i n g

as t h e o n e y o u H o w can you p r o v e j u s t u s e d B l o w that cool air is heavier l f . , ,. . o this balloon 4. than w a r m air? up to the same

a i r . N o w l e t u s find What makes the o u t w h a t m a k e s air wind blow? m o v e in l a r g e

s i z e a s t h e o n e w h o s e size y o u

a m o u n t s a c r o s s t h e s u r f a c e of t h e e a r t h . In the experiment you just performed

wrote

y o u s h o w e d t h a t w a r m a i r is l i g h t e r t h a n

down. Twist a n d hold the r u b b e r at the

c o o l a i r . W h e n a i r is c o o l , it is h e a v y .

m o u t h p i e c e so t h a t n o air c a n escape,

H e a v y air sinks to earth, a n d

b u t d o n o t k n o t it t h i s t i m e .

w a r m light air u p w a r d . F o r short, we

pushes

T h e balloon y o u just blew u p con-

say t h a t w a r m air rises, b u t w e m u s t

tains as m u c h w a r m air as did the w a r m

k e e p in m i n d t h a t t h e w a r m air rises

b a l l o o n in the p r e v i o u s experiment.

o n l y b e c a u s e it is p u s h e d u p b y t h e s i n k -

Again remove the cool balloon from the freezer. N o w were to weigh

think. Suppose

both

balloons,

would weigh more? Neither,

you

which because

y o u b l e w t h e s a m e a m o u n t of a i r i n t o both. L e t a i r o u t of t h e w a r m b a l l o o n u n t i l 18

i n g of h e a v y c o o l a i r . Because the region around the earth's e q u a t o r is t h e w a r m e s t p a r t of t h e e a r t h , t h e a i r a b o v e it is w a r m e r t h a n t h e a i r o v e r o t h e r r e g i o n s . T h i s w a r m a i r is pushed u p w a r d by the cooler, heavier

air t h a t flows in n o r t h a n d s o u t h of t h e

O n sunny days land surfaces such

as

be-

beaches and W h y is t h e s e a s h o r e c. , , , ' ,. - fields become cool in the s u m m e r ? warmer than

Eventually,

nearby water surfaces such as lakes a n d

t h i s c o o l e d a i r s i n k s , t a k i n g t h e p l a c e of

o c e a n s . T h e air in c o n t a c t w i t h t h e l a n d

the air

in t o w a r d t h e e q u a t o r .

is h e a t e d , e x p a n d s , a n d b e c o m e s l i g h t e r .

T h e s e b r o a d m o v e m e n t s of a i r a l o n g

Cooler, heavier air from over the w a t e r

e q u a t o r . A s t h i s c o o l a i r w a r m s , it, i n t u r n , is p u s h e d u p w a r d b y c o o l a i r m o v ing in. T h e rising air a b o v e t h e e q u a t o r flows

northward

and

southward,

c o m i n g c o o l a s it m o v e s . flowing

t h e s u r f a c e of t h e o c e a n t o w a r d

the

flows

toward the land and pushes the

winds.

lighter air u p w a r d . T h e rising w a r m air

T h e s e w i n d s d o n o t a c t u a l l y b l o w di-

flows t o w a r d t h e w a t e r t a k i n g t h e p l a c e

rectly n o r t h or south, b e c a u s e t h e e a r t h

of t h e c o o l a i r . T h e w a r m a i r t h a t r i s e s

is t u r n i n g f r o m e a s t t o w e s t f a s t e r t h a n

and moves out over the water becomes

the

equator

are

called

the

trade

its s u r f a c e .

As

cooled and therefore heavier. This cool

a result, the e a r t h turns b e n e a t h

the

a i r s i n k s d o w n t o t h e s u r f a c e of

air m o v i n g

over

the

southward-moving winds, causing them

w a t e r w h e r e it is f u r t h e r c o o l e d . T h e n

to move from northeast to

it flows t o w a r d t h e l a n d t o p u s h u p w a r d

southwest

and the winds are k n o w n as the north-

the air t h a t

east t r a d e winds. I n t h e e a r t h ' s S o u t h -

a n d has since b e c o m e w a r m e d by con-

ern H e m i s p h e r e , t h e directions a r e re-

tact

versed; the t r a d e winds b l o w from the

which air moves from water to land a n d

southeast to the northwest.

b a c k t o w a t e r a g a i n is c a l l e d t h e

And

the

with

flowed

the

land.

in f r o m t h e w a t e r This process,

winds are k n o w n as the southeast trade

cycle.

winds.

itself o v e r a n d o v e r a g a i n .

in wind

A c y c l e is a p r o c e s s t h a t r e p e a t s

A t n i g h t , d i r e c t i o n s of t h e w i n d c y c l e Wind blows toward land during the day and toward water at night.

19

are reversed. T h e l a n d surfaces such as

S o o n e r o r l a t e r , s o m e of t h e c h i l d r e n

s a n d , soil, a n d r o c k , w h i c h a r e q u i c k l y

o n t h e i n s i d e of t h e g r o u p w o u l d b u m p

w a r m e d by the sun also cool

into those forming the ring a r o u n d the

quickly

w h e n the sun n o longer shines on them.

whole

W a t e r surfaces are w a r m e d slowly a n d

b u m p e d hard enough, they would cause

cool slowly. A f t e r sunset, t h e l a n d sur-

o n e of t h e c h i l d r e n f o r m i n g t h e o u t s i d e

faces

the

ring t o loosen his g r a s p o n his neigh-

n e a r b y w a t e r surfaces. T h e air over the

b o r s ' w a i s t s ; a n d t h e f o r c e of t h e b u m p

w a t e r is w a r m e d b y c o n t a c t w i t h

the

w o u l d k n o c k h i m r i g h t o u t of t h e g r o u p .

flows

A n o t h e r child w o u l d t a k e his place, b u t

over the water, pushing the w a r m , light

sooner or later this second child would

air u p w a r d . T h i s w a r m air

b e b u m p e d o u t of t h e g r o u p , t o o . A s

soon

become

cooler

than

water, a n d cool air from the land flows

over

group.

If

those on

the

t h e l a n d w h e r e it b e c o m e s c o o l e d , s i n k s ,

this w e n t on, the g r o u p w o u l d

a n d flows b a c k o v e r t h e w a t e r a g a i n .

smaller a n d smaller.

The

air that

A s m o l e c u l e s c o n t i n u e t o fly off t h e

f r o m t h e w a t e r d u r i n g t h e d a y is c a l l e d

s u r f a c e of w a t e r , t h e a m o u n t of w a t e r

a

left b e h i n d b e c o m e s l e s s a n d less. T h i s

breeze

or

lake

flows

grow

landward

sea

cool

inside

breeze.

It

is

this breeze t h a t m a k e s seashores

and

lakeshores so comfortable

hot

during

weather.

W e l e a r n e d t h a t w a t e r is m a d e of v e r y small particles called How does water molecules. Moleget into the air? cules are constantly moving about and bumping into

each

o t h e r . S o m e of t h e m o l e c u l e s a t t h e s u r f a c e of t h e w a t e r a r e b u m p e d s o h a r d t h a t t h e y b r e a k a w a y a n d fly off i n t o t h e a i r . M o s t of t h e s e m o l e c u l e s f a l l b a c k , b u t s o m e a r e b u m p e d s o h a r d a n d fly so fast t h a t t h e y a r e c a u g h t a m o n g t h e m o l e c u l e s of w h i c h a i r is m a d e . T h e s e water

molecules

are

bumped

along

a m o n g the air molecules and d o

not

return to the water from which

they

came. L e t u s i m a g i n e a g a i n t h e g r o u p of children in t h e schoolyard.

Remember

that they h a d their arms a r o u n d each o t h e r s ' waists. N o w s u p p o s e t h a t all t h e children began to j u m p back and forth.

20

If you put a saucer with water in the sunlight and the same size saucer with water in the shade, you will find that water evaporates faster in a warm place (above). If you put the same amount of water in a spoon and in a saucer and put them next to each other, you will find by checking the time that water evaporates faster from a larger surface than a smaller one (below).

process in w h i c h a l i q u i d loses molec u l e s t o t h e a i r is c a l l e d

evaporation.

Y o u h a v e p r o b a b l y s e e n a p u d d l e of w a t e r o n t h e s i d e w a l k o r o n a flat r o o f "dry up." This drying was really evapor a t i o n in w h i c h all t h e m o l e c u l e s

of

w a t e r i n t h e p u d d l e flew off i n t o t h e a i r . Ordinarily, evaporation takes

place

so s l o w l y t h a t y o u d o n o t n o t i c e it. If

Water vapor in the air condenses and forms beads of water on the side of the container filled with ice.

y o u fill a s a u c e r w i t h w a t e r a n d l e a v e it u n c o v e r e d in a r o o m at o r d i n a r y

tem-

p e r a t u r e , several d a y s will p a s s b e f o r e all t h e w a t e r h a s e v a p o r a t e d . If y o u p u t

m a y b e c o m e so dry that indoor plants d i e u n l e s s t h e h u m i d i t y is i n c r e a s e d . You

may

have

heard

a

weather

the saucer into a w a r m place, such as in

r e p o r t in w h i c h

bright sunlight, the water m a y evapo-

" T h e r e l a t i v e h u m i d i t y is 7 0 p e r c e n t . "

r a t e i n a f e w h o u r s . If t h e w a t e r w e r e

This m e a n s that the air contains 7 0 per-

poured into a saucepan and heated on

cent, or seven tenths, as m u c h water as

a stove, t h e w a t e r w o u l d e v a p o r a t e in

it c a n h o l d . W h e n a i r c o n t a i n s a l l t h e

a f e w m i n u t e s . B o i l i n g is a k i n d of v e r y

w a t e r it c a n h o l d , w e s a y t h a t it is

fast e v a p o r a t i o n . F r o m t h e s e facts, y o u

saturated.

c a n see t h a t h e a t m a k e s

evaporation

take place faster. W h y does this h a p p e n ? B e c a u s e w h e n a l i q u i d is h e a t e d , its m o l e c u l e s m o v e a r o u n d f a s t e r

and

m o r e of t h e m a r e s e n t flying off i n t o t h e air.

the announcer

said,

W a r m air c a n h o l d m o r e w a t e r t h a n c o l d a i r . W h e n w a r m , m o i s t a i r is c o o l e d it g i v e s u p s o m e of i t s w a t e r . Y o u c a n see this in t h e s u m m e r t i m e w h e n

the

dew forms o n the grass after s u n d o w n . T h e g r a s s c o o l s off a f t e r t h e s u n g o e s d o w n . T h e air c o m e s in c o n t a c t with t h e

O n a hot, muggy summer day, people often say, "It's n o t W h a t is h u m i d i t y ? , . , ' . . the heat, i t s the humidity." W h a t do they mean?

Hu-

c o o l g r a s s , a n d it, t o o , is c o o l e d .

The

c o o l a i r c a n n o l o n g e r h o l d all t h e m o i s t u r e , a n d s o m e of t h e w a t e r l e a v e s t h e a i r . W e c a n s e e it a s b e a d s of

water

o n t h e b l a d e s of g r a s s . S c i e n t i s t s

say

a m o u n t of w a t e r v a p o r i n t h e a i r . W h e n

that

has

t h e r e is a l o t of w a t e r i n t h e a i r w e s a y

condensed.

t h e h u m i d i t y is h i g h . W h e n t h e a i r is

t h e w a t e r v a p o r i n t h e a i r if y o u fill a

v e r y d r y w e s a y t h e h u m i d i t y is l o w . W e

d r i n k i n g glass o r tin c a n w i t h ice a n d

feel u n c o m f o r t a b l e if t h e h u m i d i t y is

p l a c e it i n t h e k i t c h e n , o r a f a i r l y w a r m

too high. T h e perspiration on o u r bodies

r o o m . S o o n b e a d s of w a t e r w i l l f o r m

does n o t e v a p o r a t e easily b e c a u s e there

o n t h e s i d e s of t h e c o n t a i n e r . W h e n t h e

midity

is a w o r d

that

describes

the

the water vapor

in t h e

air

Y o u c a n c o n d e n s e s o m e of

is a l o t of w a t e r i n t h e a i r . B u t t o o l o w

air c o m e s into c o n t a c t with t h e ice-cold

h u m i d i t y is n o t g o o d e i t h e r . I n t h e w i n -

c o n t a i n e r , t h e a i r is c o o l e d a n d h a s t o

ter w h e n o u r houses are h e a t e d , t h e air

g i v e u p s o m e of its m o i s t u r e .

21

Obtain

two household

thermometers.

is n o t a v a i l a b l e , f a n t h e t h e r m o m e t e r s

C u t a strip one How can you i n c h w i d e a n d five measure humidity? inches long from

v i g o r o u s l y w i t h a s h e e t of c a r d b o a r d . ) After t h e fan has been blowing air on t h e t h e r m o m e t e r s f o r five m i n u t e s , n o t e

an old towel or other absorbent cloth.

t h e r e a d i n g of e a c h t h e r m o m e t e r

W r a p o n e e n d of t h e s t r i p a r o u n d t h e

w r i t e it d o w n .

b u l b of o n e of t h e t h e r m o m e t e r s

and

and

Y o u can use the table on this p a g e

t i e t h e c l o t h i n p l a c e w i t h a p i e c e of s t r i n g . T h i s is y o u r wet-bulb eter,

t h e o t h e r is y o u r dry-bulb

mometer.

thermom-

^

ther-

Hang both thermometers

at m-

t h e s a m e l e v e l . P l a c e a g l a s s of w a t e r

f: 30706050 w i t h . ^ , . . °Ut b e m g P u m P e d T r y the following

easy experiment. Put a drinking straw in a

g l a s s of

water.

A

little i n k

or

f o o d coloring will m a k e the w a t e r easier t o w a t c h . T h e w a t e r rises in t h e s t r a w a b o v e t h e l e v e l of t h e w a t e r i n t h e g l a s s . S c i e n t i s t s c a l l t h i s r i s i n g of w a t e r i n a s t r a w o r a n y n a r r o w t u b e capillary tion.

ac-

C a p i l l a r y a c t i o n is a v e r y i m p o r -

t a n t f e a t u r e of w a t e r . I t is w h a t m a k e s w a t e r r i s e t o t h e t o p of t a l l t r e e s . H o w d o e s t h i s w o r k ? W h e n t h e s t r a w is first p l a c e d i n t h e w a t e r , t h e m o l e c u l e s of w a t e r a r e a t t r a c t e d b y t h e m o l e c u l e s of the

straw

directly

above

them.

The

attraction between the water molecules a n d t h e s t r a w is g r e a t e r t h a n t h e a t t r a c tion between the water molecules them-

selves. T h e r e f o r e , t h e w a t e r m o l e c u l e s are pulled

upward.

Since

the

water

m o l e c u l e s c l i n g t o g e t h e r , w h e n s o m e of them are pulled u p w a r d they pull others with t h e m . T h u s , t h e w a t e r in t h e straw rises u p w a r d . I t c o n t i n u e s t o r i s e u n t i l t h e w e i g h t of t h e w a t e r i n t h e s t r a w is e q u a l t o t h e f o r c e of t h e u p w a r d p u l l . I n t h e s t e m s of p l a n t s a n d t h e t r u n k s of t r e e s t h e r e a r e t h o u s a n d s of t i n y t u b e s . W a t e r i n t h e soil r i s e s t h r o u g h

these

tubes by capillary action.

F o r this e x p e r i m e n t y o u will n e e d a stalk of c e l e r y , a g l a s s of How can you demonstrate capillary action in plants?

water, and some red i n k o r r e d f o o d coloring.

Put

several

d r o p s of t h e i n k o r f o o d c o l o r i n g i n t h e g l a s s of w a t e r . C u t t h e b o t t o m e n d of t h e c e l e r y s t a l k s o t h a t t h e c u t is f r e s h w h e n y o u p u t i t i n the water. N o w leave the celery in the g l a s s f o r t w o o r t h r e e h o u r s . T a k e it o u t a n d e x a m i n e it c a r e f u l l y . Y o u c a n

Capillary action causes the water to rise in the straw above the water level in the glass, and makes the colored water rise through the stalk of celery.

see b y t h e r e d c o l o r h o w h i g h in t h e stalk the w a t e r rose. C u t

across

the

stalk n e a r the b o t t o m a n d look at the

between the

slice. Y o u w i l l s e e l i t t l e d o t s of

red.

W h e n a b l o t t e r is p l a c e d o n a w e t s u r -

T h e s e a r e t h e e n d s of t i n y t u b e s t h a t g o

face t h e liquid rises in t h e tiny spaces

f r o m t h e b o t t o m of t h e c e l e r y t o

of t h e b l o t t e r b y c a p i l l a r y a c t i o n .

the

fibers

act like tiny tubes.

t o p . If y o u c u t t h e s t a l k l e n g t h w i s e y o u will see t h e s e little t u b e s w i t h r e d w a t e r i n s i d e . F l o r i s t s m a k e u s e of

capillary

B e c a u s e a i r h a s w e i g h t , p r e s s u r e is ex-

finally

erted on everything What is within the atmosphere. atmospheric A t sea level t h e prespressure? s u r e is n e a r l y 1 5 p o u n d s

reaches the petals, turning t h e m green.

o n e v e r y s q u a r e i n c h of t h e e a r t h ' s s u r -

action to color white

flowers

green for

St. P a t r i c k ' s D a y . T h e g r e e n rises

through

Capillary

the

and

a

f a c e . S u p p o s e y o u c o u l d w e i g h a col-

b l o t t e r w o r k . A b l o t t e r is a m a t m a d e

u m n of a i r t h a t c o v e r s o n e s q u a r e i n c h

of

of t h e e a r t h ' s s u r f a c e a t s e a l e v e l a n d

fibers

action

stems

coloring

is w h a t

makes

pressed together. T h e

spaces

27

I r e a c h e s t o t h e t o p of t h e e x o s p h e r e 6 0 0 miles a b o v e sea level. Y o u w o u l d t h a t t h e c o l u m n of a i r w e i g h s

find

nearly

15 p o u n d s . S i n c e t h e p r e s s u r e of t h e a i r is d u e t o its weight, y o u c a n see t h a t t h e h i g h e r y o u g o i n t h e a t m o s p h e r e , t h e less t h e air p r e s s u r e will be. A s y o u g o higher, less a i r w i l l b e a b o v e y o u t o w e i g h d o w n u p o n y o u . Six miles a b o v e t h e e a r t h ' s s u r f a c e , t h e p r e s s u r e of t h e a i r is o n l y four p o u n d s on every square inch. A t

Perform this experiment over the kitchen sink. If your glass is not small enough, the weight of the water may be greater than the air pressure, and the water might spill out of the glass.

t e n m i l e s , t h e c o l u m n of a i r a b o v e e v e r y square inch weighs only two

pounds;

b e c a u s e a i r p r e s s u r e is p u s h i n g u p w a r d

a t 1 0 0 m i l e s , o n l y t w o b i l l i o n t h s of a

on the c a r d b o a r d , a n d the air pressure

pound.

is g r e a t e r t h a n t h e w e i g h t of t h e w a t e r .

T h e air not only presses d o w n w a r d , b u t presses e q u a l l y in all directions. T h e

I n 1654, a G e r m a n scientist, O t t o v o n

t o t a l a i r p r e s s u r e o n t h e w h o l e b o d y of a h u m a n b e i n g is e q u a l t o s e v e r a l t o n s . W h y doesn't so m u c h air p r e s s u r e c r u s h our

bodies?

Because

the

process

of

Guericke, What was von Guericke's experiment?

amazed

everyone

by

ing h o w

strong

mospheric

showat-

pressure

breathing creates an air pressure inside

is. H e u s e d t w o h e m i s p h e r e s , e a c h of

t h e b o d y t h a t b a l a n c e s t h a t o u t s i d e it.

w h i c h w a s a b o u t twenty-two inches in

A s a result, w e feel n o p r e s s u r e at all.

diameter. smooth

Their

and

rims

covered

were with

ground

grease,

so

Fill a small drinking glass to the b r i m

t h a t t h e y w o u l d fit t o g e t h e r t i g h t l y a n d

with water. D o this H o w can you § l o w l y & n d carefully? show that air .. _ so t h a t y o u c a n see exerts pressure? •; t h e s u r f a c e of t h e

allow n o air to pass between them. V o n

w a t e r b u l g e s l i g h t l y o v e r t h e t o p of t h e g l a s s . P r e s s a flat p i e c e of

cardboard

o n t o p of t h e g l a s s . P r e s s t h e p a l m of y o u r h a n d o n t o p of t h e c a r d b o a r d a n d

Guericke put the rims together

and,

w i t h a n a i r p u m p h e h a d i n v e n t e d , rem o v e d the air inside the hollow sphere. So great was the air pressure on

the

outside (more than thirteen tons) that it t o o k sixteen horses, eight o n

each

side, t o pull t h e h e m i s p h e r e s a p a r t .

hand.

Y o u c a n p e r f o r m a n e x p e r i m e n t like

T u r n the glass upside d o w n quickly so

t h a t of v o n G u e r i c k e . Y o u w i l l n e e d

t h a t n o w a t e r spills. R e m o v e y o u r h a n d

t w o p l u n g e r s of t h e k i n d t h a t a r e u s e d

from beneath the cardboard. T h e card-

t o f o r c e w a t e r t h r o u g h d r a i n s . Y o u will

b o a r d will r e m a i n in p l a c e , h o l d i n g t h e

also need a friend to help you. Thor-

w a t e r i n t h e g l a s s . T h e w e i g h t of

oughly wet both plungers. Ask

grasp t h e glass with the o t h e r

the

water cannot push the cardboard down 28

f r i e n d t o sit i n a c h a i r a n d h o l d

your the

plunger handle b e t w e e n his knees, the

a 6 0 0 - m i l e c o l u m n of a i r c a n b e u s e d

r u b b e r c u p u p w a r d . P l a c e t h e c u p of

to measure atmospheric pressure.

your plunger u p o n the other one, and slowly a n d carefully p u s h d o w n

If y o u w e r e t o fill a g l a s s t u b e m o r e

until

than 30 inches long with mercury and

m o s t of t h e a i r h a s b e e n e x p e l l e d f r o m

p l a c e it u p s i d e d o w n i n a d i s h of m e r -

the plunger cups. N o w ,

c u r y , s o m e of t h e m e r c u r y i n t h e t u b e

e a c h of

you

g r a s p a h a n d l e , a n d s e e h o w difficult it is t o p u l l t h e p l u n g e r s a p a r t . If y o u h a v e o n l y o n e p l u n g e r , p l a c e it o n a s m o o t h w e t s u r f a c e a n d

push

d o w n h a r d . Y o u w i l l s e e h o w h a r d it is t o p u l l it free. A l l t h e f o r c e h o l d i n g t h e p l u n g e r t o t h e s u r f a c e is d u e t o a t m o s pheric pressure. W e l e a r n e d t h a t a c o l u m n of a i r o n e inch square How do we measure a n d 6 0 0 miles atmospheric pressure? h i g h weighs 15 p o u n d s . S u p p o s e w e c o u l d p u t t h i s colu m n of a i r o n o n e s i d e of a s c a l e a n d a c o l u m n of m e r c u r y o n e i n c h s q u a r e o n t h e o t h e r s i d e of t h e s c a l e . T o b a l a n c e

You can perform an experiment similar to von Guericke's. For the spheres substitute two plungers and for the sixteen horses, yourself and a friend.

t h e air, t h e m e r c u r y w o u l d h a v e t o b e 30 inches high. T h e fact that a 30-inch c o l u m n of m e r c u r y w e i g h s a s m u c h a s

Sixteen horses could not pull von Guericke's steel spheres apart.

w o u l d r u n o u t u n t i l t h e w e i g h t of t h e m e r c u r y left i n t h e t u b e is b a l a n c e d b y t h e w e i g h t of t h e a i r p r e s s i n g o n

the

m e r c u r y in t h e dish. N o air w o u l d b e pressing on the m e r c u r y in the

tube

because

com-

pletely

the

space

empty.

above

This

i t is

arrangement

g l a s s t u b e a n d m e r c u r y is c a l l e d a cury

of mer-

barometer.

Let us suppose that we are carrying a mercury

barometer

from

sea

level

u p a m o u n t a i n . W h e n w e b e g i n o u r ascent, the mercury column measures 30 inches. W h e n w e h a v e c l i m b e d 9 4 0 feet, t h e m e r c u r y c o l u m n is o n l y 2 9 i n c h e s high. W h e n we have climbed to 8,360 feet — a little m o r e t h a n o n e a n d a half m i l e s a b o v e s e a l e v e l — w e find t h a t t h e c o l u m n of m e r c u r y is o n l y 2 2

inches

high. W h y d o e s t h e c o l u m n of m e r c u r y b e c o m e shorter as we carry the b a r o m e t e r h i g h e r ? B e c a u s e t h e c o l u m n of a i r t h a t balances the mercury becomes

shorter

a n d lighter as w e go higher. W e k n o w

aneroid

that

means "without liquid." This barometer

at

sea level t h e

column

of

air

barometer. T h e word "aneroid"

w e i g h s n e a r l y 15 p o u n d s . A t 8 , 3 6 0 f e e t

does

a b o v e s e a l e v e l t h e c o l u m n of a i r w e i g h s

m a i n p a r t of a n a n e r o i d b a r o m e t e r is

a l m o s t 1 1 p o u n d s a n d is b a l a n c e d b y a

a n airtight c a n from w h i c h as m u c h air

2 2 - i n c h c o l u m n of m e r c u r y t h a t

also

as possible has been p u m p e d . A spring

Thus, when we measure the height

c a n f r o m c o l l a p s i n g u n d e r p r e s s u r e of

of a c o l u m n of m e r c u r y i n a g l a s s t u b e ,

t h e o u t s i d e a i r . A p o i n t e r is a t t a c h e d

w e a r e a l s o m e a s u r i n g t h e p r e s s u r e of

b y l e v e r s a n d a c h a i n t o t h e t o p of t h e

t h e c o l u m n of a i r a b o v e t h e m e r c u r y .

c a n , a n d is a r r a n g e d s o t h a t i t p o i n t s t o

W e usually express the pressure in units

a scale divided into inches.

w e i g h s a l m o s t 11 p o u n d s .

not

contain

any

mercury.

The

a t t a c h e d t o t h e t o p of t h e c a n k e e p s t h e

of h e i g h t of t h e m e r c u r y c o l u m n .

We

W h e n t h e a i r p r e s s u r e i n c r e a s e s , it

say, for example, t h a t t h e a t m o s p h e r i c

p u s h e s t h e t o p of t h e c a n d o w n a lit-

p r e s s u r e a t t h e t o p of t h e E m p i r e S t a t e

tle, a n d t h e p o i n t e r m o v e s t o w a r d t h e

B u i l d i n g is " 2 9 i n c h e s of m e r c u r y , " o r

higher n u m b e r s o n t h e scale. W h e n the

simply, " 2 9 inches." Another

30

k i n d of b a r o m e t e r

air pressure decreases, the spring pushes is

the

t h e t o p of t h e c a n u p , a n d t h i s m o v e -

_13L In the middle of the seventeenth century Italian mathematician and physicist Evangelista Torricelli discovered the principle of the barometer, and thereby discovered the force of atmospheric pressure. He calculated that in order to support a column of mercury about thirty inches high, the atmosphere had to press downward with a weight of nearly fifteen pounds per square inch. About the same time Otto von Guericke of Magdeburg began investigating air pressure. Out of his work came the water barometer. A brass tube that was over thirty-four feet long, with a closed glass section at one end, was filled with water. Von Guericke then inverted the tube, placing the open end in a tub of water. As the pressure of the atmosphere changed the water in the tube rose and fell.

5

T

ANEROID BAROMETER

a MERCURY BAROMETER m e n t , t o o , is m e a s u r e d o n t h e s c a l e b y the pointer, which moves toward

the

lower numbers. W e learned that a barometer at 8,360 f e e t w i l l m e a s u r e 2 2 i n c h e s of m e r c u r y . T h e r e f o r e , if w e h a v e a b a r o m e t e r t h a t measures 2 2 inches, we k n o w w e

are

8 , 3 6 0 feet a b o v e sea level. T h u s w e c a n use a b a r o m e t e r to m e a s u r e

altitude.

h o w w e b r e a t h e . L e t u s see h o w

very i m p o r t a n t activity takes place. Stretching across t h e entire b o d y cav-

M a n y airplanes use special barometers,

ity b e n e a t h

called altimeters, to m e a s u r e the plane's

m u s c l e c a l l e d t h e diaphragm

altitude.

fram).

When

breath,

the

You

know

that

human

H o w d o e s air g e t , , into our l u n g s ?

beings

must

b r e a t h e in order ,. T-» ^i t o live. B r e a t h i n g is s o

natural

that hardly anyone stops to ask

just

this

t h e l u n g s is a l a r g e you

inhale,

diaphragm

flat

(dy'-uhor

take

moves

a

down-

w a r d a n d the ribs m o v e u p w a r d

and

o u t w a r d . T h i s i n c r e a s e s t h e size of t h e b o d y c a v i t y . A s a r e s u l t t h e r e is l e s s a i r pressure within the b o d y cavity

than

outside the body. Atmospheric pressure

31

LUNGS

TRACHEA

outside the b o d y pushes air through the nose or m o u t h a n d d o w n into the lungs. W h e n you exhale, or blow out breath, the diaphragm moves

your

upward

a n d t h e r i b s m o v e d o w n w a r d a n d inward.

This

results

in

more

pressure

within the body cavity than outside the b o d y . T h i s p r e s s u r e p u s h e s a i r o u t of the lungs t h r o u g h the nose or m o u t h . BRONCHIAL TUBES DIAPI RAGM The diagram at left shows you how we breathe.

Air

a n d

W a t e r

P e r h a p s you have seen y o u r m o t h e r use an egg timer w h e n How did ancient she boils a n egg. man use water T h e s a n d in o n e to measure time? e n d of t h e t u b e runs t h r o u g h a tiny opening into

at

W o r k

h a d to e n d his speech w h e n the jar w a s empty. T h e r e w e r e s e v e r a l o t h e r k i n d s of water clocks. O n e was very similar to the one we just described except that

the

t h e j a r w a s m a d e of g l a s s a n d t h e r e w e r e

l o w e r e n d of t h e t u b e i n e x a c t l y t h r e e

m a r k i n g s o n t h e s i d e of it. T h e m a r k s

m i n u t e s . W h e n a l l t h e s a n d is i n

the

w e r e a r r a n g e d so t h a t e a c h o n e stood

mother

for a n h o u r . A s the water dripped out

k n o w s that the egg h a s boiled for three

of t h e j a r , t h e w a t e r l e v e l b e c a m e l o w e r ,

m i n u t e s . A n e g g t i m e r is v e r y m u c h l i k e

m a r k i n g the hours as they passed. Some

a device the ancient G r e e k s used

water clocks h a d a

lower end

of t h e t u b e , y o u r

measure time. T h e

Greeks

let

to

water

drip slowly from a jar with a tiny hole i n it. W h e n a l l t h e w a t e r h a d d r o p p e d out the Greeks

knew

that

a

certain

floating

figure

of a

w o m a n whose arm pointed to the hour m a r k e d o n t h e s i d e of t h e j a r . I n a n o t h e r k i n d of w a t e r c l o c k , t h e water from the container dripped into

a m o u n t of t i m e h a d g o n e b y . T h e y u s e d

a cylinder. In the cylinder was a

this " w a t e r clock," called a

clepsydra

i n g p i s t o n w i t h a l o n g s h a f t o n t o p of

( k l e p ' - s i - d r a ) , t o m e a s u r e t h e l e n g t h of

it. C u t i n t o t h e s h a f t w e r e g e a r t e e t h .

speeches in t h e law courts. A

T h e s e t e e t h fitted i n t o a c i r c u l a r g e a r .

32

speaker

float-

A s the piston

floated

upward, the teeth

on the shaft m o v e d t h e circular gear. A pointer attached

to the circular

gear

m o v e d slowly a r o u n d in a circle, pointing t o n u m b e r s o n a dial o u t s i d e

the

gear. W h e n the piston w a s at the bott o m of t h e c y l i n d e r , t h e p o i n t e r p o i n t e d to 24, w h i c h indicated midnight. water filled floated

dripped it

little

into by

the little,

cylinder the

slowly u p w a r d . A s the

As and

cylinder piston

m o v e d u p w a r d , its g e a r t e e t h t u r n e d t h e circular gear, and the pointer pointed to the h o u r s as they passed, b e g i n n i n g with the numeral one. In 24 hours the

Water is supplied continuously from pipe A into reservoir B, from where it slowly drips into tank C. An overflow tube G keeps the level in the reservoir constant. As the level of water in tank C rises, rack E is pushed up by float D. The rack turns geared wheel F, and the hand of the clock moves the dial to indicate the hour.

c y l i n d e r h a d filled, a n d t h e p o i n t e r h a d t u r n e d once a r o u n d the dial. A t the e n d of 2 4 h o u r s , t h e w a t e r c l o c k h a d t o b e

E v e r since ancient times m a n has used

set b y e m p t y i n g t h e c y l i n d e r . How do windmills work?

t h e f o r c e of m o v i n g

air

and

for

water

to

work

h i m . W i n d is m a d e t o d o w o r k b y t u r n i n g t h e sails

of w i n d m i l l s . T h e f a m o u s w i n d m i l l s of the Netherlands and Belgium had huge

In the Greek clepsydra, the water dopped out of the vessel.

c l o t h sails, s o m e of w h i c h w e r e

two

stories high. E a c h windmill h a d

four

s a i l s set i n t h e s h a p e of a c r o s s

and

a t t a c h e d t o a s h a f t . T h e sails d i d n o t face the wind directly, but were t u r n e d at a slight angle to the direction

from

which the wind was blowing. W h e n the

In the Chinese and Indian waterclocks a small brass bowl with its bottom pierced was floated on a basin of water. The floating bowl gradually filled with water, and, after a measured interval of time, sank. An attendant then struck a gong and set the bowl afloat again.

33

wind struck

t h e sail, t h e m o v i n g

s l i p p e d p a s t it. B e c a u s e t h e s a i l

air was

the w i n d w a s blowing. A s a result, the sail t u r n e d

the shaft.

The

shaft

was

t u r n e d s l i g h t l y s i d e w i s e , it w a s p u s h e d

attached to grindstones or to a pump.

in a direction crosswise t o t h a t in which

W h e n a stiff w i n d b l e w a g a i n s t t h e sails, the windmill ground grain or p u m p e d water. A l m o s t all t h e o l d w i n d m i l l s

have

been replaced by modern ones that have s t e e l b l a d e s i n s t e a d of c l o t h sails. T w o t o s i x t e e n of t h e s e b l a d e s a r e set o n a shaft at scientifically m e a s u r e d

angles,

so t h a t t h e b l a d e s spin r a p i d l y even in a slow wind. B e c a u s e these blades w o r k so m u c h better, they need not be l a r g e a s t h e o l d c l o t h sails. V e r y m o d e r n windmills have blades

as few

longer

t h a n a m a n is t a l l . Steel

windmills

are

used

in

many

p a r t s of t h e w o r l d , a n d t h e i r m a i n u s e s are to p r o d u c e electricity a n d to p u m p water for irrigating crops. T o p r o d u c e e l e c t r i c i t y , t h e s h a f t of t h e w i n d m i l l is a t t a c h e d t o a d y n a m o . P e o p l e w h o live far

from

a

large

electric

generating

Left, windmill with canvas sails, below, cutaway of a windmill used to pump water out of the fields.

plant m a y have their own windmills on

T h e water pushed against the paddle,

a tall steel t o w e r . T h e w i n d m i l l

m o v i n g it i n t h e d i r e c t i o n t h e

pro-

stream

duces electricity for lighting h o m e s a n d

was

running household appliances. T o p u m p

t u r n a little. A s o n e p a d d l e t u r n e d u p

water for irrigation, the electricity m a y

a n d o u t of t h e w a t e r , t h e n e x t p a d d l e

r u n t h e m o t o r of a w a t e r p u m p . O r , t h e

dipped

windmill m a y be connected directly to

pushed forward by the stream. T h u s the

t h e s h a f t of t h e p u m p .

wheel kept turning. T h e turning

flowing,

and caused the wheel to

beneath

the

water

and

was shaft

w a s m a d e t o d o w o r k , p e r h a p s b y atT o m a k e w a t e r d o w o r k for him, m a n invented the water How do water w h e e l . O n e of t h e wheels work? e a r l i e s t t y p e s of w a t e r

t a c h i n g it t o a g r i n d s t o n e t o g r i n d c o r n or wheat. A v e r y c l e v e r k i n d of w a t e r was

called

Archimedes' Greek

screw,

after

w h e e l s c o n s i s t e d of t w o d i s c s of w o o d

Archimedes,

set a f o o t o r t w o a p a r t a t o n e e n d of a

a n d s c i e n t i s t , w h o l i v e d i n t h e c i t y of

r o u n d shaft. I n the space b e t w e e n t h e

S y r a c u s e o n t h e i s l a n d of Sicily a b o u t

d i s c s w e r e s e v e r a l flat p i e c e s of w o o d ,

2,300 years ago. Archimedes' screw h a d

called p a d d l e s . T h e s e p a d d l e s w e r e at-

t w o m a i n parts: a large, hollow, w o o d e n

tached to b o t h the shaft a n d the discs

cylinder, a n d a water wheel. T h e wheel

a n d were long e n o u g h to stick out past

was built a r o u n d the w o o d e n

cylinder

t h e r i m s of t h e d i s c s .

close to o n e end. Inside the

cylinder

T h e wheel was placed on a wooden

a

wheel

mathematician

was a w o o d e n spiral that r a n the whole

the

l e n g t h of t h e c y l i n d e r , l i k e t h e g r o o v e s

height to allow the lowermost paddle to

o n a s c r e w . T h e e n d of t h e c y l i n d e r ,

r e a c h b e n e a t h t h e s u r f a c e of t h e w a t e r .

with the water wheel attached, rested

framework

over a stream at just

Left, a water wheel is said to be undershot when water turns the wheel by striking its blades from the bottom. It is called an overshot water wheel, at right, when it is turned by water falling upon the paddles from above.

35 J

Water mill driven by undershot wheel.

u n t i l it r a n o u t of t h e u p p e r e n d of t h e cylinder. This was the way water was transported to irrigate crops. Obtain

a

straight

H o w can you m a k e . IO a water wheel?

knitting

needle.

P u s h the needle . , ., t h r o u g h t h e cen° t e r of a c o r k .

P u s h half a d o z e n steel p e n p o i n t s into t h e c o r k a t e v e n l y s p a c e d i n t e r v a l s circling a r o u n d the cork. B e n d a heavy p i e c e of w i r e

( a wire clothes

hanger

will d o ) i n t o t h e s h a p e s h o w n in t h e Cutaway view of Archimedes' screw.

illustration. Rest the knitting needle on the wire frame you have just made.

o n a s u p p o r t s e t o n t h e b o t t o m of t h e

U s i n g a p e n c i l p o k e a h o l e in t h e cen-

river. T h e other e n d r e a c h e d t o t h e river

t e r of o n e s i d e of a m i l k c a r t o n . P l a c e

b a n k . T h e cylinder was tilted

the carton in a sink so that water from

upward

the

a f a u c e t will r u n i n t o t h e h o l e in the

the

t o p of t h e c a r t o n . W h e n a s t r e a m of

l o w e r e n d of t h e s p i r a l s c o o p e d u p w a t e r

w a t e r b e g i n s t o p o u r o u t of t h e h o l e in

a n d m o v e d it u p w a r d a l o n g t h e s p i r a l

t h e s i d e of t h e c a r t o n , p l a c e t h e w i r e

stream

36

toward

slightly

turned

the shore. W h e n the

water

wheel,

frame strikes

b e n e a t h it, s o t h a t t h e the

penpoints.

stream

Your

water

wheel will spin r a p i d l y . M o d e r n w a t e r w h e e l s a r e m a d e of m e t a l and are called How does water water turbines. produce electricity? T h e m a i n u s e of w a t e r t u r b i n e s is t o p r o d u c e

electric-

ity. I n m o u n t a i n o u s c o u n t r y ,

streams

r u n d o w n t h e s i d e s of m o u n t a i n s v e r y swiftly. S o m e of t h e w a t e r f r o m

these

s t r e a m s is s e n t t h r o u g h p i p e s . A t

the

e n d of t h e p i p e s a r e t u r b i n e s t h a t t u r n v e r y r a p i d l y w h e n t h e swiftly

moving

w a t e r strikes t h e m . I n flat c o u n t r y , w a t e r m a y b e g i v e n the force needed to turn turbines rapidly

WIRE SUPPORT

by storing the water behind high dams. A b o u t h a l f w a y d o w n f r o m t h e t o p of

How to make your own water wheel.

the d a m are openings that lead to long, wide pipes. T h e s e pipes, called stocks, dam.

pen-

r u n d o w n t o t h e b o t t o m of t h e Water

plunging

down

through

each penstock strikes a turbine. spinning shaft

of e a c h t u r b i n e is

tached to a machine called a o r electric electricity.

generator,

which

The at-

dynamo, produces

Water is put to work to help create electricity.

BLOW 1

CENTER

SPOOL

Xi / X

II

CARD

T a k e a small card, push a pin through its c e n t e r , a n d p u t t h e p i n W

^

a t

ls

. . . . Principle? n

w h e n t h e s p e e d is l o w , t h e p r e s s u r e is high.

Remember

that

a i r is a

fluid.

i n t o t h e h o l e of a s p o o l .

W h e n y o u blew t h r o u g h the hole in the

H o l d t h e s pr o o l s o t h a t t h e

s p o o l , y o u r b r e a t h — w h i c h is a i r —

h o l e t h r o u g h it is v e r t i c a l

m o v e d swiftly a c r o s s t h e u p p e r surface

and hold the card against the

spool.

of t h e c a r d a n d r e d u c e d t h e a t m o s p h e r i c

N o w b l o w t h r o u g h the hole in t h e spool,

pressure on that surface. T h e

atmos-

at the same time releasing your hold on

pheric pressure on the lower

surface

the card. Y o u might expect the card to

remained

be blown a w a y from the spool.

How-

sure pushing u p w a r d on the card was

e v e r , y o u w i l l find t h a t a s l o n g a s y o u

greater than the pressure pushing down-

b l o w t h r o u g h t h e s p o o l t h e c a r d will

ward,

remain n e a r the spool. A s soon as you

y o u r b r e a t h m o v i n g swiftly a c r o s s t h e

s t o p b l o w i n g , t h e c a r d w i l l fall.

What

s h e e t of p a p e r l o w e r e d t h e p r e s s u r e o n

h o l d s t h e c a r d u p w h e n y o u r b r e a t h is

its u p p e r surface, a n d t h e g r e a t e r pres-

passing t h r o u g h the hole in t h e spool?

sure on the lower surface pushed

Some force stronger than gravity must

paper up.

unchanged.

the card

Since the

stayed

up.

pres-

Likewise,

the

H a v e you ever noticed h o w a shower

be holding the card up. L e t us see this s a m e force a t w o r k

c u r t a i n m o v e s i n t o w a r d t h e s p r a y of

a g a i n . C u r l o n e e n d of a s h e e t of p a p e r

water?

The

a r o u n d a pencil. H o l d the pencil near

from

y o u r lips a n d b l o w y o u r b r e a t h a c r o s s

a l o n g w i t h it. T h e s w i f t l y m o v i n g a i r de-

t h e p a p e r . T h e p a p e r will rise, a n d will

c r e a s e s t h e p r e s s u r e o n t h e i n s i d e of t h e

r e m a i n e x t e n d e d in the air as long as

s h o w e r c u r t a i n a n d t h e p r e s s u r e of t h e

you continue to blow.

outside air pushes t h e curtain

the

water

shower's

squirting nozzle

outward

moves

air

inward.

B o t h of t h e s e e x p e r i m e n t s i l l u s t r a t e a scientific p r i n c i p l e t h a t w a s ered

by

a

Swiss

discov-

mathematician

physicist, D a n i e l Bernoulli, w h o

A n a i r p l a n e w i n g is flat o n t h e b o t t o m

and

W h a t holds an airplane up?

lived

m o r e t h a n 2 0 0 y e a r s a g o . B e r n o u l l i disc o v e r e d t h a t w h e n t h e s p e e d of a

fluid

is h i g h , t h e p r e s s u r e of t h e fluid is l o w ;

38

air

flowing

and curved on

top.

A s the plane moves t h r o u g h t h e air, the

o v e r t h e t o p of t h e

moves faster t h a n the air

flowing

wing along

LESS AIR PRESSURE

•

be straight. T h e p a p e r has the general s h a p e of a s e c t i o n c u t f r o m a n a i r p l a n e w i n g . T h e r o u n d e d e n d is t h e f o r w a r d end. S u s p e n d t h e p a p e r b y t h r e e p i e c e s of string, t w o passed t h r o u g h the forward e n d a n d o n e t h r o u g h t h e r e a r p a r t of

: : : r r t " 4 " 4 - - " ^ : f :

the folded paper. A t t a c h the strings to sticks, as s h o w n in t h e illustration, o r

GREATER AIR PRESSURE

t h e b o t t o m . T h e a i r t h a t flows o v e r t h e t o p of t h e w i n g m u s t p a s s o v e r

SHEET OF PAPER

the

c u r v e i n t h e s a m e l e n g t h of t i m e a s a i r beneath

the

wing

passes

straight undersurface.

along

Since a

the

curved

l i n e is l o n g e r t h a n a s t r a i g h t l i n e b e tween

the

same

two

points,

the

air

SHEET OF CARDBOARD

moving the longer distance must move faster t h a n the air m o v i n g the shorter distance. Therefore,

air moves

o v e r t h e u p p e r s u r f a c e of a n

faster

airplane

wing than over the lower surface. According

to

Bernoulli's

Principle,

the

p r e s s u r e o n t h e u p p e r s i d e of t h e w i n g is l e s s t h a n t h e p r e s s u r e o n t h e u n d e r s i d e of t h e w i n g . T h u s , t h e h i g h e r p r e s s u r e of t h e a i r b e n e a t h a n

airplane's

w i n g s h o l d s :t u p .

F o r this e x p e r i m e n t y o u will n e e d a p i e c e of H o w can you s h o w why its wings will ..,/ . . , lift an airplane?

m d

cardboard &

§ h e e t

o f

, paper the same r \ „ width. C u t the

c a r d b o a r d s o t h a t i t is a b o u t h a l f

Experiment that shows why its wing lifts an airplane.

as

u s e f o u r p i l e s of b o o k s t o h o l d t h e e n d s

long as t h e p a p e r . N o w staple o r glue

of t h e s t r i n g s . P l a c e a n e l e c t r i c f a n i n

t h e c a r d b o a r d t o t h e s h e e t of p a p e r a s

f r o n t of t h e " a i r p l a n e w i n g " a n d t u r n

s h o w n in t h e illustration. B e n d t h e pa-

t h e fan o n . T h e w i n g will rise slightly

per back over the cardboard and staple

as the m o v i n g air lowers the pressure o n

it a b o u t o n e i n c h f r o m t h e e n d of t h e

its u p p e r s u r f a c e a n d t h e air

c a r d b o a r d . T h e u p p e r surface of

p r o v i d e s lift.

the

beneath

p a p e r will b e c u r v e d , t h e l o w e r o n e will 39

This is the grip for a curve ball. As the ball is delivered, the hand is twisted inward from the wrist at the same time as the wrist snaps. The ball rolls off the outside of the first finger, thus developing the spin that makes it change direction on its way to the plate.

A pitcher c a n t h r o w a baseball so that

t h a t w h e n a i r m o v e s r a p i d l y , its p r e s -

it w i l l c u r v e u p Qr d o w n . , , ward, or to the

s u r e is l o w e r e d . T h u s , t h e r a p i d l y m o v -

How does a baseball pitcher throw a . 1IO curve b a l l ?

w a r d

i n g a i r o n t o p of t h e b a s e b a l l h a s lower pressure t h a n the air

a

beneath.

T h e b a l l , t h e n , is p u s h e d u p w a r d i n a r i g h t o r left. H e

d o e s this t o m a k e it h a r d for t h e b a t t e r

curved path. By spinning the ball from b o t t o m to

to hit the ball. H o w does he m a k e the

t o p , left t o r i g h t , o r r i g h t t o left,

ball curve? T h e pitcher wraps his

fin-

p i t c h e r c a n m a k e it c u r v e i n w h a t e v e r

gers a r o u n d the ball in a m a n n e r

that

a

direction he wishes.

m a k e s t h e b a l l s p i n w h e n it l e a v e s h i s hand. T h e spinning ball carries

along

A s i p h o n is a t u b e s h a p e d

t h e a i r c l o s e t o it, s o t h a t t h e a i r s p i n s , too. Suppose the ball spins in the s a m e d i r e c t i o n a s it is m o v i n g . T h e a i r o n t o p

like t h e letter ™ . make a siphon?

i n g . T h e a i r b e n e a t h t h e b a l l is c a r r i e d in the direction opposite the b a l l . T h e r e f o r e , it m o v e s m o r e

moving slowly

t h a n the air on top. W e h a v e learned 40

"J"

u pr s i d e d o w n . W i t h it

w i l l m o v e b o t h a s f a s t a s t h e b a l l is s p i n n i n g a n d a s f a s t a s t h e b a l l is m o v -

something

you

can

make

w a t e r flow u p o v e r t h e w a l l of a c o n tainer,

without

lifting

the

water

by

p u m p i n g . F o r this e x p e r i m e n t y o u will need a rubber

or plastic tube

about

t h r e e f e e t l o n g , a p a i l of w a t e r , a n d a n

e m p t y p a i l . P l a c e t h e p a i l of w a t e r o n

e m p t i e d the pail. Y o u m a y w o n d e r w h y

a table and put the empty pail on the

atmospheric

floor

tube

w a t e r f r o m flowing o u t of t h e l o w e r e n d

completely with water. Y o u c a n d o this

of t h e t u b e . I t is b e c a u s e t h e w e i g h t of

b y d r o p p i n g it i n t o t h e p a i l of w a t e r o r

t h e c o l u m n of w a t e r i n t h e t u b e c a u s e d

b y filling i t w i t h w a t e r f r o m t h e f a u c e t .

a d o w n w a r d force greater than the up-

N o w p i n c h b o t h e n d s of t h e t u b e s o t h a t

w a r d f o r c e of t h e a t m o s p h e r i c p r e s s u r e .

next to the table. Fill the

pressure

did

not

keep

no water can escape, and quickly place o n e e n d of t h e t u b e i n t h e p a i l of w a t e r

Archimedes

the

Greek

well b e l o w t h e w a t e r ' s surface. L e t t h e other end hang down into the

empty

p a i l . L e t g o of b o t h e n d s of t h e t u b e a t the same

time.

The

water

will

flow

t h r o u g h t h e t u b e , u p o v e r t h e s i d e of the pail on the table, a n d d o w n into the l o w e r p a i l . If y o u k e e p t h e u p p e r

end

of t h e t u b e c l o s e t o t h e b o t t o m of t h e p a i l o n t h e t a b l e , all t h e w a t e r w i l l s o o n b e in t h e pail o n t h e

floor.

H o w did A r c h i m e d e s solve the problem of King Hiero's crown?

scientist lived , 300

2

who

about years

ago, w a s a kinsman

of

King

H i e r o , t h e r u l e r of S y r a c u s e . A c c o r d i n g t o legend, o n e d a y in t h e y e a r 2 5 0 B . C . , t h e k i n g b o u g h t a c r o w n t h a t h e susp e c t e d w a s m a d e of g o l d m i x e d

with

s i l v e r . G o l d is m o r e e x p e n s i v e t h a n silv e r , s o if K i n g H i e r o h a d p a i d f o r a

A s i p h o n is v e r y u s e f u l f o r e m p t y i n g an

aquarium

water from

or even for

taking

the

a wash bowl that has

a

stopped-up drain. Chemists use siphons in l a b o r a t o r i e s t o t r a n s f e r liquids f r o m o n e bottle to a n o t h e r . L e t us see h o w this

simple,

but

very

useful,

device

works. When you removed your How does a . . ._ siphon w o r k ?

finger

the lower end , tube, some flowed

from of

the . _ water

. out, leaving an

e m p t y s p a c e b e h i n d it. Y o u

remember

t h a t t h e a t m o s p h e r e is p r e s s i n g o n a l l t h i n g s o n t h e s u r f a c e of t h e e a r t h . T h e a t m o s p h e r i c p r e s s u r e o n t h e s u r f a c e of the w a t e r in the pail p u s h e d w a t e r u p t h e t u b e t o fill t h e e m p t y s p a c e t h e r e . M o r e w a t e r flowed o u t , a n d w a s a g a i n replaced, due to atmospheric pressure. This water

process flowed

continued through

until the

all

tube

the

How to make your own siphon.

and 41

Both crowns weighed the same, but since more water spilled from the bowl in which the new crown was immersed, Archimedes concluded that the new crown was larger and, therefore, could not be pure gold. pure gold crown, he h a d been cheated.

While

Archimedes

was

thinking

T h e k i n g a s k e d A r c h i m e d e s t o find a

a b o u t this p r o b l e m , h e w e n t to the pub-

w a y t o tell w h e t h e r t h e c r o w n really did

l i c b a t h s of S y r a c u s e . A s h e s a t d o w n

h a v e s i l v e r i n it. A r c h i m e d e s k n e w t h a t

i n h i s b a t h , h e w a t c h e d t h e w a t e r spill

s i l v e r w e i g h s less t h a n g o l d , s o it s h o u l d

o v e r t h e s i d e s of t h e t u b . T h i s

h a v e b e e n easy t o tell w h e t h e r t h e n e w

gave h i m the idea h e n e e d e d to solve

crown had

the king's problem.

silver m i x e d in t h e

gold.

sight

Archimedes

Archimedes h a d only to weigh the new

supposedly

c r o w n a n d a n o t h e r c r o w n t h e s a m e size

a n s w e r t h a t h e j u m p e d o u t of t h e b a t h

m a d e of p u r e g o l d . If t h e n e w

a n d r a n n a k e d t h r o u g h t h e streets shout-

crown

so excited

was

w e r e m a d e of g o l d m i x e d w i t h s i l v e r , i t

ing,

would

G r e e k for, " I h a v e f o u n d

weigh

less t h a n

a

pure

gold

c r o w n of t h e s a m e size. B u t t h e k i n g ' s problem

was not

this easy to

"Heureka!

at finding

Heureka!"

the

which it! I

is

have

found it!"

solve.

Archimedes went to King Hiero and

Archimedes found that the new crown

t o l d h i m h e c o u l d s o l v e t h e p r o b l e m of

weighed t h e s a m e as a p u r e gold c r o w n

the new crown. H e asked the king to

t h a t a p p e a r e d t o b e t h e s a m e size. B u t

send for t h e c r o w n s a n d for t w o bowls

he couldn't be sure the crowns

were

of e q u a l size a n d l a r g e e n o u g h t o c o n -

really t h e s a m e size b e c a u s e t h e y w e r e

tain t h e c r o w n s . H e also a s k e d for t w o

b o t h e l a b o r a t e l y c a r v e d a n d it w a s n o t

larger bowls in which he placed

possible to measure t h e m

smaller bowls. H e then

accurately.

filled

the

the two

H e w o u l d h a v e t o find a w a y t o m e a s u r e

small bowls to the brim with water, a n d

t h e c r o w n s . If t h e n e w c r o w n h a d s i l v e r

placed a crown in each bowl.

i n it, it w o u l d h a v e t o b e l a r g e r

s p i l l e d o v e r t h e s i d e s of b o t h b o w l s i n t o

than

t h e c r o w n of p u r e g o l d , s i n c e t h e y b o t h

the larger bowls. Archimedes

weighed the same.

measured

42

the

amount

of

Water

carefully

water

that

spilled f r o m e a c h bowl. H e f o u n d t h a t

j e c t , t h e o b j e c t w e i g h s less. B e c a u s e of

m o r e w a t e r spilled f r o m

A r c h i m e d e s ' P r i n c i p l e , w e c a n tell h o w

the bowl in

which he had put the new crown. This

m u c h less t h e object weighs. W e

meant that the new crown was larger

say A r c h i m e d e s ' Principle in

t h a n t h e c r o w n of p u r e g o l d . S i n c e t h e

w a y : the loss of weight

crowns weighed the same, Archimedes

a liquid

is equal

k n e w that the new crown, being larger,

liquid

displaced.

m u s t b e m a d e of a l i g h t e r m e t a l

than

gold. T h e k i n g w a s right; as h e

sus-

pected, the new crown was not

pure

To

test

weight

will

W h e n A r c h i m e d e s w a s sitting in bath, W h a t

is

his

he

no-

in

of

Principle,

How can you prove Archimedes' Principle?

gold.

another

of an object

to the

Archimedes'

can

the

you

need ,

a a

large pan,

a

b u c k e t

,

spring scale, a brick, a large bottle, a funnel, a n d s o m e string.

ticed Al .

some,

thing

else:

the bucket to the brim with water. D o

his b o d y w a s lighter t h a n it w o u l d h a v e

n o t spill a n y i n t o t h e p a n . If y o u a c c i -

been h a d there been n o water in

d e n t a l l y spill s o m e w a t e r , m o p i t

Archimedes Principle?

bathtub.

You probably

have

the

noticed

P l a c e t h e b u c k e t i n t o t h e p a n a n d fill

with a cloth. Tie the string

up

securely

the s a m e thing w h e n sitting in a bath-

a r o u n d the brick so that you can h o o k

tub. Archimedes

it o n t o t h e s p r i n g scale. H o l d t h e scale

understood

that

he

w e i g h e d less w h e n i n t h e w a t e r b e c a u s e

high e n o u g h so that the brick

hangs

s o m e of t h e w a t e r w a s p u s h i n g u p w a r d

f r e e . N o t e t h e w e i g h t of t h e b r i c k a n d

on his body. T h i s upward-pushing force

m a r k it d o w n o n paper.

force. A r c h i m e d e s ,

Slowly lower the brick into the water

being a mathematician, wanted to k n o w

i n t h e b u c k e t . W h e n t h e w h o l e b r i c k is

is c a l l e d a buoyant

h o w strong the buoyant force was. H e f o u n d t h a t t h e s t r e n g t h of t h e b u o y a n t force o n his b o d y w a s equal

to

the

w e i g h t of t h e w a t e r t h a t s p i l l e d

over

t h e sides of t h e t u b . T h i s w a t e r spilled because

Archimedes'

body

took

up

V

space in the t u b that h a d b e e n t a k e n u p by water before h e h a d climbed

into

the tub. W h e n an object takes u p space in a liquid in this m a n n e r , w e say t h a t t h e o b j e c t displaces

the liquid. Archi-

m e d e s ' r u l e — o r p r i n c i p l e — is t h a t an object force

in a liquid that

is equal

is buoyed to the weight

up

by of

a the

displaced liquid. W h e n a b u o y a n t force acts o n a n ob-

The experiment proves Archimedes' Principle. Instead of a spring scale you can use a kitchen scale, as illustrated. Slip the strings holding the brick over the platform of the scale.

43

beneath water, but not touching

the

r e m e m b e r , s a y s t h a t t h e l o s s of w e i g h t

b o t t o m of t h e b u c k e t , a g a i n n o t e

the

of a n o b j e c t i n a l i q u i d is e q u a l t o t h e

w e i g h t of t h e b r i c k o n t h e s c a l e . T h e brick

will

weigh

less.

Subtract

w e i g h t of t h e b r i c k i n t h e w a t e r

the from

t h e w e i g h t of t h e b r i c k i n a i r . T h e diff e r e n c e w i l l e q u a l t h e a m o u n t of w e i g h t the brick lost w h e n in water. water.

T a k e t h e b u c k e t o u t of t h e p a n , b e i n g c a r e f u l n o t t o spill a n y m o r e w a t e r f r o m the bucket into the pan. Weigh the botusing the funnel,

carefully

pour the water from the p a n into the bottle. W e i g h t h e bottle a n d the water t o g e t h e r . S u b t r a c t t h e w e i g h t of t h e b o t t l e f r o m t h e w e i g h t of t h e b o t t l e

and

w a t e r t o g e t h e r . T h i s will give t h e w e i g h t of t h e w a t e r d i s p l a c e d b y t h e b r i c k . If y o u h a v e b e e n careful, this w e i g h t will e q u a l t h e a m o u n t of w e i g h t t h e b r i c k lost w h e n p l a c e d in water. T h i s proves Archimedes'

Principle which,

A b a t t l e s h i p is m a d e of s t e e l a n d is extremely heavy. T h e Why do Tj s s N e w Jersey battleships float? . , __ n f v A weighs 57,000 tons

R e m o v e the brick from the

tle, t h e n ,

w e i g h t of t h e d i s p l a c e d l i q u i d .

as

you

w h e n it is fully l o a d e d a n d t h e a i r c r a f t c a r r i e r U . S . S . Enterprise

weighs 85,000

t o n s . W h e n w e t h i n k of t h e f a c t t h a t t h e s t e e l of w h i c h t h e y a r e c o n s t r u c t e d is s o m u c h h e a v i e r t h a n w a t e r , it d o e s n o t seem reasonable that objects as heavy as ships should steel ships

Why, then,

do

P r i n c i p l e will give

us

float?

Archimedes' the answer.

float.

You

remember

that

the

b u o y a n t f o r c e o n a n o b j e c t i n w a t e r is e q u a l t o t h e w e i g h t of t h e w a t e r

dis-

p l a c e d . If t h e w e i g h t of t h e w a t e r disp l a c e d is m o r e t h a n t h e w e i g h t of t h e object, t h e b u o y a n t force will b e g r e a t e r

A steel ship floats because the part of the ship below the surface displaces a volume of water that weighs as much as the whole ship. The ship is buoyed up with a force equal to the weight of this displaced water.

t h a n t h e w e i g h t of t h e o b j e c t , a n d t h e

t h e b u o y a n t f o r c e is g r e a t e n o u g h

object

k e e p t h e fish a f l o a t n e a r t h e

will

float.

The

Enterprise

is

to

surface,

n e a r l y one-fifth of a m i l e l o n g , a s w i d e

h o w c a n t h e fish o v e r c o m e t h e b u o y a n t

a s a c i t y b l o c k , a n d is n i n e s t o r i e s h i g h .

force a n d remain near the b o t t o m ? W h y

T h e w e i g h t of t h e w a t e r d i s p l a c e d b y

doesn't the b u o y a n t force always push

t h e v o l u m e of t h i s h u g e s h i p is m o r e

t h e fish u p n e a r t h e s u r f a c e

than 85,000 tons; therefore, the buoy-

t h e fish s w i m s d e e p e r ? T h e a n s w e r t o

a n t f o r c e a c t i n g o n t h e s h i p is

t h e s e q u e s t i o n s is t h a t t h e fish h a s a n

more

whenever

than 85,000 tons. A s a result, the great

o r g a n c a l l e d a swim

m a s s of s t e e l floats. If s o m e g i a n t h a n d

of w h i c h t h e fish c a n c h a n g e i t s size.

w e r e t o c r u s h t h e Enterprise

T h e s w i m b l a d d e r is a s a c of

into a solid

bladder

by means elastic

l u m p of s t e e l , it w o u l d d i s p l a c e a m u c h

tissue b e t w e e n

s m a l l e r v o l u m e of w a t e r . T h e w a t e r dis-

b a c k b o n e . T h e b l a d d e r is filled w i t h a i r

p l a c e d b y t h e s o l i d l u m p of s t e e l w o u l d

and can be m a d e larger and

w e i g h less t h a n 8 5 , 0 0 0 t o n s ,

therefore

W h e n t h e s w i m b l a d d e r is l a r g e s t , t h e

t h e b u o y a n t f o r c e w o u l d b e less t h a n

size of t h e fish is i n c r e a s e d ; t h i s m e a n s

85,000 tons and would not be enough

t h a t t h e fish d i s p l a c e s t h e m o s t

t o h o l d t h e steel u p in t h e w a t e r .

t h a t it c a n , a n d t h e b u o y a n t f o r c e

on

t h e fish is g r e a t e s t ; a s a r e s u l t , t h e

fish

your m o t h e r uses in the kitchen. F r o m

floats

the

fish

t h e foil, m a k e a b o a t a b o u t five i n c h e s

m a k e s its s w i m b l a d d e r s m a l l , t h e

fish

l o n g a n d t w o i n c h e s w i d e . If t h e b o a t

d i s p l a c e s less w a t e r , a n d t h e

d o e s n o t l e a k , it w i l l float i n w a t e r . T h e

f o r c e u p o n it is l e s s ; a s a r e s u l t , t h e fish

O b t a i n a s h e e t of m e t a l foil s u c h a s

water

the

boat

displaces

weighs

the

s a m e as t h e m e t a l foil; t h e r e f o r e ,

the

b u o y a n t f o r c e is e q u a l t o t h e w e i g h t of

at

the

the

stomach

surface.

When

s q u e e z e t h e b a l l flat, a n d d r o p it i n t o w a t e r . I t will sink, b e c a u s e n o w t h e b a l l t h e foil, a n d a s a r e s u l t t h e

than

buoyant

f o r c e is t o o s m a l l t o h o l d t h e foil u p i n the water. You

h a v e seen

fish

in

an and

/'SWIM BLADDER INFLATED

aquarium

swimming How does a fish move up and down in the water?

down

up in

the water

and

floating

at

w h a t e v e r level they choose. H o w c a n a fish float i n w a t e r n e a r t h e s u r f a c e a n d a l s o n e a r t h e b o t t o m of t h e w a t e r ? If

the

smaller.

water

buoyant

When the swim bladder is inflated the fish displaces more water. This causes it to rise to the surface.

t h e foil. R o l l t h e foil i n t o a t i g h t b a l l ,

displaces w a t e r t h a t w e i g h s less

and

SWIM BLADDER DEFLATED

Schematic drawing that shows how the diving tanks in a submarine function. By filling or emptying the tanks, the submarine can submerge or surface..

m a r i n e c o m m a n d e r c a n c h o o s e t h e level

sinks t o l o w e r levels. B y c h a n g i n g t h e

Once a boy took a heavy earthenware

size of its s w i m b l a d d e r , a fish c a n

What is Pascal's Principle?

A s u b m a r i n e c a n n o t c h a n g e i t s size a s a

fish ^

b m

marine

can, §ub.

can

float.

j u g t o a well.

float

a t w h a t e v e r l e v e l it c h o o s e s .

How does a submarine move up and down , .. in the water?

at w h i c h t h e s u b m a r i n e will

filled

He

the jug until

it o v e r f l o w e d . T h e n

h e p u t t h e s t o p p e r i n t o t h e m o u t h of the jug and, to m a k e

sure that

the

s t o p p e r w o u l d n o t fall o u t , h e s t r u c k it s h a r p l y w i t h t h e p a l m of h i s h a n d . H e was astonished when the whole bottom

m a k e u s e of

fell o u t of t h e j u g ! W h y d i d t h e

jug

Archimedes' Principle in another way,

b r e a k w h e n its s t o p p e r w a s s t r u c k

s o a s t o c h a n g e t h e l e v e l a t w h i c h it

blow

floats.

are

s t r o n g e n o u g h t o b r e a k a j u g m a d e of

large t a n k s into w h i c h sea w a t e r c a n b e

heavy earthenware? In order to under-

pumped. W h e n the tanks are

empty,

s t a n d w h a t h a p p e n e d , w e will h a v e t o

a t t h e s u r f a c e of

learn something concerning liquids that

t h e s e a . T h i s m e a n s , of c o u r s e , t h a t t h e

was discovered by a y o u n g F r e n c h phys-

weight

icist, m a t h e m a t i c i a n ,

Inside a submarine,

the submarine of

floats

water

the

there

submarine

dis-

that

would

not

ordinarily

and

n a m e d Blaise Pascal, w h o lived

submarine, and therefore the buoyant

3 0 0 years ago.

to the submarine's weight. When

a submarine

be

philosopher

p l a c e s is t h e s a m e a s t h e w e i g h t of t h e f o r c e a c t i n g o n t h e s u b m a r i n e is e q u a l

a

some

W h a t P a s c a l l e a r n e d w a s t h i s : if y o u a p p l y p r e s s u r e a n y w h e r e t o a fluid h e l d

commander

in a c o n t a i n e r , t h e p r e s s u r e will p a s s

wishes t o t a k e his craft b e n e a t h t h e sur-

through

f a c e of t h e s e a , h e p u m p s w a t e r i n t o t h e

w i t h o u t a n y l o s s of s t r e n g t h . T h i s dis-

tanks within

c o v e r y is k n o w n a s P a s c a l ' s P r i n c i p l e .

the

submarine.

As

the

the

fluid

in

every

direction

t a n k s fill, t h e w e i g h t of t h e s u b m a r i n e

Suppose w e h a v e a big iron t a n k in

b e c o m e s g r e a t e r t h a n t h e w e i g h t of t h e

t h e s h a p e of a c y l i n d e r . I t is l y i n g o n i t s

w a t e r it displaces; t h e b u o y a n t

side a n d is six feet h i g h a n d t e n

force

feet

a c t i n g o n t h e s u b m a r i n e b e c o m e s less,

l o n g . T h e t a n k is filled w i t h w a t e r .

and

s m a l l p i s t o n is f i t t e d i n t o o n e e n d of t h e

the

craft

sinks.

By

varying

the

a m o u n t of w a t e r i n t h e t a n k s , t h e s u b 46

A

tank a n d a large piston into the other

t o m , t h e n 1 , 6 0 0 p o u n d s of f o r c e s u d -

end. T h e pistons are square, and

denly pushed on the bottom. N o won-

the

small one has a surface just one inch

der the jug broke!

across, while the large o n e has a surface t e n i n c h e s a c r o s s . A s q u a r e t h a t is o n e

H a v e y o u e v e r s e e n a m a n fix a flat t i r e o n t h e w h e e l of

i n c h a c r o s s h a s a n a r e a of o n e s q u a r e i n c h ; a s q u a r e t h a t is t e n i n c h e s a c r o s s h a s a s u r f a c e a r e a of 1 0 0 s q u a r e i n c h e s .

How does a hydraulic jack work?

of a b i g or

truck

trailer?

the

F i r s t , h e h a d t o lift t h e w h e e l off t h e

small piston. H o w m a n y m e n will b e

g r o u n d . T o d o this h e used a tool called

needed to push against the big piston

a j a c k . H e p l a c e d t h e u p r i g h t p a r t of

i n o r d e r t o k e e p it f r o m b e i n g p u s h e d

t h e j a c k u n d e r a p a r t of t h e

o u t w a r d a t i t s e n d of t h e t a n k ? A h u n -

frame; then he p u m p e d a handle up and

dred men! T h e water transmits to each

d o w n ; a s a r e s u l t , t h e u p r i g h t p a r t of

of t h e 1 0 0 s q u a r e i n c h e s o n t h e s u r f a c e

t h e jack m o v e d slowly u p w a r d , pushing

of t h e b i g p i s t o n t h e s a m e a m o u n t of

t h e t r u c k ' s h e a v y f r a m e w i t h it.

pressure that was applied to the

j a c k t h a t is n e e d e d t o lift a n o b j e c t a s

N o w suppose a m a n pushes on

one

truck's

The

s q u a r e i n c h of t h e s m a l l p i s t o n . If t h e

h e a v y a s a t r u c k is a hydraulic

m a n p u s h e d o n t h e o n e - i n c h - s q u a r e pis-

"Hydraulic" means "operated by

t o n w i t h a p r e s s u r e of 6 0 p o u n d s , t h a t

t e r . " I n s i d e t h e j a c k is a t h i c k i r o n cy-

60

have

l i n d e r filled w i t h w a t e r . E v e r y t i m e t h e

passed through the water to every other

m a n pushed the handle down, a small

s q u a r e i n c h i n t h e t a n k . T h e l a r g e pis-

piston p u s h e d against the w a t e r in t h e

ton h a d 100 square inches, so 6 0 p o u n d s

cylinder, a n d the p u s h passed t h r o u g h

of p r e s s u r e p a s s e d t h r o u g h t h e

water

the water to a large piston, where the

t o e a c h o n e of t h e 1 0 0 s q u a r e i n c h e s of

s t r e n g t h of t h e p u s h w a s s o g r e a t l y i n -

the large piston. O n e h u n d r e d times 60

creased that the heavy truck could be

is 6 , 0 0 0 , s o 6 , 0 0 0 p o u n d s of p r e s s u r e

lifted u p w a r d .

pounds

of

pressure

would

was pushing against the large

piston.

Many

machines

have

jack.

been

wa-

con-

N o w o n d e r it w o u l d t a k e 1 0 0 m e n t o

s t r u c t e d t o m u l t i p l y f o r c e b y m e a n s of

hold the large piston against the pres-

P a s c a l ' s P r i n c i p l e . O n e of t h e s e is t h e

sure exerted by one m a n .

h y d r a u l i c p r e s s t h a t is u s e d t o b a l e c o t why

the

t o n . Y o u k n o w t h a t c o t t o n is v e r y l i g h t

Suppose

the

a n d fluffy. I t w o u l d b e v e r y a w k w a r d t o

s t o p p e r h a d a b o t t o m s u r f a c e of

one

s h i p l a r g e a m o u n t s of c o t t o n j u s t a s it

N o w you can understand earthenware

jug

broke.

square inch. L e t us say that the b o t t o m

comes from the cotton plant; the

of t h e j u g h a d a n a r e a of 8 0

cotton would take u p so m u c h

room.

S o , c o t t o n is p l a c e d i n l a r g e

burlap

square

inches. A l s o , let us say t h a t t h e h i t t h e s t o p p e r w i t h a p r e s s u r e of pounds.

If a p r e s s u r e

of

20

boy 20

pounds

sacks a n d compressed by a

fluffy

hydraulic

press. W h e n the baler pushes a small

passed through the water to every one

p i s t o n , a l a r g e o n e is m o v e d

upward,

of t h e 8 0 s q u a r e i n c h e s of t h e j u g ' s b o t -

s q u e e z i n g a i r o u t of t h e c o t t o n b y p u s h 47

The way in which the jack works to hoist automobiles in a service station, is based on Pascal's Law. Today oil is used instead of water in many hydraulic appliances.

ing the sack against a

flat

of

fing o u t a g a i n . T h e h y d r a u l i c lift t h a t

around

the

raises cars a b o v e t h e g r o u n d in service

c o m p r e s s e d c o t t o n t o k e e p it f r o m

fluf-

stations also uses Pascal's Principle.

iron. Wires are w r a p p e d

square

Air,

W a t e r ,

Y o u h a v e l e a r n e d t h a t air a n d w a t e r a r e very important How can you help s u b s t a n c e s > W i t h . keep air and ., ., . out them, there water clean? w o u l d b e n o lite at all. S o m e t i m e s a i r a n d w a t e r polluted,

become

t h a t is, t h e y . b e c o m e d i r t y a n d

even d a n g e r o u s to use. W h e n this happ e n s , m a n h a s t o find a w a y of m a k i n g t h e m clean a n d safe again. P e r h a p s y o u have seen a polluted p o n d or

stream

f r o m w h i c h n o o n e is a l l o w e d t o d r i n k w a t e r o r s w i m in b e c a u s e h e w o u l d get s i c k if h e d i d . I n s o m e c i t i e s t h e a i r , t o o , b e c o m e s v e r y d i r t y . S o m e t i m e s it gets so dirty that people

cough

and

A s t h e p o p u l a t i o n of o u r c o u n t r y increases, m o r e a n d m o r e waste materials

48

lakes

Y o u and

rivers.

Many

people

are

needed to w o r k at keeping our air a n d water clean. W e need engineers to build d a m s a n d sewage systems. Chemists are n e e d e d t o t e s t w a t e r a n d a i r t o s e e if t h e y a r e safe, a n d r e s e a r c h e r s a r e n e e d e d to discover new ways to m a k e clean.

People

are

needed

to

them advise

farmers on the best way to plant crops so t h a t soil e r o s i o n will b e p r e v e n t e d . W e n e e d o t h e r p e o p l e t o p l a n cities a n d t o w n s so t h e factories a n d d u m p s that might pollute the air a n d w a t e r are not built near homes. T h e s e t a s k s offer

much

interesting

work that can be a rewarding

lifelong

career. P e r h a p s y o u will o n e d a y enter

c h o k e a n d their eyes w a t e r .

will b e e m p t i e d i n t o t h e a i r a n d

a n d

into

one

of

these

useful

and

challenging

fields w o r k i n g t o k e e p o u r a i r a n d w a t e r c l e a n a n d safe.

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