UNIVERSITY OF CALIFORNIA 
 AT LOS ANGELES
 
 NATURAL PHILOSOPHY.
 
 NATURAL PHILOSOPHY 
 
 COMMON AND HIGH SCHOOLS. 
 
 LE EOT O. COOLET, PH. D., 
 
 PROFES808 OP NATUBAL SCIENCE IN TUB MEW YOUK STATE NORMAL SCHOOL. 
 
 NEW YORK: 
 SCRIBNER, ARMSTRONG & CO., 
 
 SUCCESSORS TO 
 
 CHARLES SCRIBNER & CO., 
 
 654 BROADWAY. 
 1872.
 
 Entered according to Act of Congress, in the year 1871, 
 
 BY LK KOY C. COOLKY, 
 lu the Office of the Librarian of Coiigress, at Washington
 
 CT] 
 
 CO 
 
 oo 
 
 CD 
 
 PREFACE 
 
 THE great aim of this little book is to present the most 
 ^elementary facts of Natural Philosophy, iu such a way as to 
 Sexercise the child constantly in observing phenomena and in 
 3 drawing inferences from what he observes. 
 3 "Whenever a child is old enough to ask such questions, as, 
 " What makes the thunder," or, "Where does the rain come 
 
 (J from," or to exclaim " How pretty the clouds are this even- 
 
 2 . 
 
 ~~ ing," it is old enough to begin the study of natural philosophy. 
 
 OO 
 
 ,-. When snch questions are asked the mind is awake to see the 
 
 ^phenomena of nature, and "is ready to receive instruction. 
 
 ** They show the presence of a desire to know, and the absence 
 
 of power to learn without assistance ; and in this way they 
 
 jlead us to believe that the time has cohae when the work of 
 
 minstruction should begin. 
 
 ^g 
 
 C Moreover, the study of natural philosophy is easy, and in- 
 
 Cteresting to young pupils, because, when properly presented, 
 
 it brings ne\v sights to the eye, and new sounds the ear, 
 
 in a way to be especially pleasant to children. The simplest 
 
 experiments awaken enthusiasm in the mind of a child, and 
 
 such as he may be able to repeat by himself are the source 
 
 of the greatest delight. 
 
 233560
 
 6 PREFACE. 
 
 This study is not only easy and interesting, it is also in the 
 highest degree beneficial to the young, partly because of the 
 valuable facts it imparts, but even more on account of the 
 mental power it devclopes. The object of primary education 
 should be to discipline the senses to habits of quick and ac- 
 curate observation, and the mind to the habit of forming 
 correct judgments from facts which the senses reveal. 
 Natural Philosophy furnishes abundant materials of the most 
 excellent kind, by means of which these objects may be ac- 
 complished. There are curious motions, beautiful colors nnd 
 harmonious sounds, together with numerous other phenom- 
 ena, which can be easily presented in the form of simple ex- 
 periments, by which the skilful teacher can cultivate the 
 power of the senses to furnish correct impressions, and at 
 the same time develope the power of basing accurate judg- 
 ments upon the impressions received. In a word, this study 
 when properly presented is eminently fitted to teach even 
 young pupils how to gain knowledge for themselves by ob- 
 serving events. 
 
 To this end, the following plan ought to prevail in present- 
 ing elementary facts. An easy experiment or some phenom- 
 enon of common occurrence, is to be introduced and the atten- 
 tion of the child directed to certain appearances and condit- 
 ions, after which he may be called upon to notice the truth 
 which these appearances suggest. A concise and accurate 
 statement of the fact or principle itself, in form to be easily 
 remembered, may finally complete the investigation. 
 
 Many of the experiments are such as pupils can make for 
 themselves: let them be encour.iged to do so; if they are,
 
 PREFACE. 7 
 
 they will soon be bringing to the notice of the teacher, 
 others which the text does not describe, and if the teacher 
 will visit such efforts with marks of especial notice or reward 
 ho will soon find an enthusiasm in his class, which will 
 make the pursuit of this study delightful and profitable to 
 the end. 
 
 Another feature of this little book, which, it is believed, 
 will commend it to the favor of both pupil and teacher, is the 
 system of questions which runs through it. Every impor- 
 tant topic in the discussion of each subject is introduced by 
 a question, instead of by a formal title, as is customary. 
 These questions will prove to be excellent guides, and really 
 very important helps to the young pupil. The teacher will 
 also find them serviceable in conducting the exercises of the 
 class room, to which they are especially adapted, by being in 
 immediate connection with the text, and in bold type easily 
 'caught by the eye, instead of at the bottom of the page or the 
 back of the book, in fine print and compact lists. The eye 
 catching them quickly, is not confined to the book ; their 
 use, therefore, will not be at the sacrifice of the vivacity and 
 vigor of the exercise. 
 
 Albany, 1871.
 
 NATURAL PHILOSOPHY. 
 
 PROPERTIES OF MATTER 
 
 Describe the experiment with cochineal. If, to 
 
 try an easy experiment, we take a single grain weight of 
 cochineal, and dissolve it in as much as a thimbleful of 
 water, and then pour this small quantity into a gallon of 
 clear water, the whole gallon will receive a beautiful crim- 
 son color. 
 
 Into how many pieces has the grain of cochi- 
 neal been divided ? Now a gallon of water is said to 
 contain as many as 60,000 drops, and to color a single 
 drop, all through, will take as many as 100 little particles 
 of cochineal. If this is true, then the grain of cochineal 
 must be divided into not less than six millions of pieces ! 
 
 Can other bodies be divided ? If an apple be cut 
 into 100 pieces, each piece will of course be very small 
 indeed, but yet it will not be BO small that it can not be 
 divided into pieces smaller yet. 
 
 The blow of a hammer may break a pane of glass into 
 a thousand parts, but each one of these little pieces may 
 by another blow be broken into pieces still smaller. 
 i*
 
 10 NATURAL PHILOSOPHY. 
 
 What is divisibility ? Every body of matter may 
 be cut or broken into pieces. This is one of the qualities 
 or properties of matter, and we call it divisibility. 
 
 Divisibility is the property of matter in virtue of 
 which a body may be separated into parts. 
 
 Are examples of great divisibility common ? 
 There are bodies all around us so small that we can not 
 see them. They are in the air we breathe and in the water 
 we drink. Some of them are alive and some are not. 
 Many of them are so very small that we need the most 
 powerful microscope to see them at all. Yet every one is 
 made up of pieces or parts which are of course smaller 
 than itself. 
 
 For example : the dust which clings to one's finger 
 when he holds a butterfly or a moth is made up of 
 very small particles, and yet each of these little particles 
 of dust, which we can scarcely see with the naked eye, is 
 found, by using the microscope, to be made up of a 
 thousand or more little balls. 
 
 Are living creatures so very small? And then, 
 too, there are living creatures so small, that it may need as 
 many as a million of them to make a pile as large as a 
 mustard-seed. Hosts of them are living in the air and in 
 the water all around us. They are so very very smali 
 that it has been said that a thousand of them might sWim 
 or fly sid? by side through the eye of a needle. 
 
 And yet each of these little creatures must be made up 
 of still smaller parts, or else they could not move about 
 nor devour their food, as all of them are able to do. We 
 can not even imagine how very small these parts must 
 be. 
 
 What are molecules ? If we keep on dividing a 
 "oody into smaller and smaller pieces, we shall at last get
 
 PROPERTIES OF MATTER. H 
 
 to a piece so very small that it can not be divided again 
 without changing it into some other kind of matter. 
 These smallest pieces are called molecules. 
 
 Molecules are particles of matter which can not be 
 divided without changing their nature. -^ 
 
 Does every body occupy space ? Every little par- 
 ticle of dust, and even a molecule, must have some size. 
 You can not even think of a body which should have no 
 size at all. The very smallest body you can think of fills 
 up a little room or space. And then every larger thing, a 
 %hot for example, or a cannon-ball ; the worl<J itself, so 
 many millions of times larger than the ball ; and the sun, 
 which is fourteen hundred thousand times larger than the 
 world, each of these bodies has its own particular size. 
 or, in other words, each one fills a certain portion of 
 space. 
 
 To occupy space is one of the properties of all matter : 
 it is called extension. 
 
 Extension is the property of matter in virtue of which 
 a body occupies a certain portion of space. 
 
 Can wood and -water fill the same place at once ? 
 
 If we fill a goblet with water, and then gently push a 
 small stick down into it, the water runs over. We see 
 in this way that wood and water can not be put into the 
 same place at the same time. 
 
 Can water and air fill the same place at once ? 
 Boys sometimes turn a goblet bottom upward, and then 
 push it down into a vessel of water to see the air keep the 
 water out of the goblet. The air will not let the water 
 in, because they can not both be in the same place at 
 once.
 
 12 NATURAL PHILOSOPHY. 
 
 Can not a nail be driven into the same place 
 with wood ? When a nail is driven into wood, the par- 
 ticles of the wood are squeezed nearer together so as to 
 make room for the nail to enter. The wood and the nail 
 do not fill the same space at the same time. 
 
 What is impenetrability? No two bodies can 
 ever be in exactly the same place at once. This is 
 one of the properties of matter. It is called impenetra- 
 
 Impenetrability is the property of matter which does 
 not allow two bodies to occupy the same space at the same 
 time. 
 
 Can not water and sugar be put into the same 
 space at once? Try an easy experiment in this way: 
 till a goblet to the very brim with water, so that not a 
 drop more can be added without running over. Then 
 take some fine sugar, and very slowly sprinkle it into 
 the water. Quite a large quantity may be put into the 
 goblet before a single drop of water will overflow. It 
 would at first appear that water and sugar have been here 
 put into the same space at the same time. 
 
 Describe the experiment with shot. Take 
 another goblet and fill it with shot, and then pour fine 
 sand upon the shot. The sand will, of course, fall down 
 between the shot, and a large quantity will be poured 
 in before any will run over. Now the sand only fills up 
 the space between the shot ; no one would think of saying 
 that the sand and shot fill the same space at once. 
 
 How does this explain the experiment with 
 sugar and water? Now the sugar and water act in 
 the same way as the sand and sHot. The fact is that 
 water is made up of little balls 01 molecules, and the
 
 PROPERTIES OF MATTER. 13 
 
 molecules of sugar are little enough to fall into the spaces 
 between them. The two things do not occupy the same 
 space at once. 
 
 What should we learn from the experiment? 
 We see that there must be spaces between the molecules 
 ot water into which the sugar falls. It is true also of all 
 other bodies that there are spaces between their mole- 
 cules. On this account bodies are said to be porous. 
 
 Porosity is the property of matter in virtue of which 
 there are spaces between its molecules. 
 
 How was gold shown to be porous ? A long time 
 ago, at Florence, a hollow globe of gold was filled with 
 water, shut up perfectly tight, and then put under immense 
 pressure! The water actually oozed through the gold, 
 and, like a gentle dew, covered the outside. The water 
 must have come through between the molecules of the 
 gold, so that we know that even gold is porous. 
 
 What is elasticity ? We all knoAv that a piece of 
 india-rubber can be pulled out to a great length, and that 
 it will afterward spring back again. We have also doubt- 
 less seen a steel wire straighten itself quickly after being 
 bent. The quality or property which causes these bodies 
 to spring back is called elasticity. 
 
 Elasticity is the property of matter in virtue of 
 which a body springs lack after having yielded to some 
 force. 
 
 Is glass elastic ? The balls which boys use to play 
 " marbles " w r ith are sometimes made of glass, and every 
 one knows how well one of these balls will bound upward 
 when thrown upon the floor or pavement. Now the little 
 ball is actually flattened at the moment when it strikes 
 the floor, but the next instant the flattened part springs
 
 14 NATURAL PHILOSOPHY. 
 
 back, and it is this springing back which throws the ball 
 into the air. This shows that glass is very elastic. 
 
 Are all bodies elastic ? All bodies are more or less 
 elastic. Lead and clay have but little elasticity, but even 
 "lead is elastic, for we nnd that two lead balls after being 
 struck together w : ll bound a little. 
 
 On the other hand, glass and ivory are among the most 
 elastic solids we know of. 
 
 What becomes of water that boils away ? When 
 water boils, steam is formed. Now let us prove that all 
 the water which disappears is changed into steam. 
 
 ."We will first notice that when a cold body is held in 
 a cloud of steam it is soon wet, which shows that by cool- 
 ing the steam we get back water from which it was made. 
 
 Now let us put just one pound of water into a vessel 
 and boil it. Let the steam pass through a pipe into 
 another vessel which is kept cold. The steam goes over 
 into the cold vessel, and is changed back into water. 
 When the water is all " boiled away " from the first 
 vessel we shall find just one pound of water in the other. 
 All the water which disappeared was changed into steam. 
 Not a particle was destroyed. 
 
 Is this true when the steam goes into the air ? 
 When the steam goes oft' into the air, as it usually does, 
 its particles are so widely scattered that they disappear 
 entirely, but every one of them is still in existence some- 
 where. 
 
 What becomes of wood when it burns ? When 
 wood is burned, one part of it is changed into ashes, and 
 the rest changes into smoke and vapor. Not a single 
 particle of it is ever destroyed. 
 
 What is indestructibility ? Man may thus change
 
 PROPERTIES OF MATTER. 15 
 
 the shape and condition of bodies, but he can not destroy 
 a single molecule of any thing. On this account matter 
 is said to be indestructible. 
 
 Indestructibility is the property of matter in virtue of 
 which it can not be destroyed. 
 
 What is compressibility? When we squeeze a 
 sponge in the hand we press its particles nearer and 
 nearer together, and finally make the sponge much 
 smaller than it was. Bodies which can be made smaller 
 by pressure are said to be compressible. 
 
 Compressibility is the property of matter in virtue of 
 which a body can be made smaller l>y pressure. 
 
 Is air compressible ? Air is very compressible. You 
 may learn this from so humble a thing as a pop-gun. 
 Before the stopper is blown out, the air behind it you 
 can see to be crowded into perhaps not half the space it 
 filled at first. 
 
 Is water compressible ? Water is very slightly 
 compressible. Very great pressure is needed to compress 
 it enough to show that it is compressible at all. A press- 
 ure which would compress air into less than a hundredth 
 part of its natural bulk would not compress water, enough 
 to be noticed. 
 
 Are all bodies compressible ? All bodies are more 
 or less compressible. Air is one of the most compressi- 
 ble of all substances, and water is one which is among 
 the least compressible. 
 
 Are molecules compressible ? All that pressure 
 can do to a body is to push its molecules nearer together : 
 we do not suppose that it makes the molecules themselves 
 any smaller. 
 
 When air is compressed into a hundredth part of its
 
 ] tf NATURAL PHILOSOPHY. 
 
 natural volume, its molecules have been pushed a hundred 
 times nearer together than they were at first. 
 
 What is density ? Now when the air is so much com- 
 pressed there is much more of it in a given space, a cubic 
 inch for example, than there was before. In this con- 
 dition it would be said to be more dense. 
 
 Density has reference to the quantity of matter in a, 
 given bulk. 
 
 What is inertia ? Masses of matter have no power 
 to move themselves nor to stop themselves when once in 
 motion. The clouds move along in the sky, not because 
 they choose to do so, but because they are pushed along 
 by the wind. An apple falls from the tree, because it is 
 pulled down by an influence, soon to be described, called 
 gravitation ; and rocks rest in their places, not because 
 they have any power in themselves to do so, but because 
 they are held there by forces acting upon them. Bodies 
 of matter have no power to change their own condition, 
 and on this account they are said to be inert. Inertia in 
 the property of matter which does not allow a body to 
 change its own condition of rest or motion. 
 
 Are molecules ever at rest? Masses of matter 
 are often at rest ; it is believed that molecules never are. 
 On the contrary, it is thought that the molecules of every 
 body are forever in motion. 
 
 You have perhaps seen a cluster of bees at the door of 
 their hive, or of ants at the entrance to their nest, all hud- 
 dled together and hurrying over and around each other in 
 constant and curious motions. Now if our eyes were power- 
 ful enough to see the little molecules of which a block of 
 wood is composed, it is thought that we should witness a
 
 PROPERTIES OF MATTER. 17 
 
 scene of activity still more curious and constant, for every 
 molecule in all the vast number which the block contains 
 is in rapid motion. Philosophers believe that not a single 
 one in all the world was ever for a moment still. 
 
 In -what respect are the properties so far de- 
 scribed, alike? If we think again of the properties 
 which we have just examined, we find that they are all 
 of such a character tnat a body may show that it has 
 them without any change taking place in its nature. 
 
 A smart blow with a hammer shatters a stone into 
 fragments, and the experiment teaches us that the stone 
 lias the property of divisibility. But then- every piece 
 will be a piece of stone, and of just the same kind as be- 
 fore the blow ; and so we find that a body may show the 
 property of divisibility without any change in its nature. 
 
 What are physical properties ? All such proper- 
 ties, that is, all properties which a body may show with- 
 out any change in its nature, are called physical proper- 
 ties. 
 
 What are chemical properties ? But all proper- 
 ties are not like these. Explosibility, for example, is one 
 which a body can not show without a change in its nature. 
 
 Suppose a little gunpowder lies upon the table. You 
 do not know whether it is explosive or not : it may be 
 damp, and hence not explosive. Bui you touch it with a 
 lighted match a bright flash and a sudden puff occurs, 
 and you say that the powder is explosive. Now all that 
 is left upon the table at the spot is a dark stain. The 
 powder itself has been changed into gases which have 
 passed off and hidden in the air. No body of matter can 
 show that it has the property of explosibility without 
 changing to something else, and for this reason explosibil-
 
 13 NATURAL PHILOSOPHY. 
 
 ity is called a chemical property. Chemical properties 
 are those which a body can not show without a change in 
 its nature. 
 
 What is natural philosophy? Natural philosophy 
 is the science which treats of the physical properties of 
 matter and explains those things which occur without 
 any change in the nature of bodies. 
 
 The chemical properties of matter are to be described 
 in the science of chemistry : we need give no further 
 attention to them now.
 
 ATTRACTION. 
 
 What is the effect of rubbing sealing-wax -with 
 flannel? If we briskly rub a stick of sealing-wax with a 
 piece of flannel or silk, we seem to give it a power which 
 it did not have before, for if we hold it near to small bits 
 
 Fig. 1. 
 
 of cotton we see them fly quickly toward it, or if we pre- 
 sent it to a pith-ball hung by a silk thread, the ball will 
 be drawn aside or lifted by it. (Fig. 1.) 
 
 What may be seen on the surface of quiet water ? 
 If we observe the surface of quiet pools of water, we
 
 20 NATURAL PHILOSOPHY. 
 
 notice that sticks and straws will not stay for any great 
 length of time upon the middle parts of the surface, but 
 that, instead of this, they will be gathered together around 
 the edges. 
 
 Or, if we wish to try an' experiment, we may put a 
 number of bits of wood here and there upon the surface of 
 water in a large pail or tub standing in a quiet place 
 where it may rest over night. In the morning we will 
 find the bits of wood huddled together, or against the side 
 of the vessel : not one of them staying alone where it was 
 placed. 
 
 What other facts of the same kind ? Other facts 
 of the same kind are still more familiar. A stone moves 
 toward the ground when 'not supported. Leaves fall to 
 the earth in autumn, and rain-drops and hail stones will 
 not abide in the sky. 
 
 What do these experiments and facts illustrate ? 
 Now all these experiments and facts illustrate the ten- 
 dency of all bodies of matter to approach each other. If 
 they were not kept apart by some other forces this 
 tendency would cause all bodies to rush together. The 
 influence that would bring them together is called attrac- 
 tion. 
 
 Name varieties of attraction. Attraction shows 
 itself in many ways, and when acting in different ways it 
 is called by different names. When magnets attract each 
 other the influence is called magnetic attraction. The 
 influence of the sealing-wax upon the pith-ball (Fig. 1) is 
 called electrical attraction. Besides these there are other 
 varieties, called cohesion, adhesion, and iOMiiation. 
 
 With the last three varieties we must now become 
 acquainted ; but of the first two we shall learn more at 
 another time.
 
 ATTRACTION. 21 
 
 Why is a rod of iron so strong ? It is by no means 
 easy to break a rod of iron. Every child knows this, but 
 there are very few who can give a reason why the iron is 
 so strong. 
 
 Just think of the rod being made up of molecules, as 
 we have learned that all bodies are. These molecules 
 would fall apart if there were not something to hold 
 them together. They are held together by attraction, 
 and the iron is strong just because this attraction is very 
 strong. 
 
 How are the molecules of any body held to- 
 gether ? Just as there is attraction among the molecules 
 of iron, so there is among the molecules of any other solid 
 body an attraction which holds them together. This at- 
 traction acts continually. Were it to stop its action for 
 the briefest moment, solid bodies would be seen instantly 
 crumbling to pieces. Chairs, stoves, tables, and indeed 
 the very walls of the house, would fall to powder finer 
 and looser than ashes or flour. 
 
 The attraction which holds the molecules of a body 
 together is called cohesion^ 
 
 Is the cohesion alike in all bodies ? Cohesion is 
 much stronger in some bodies than in others. Iron is 
 very cohesive but lead is not. It is easy to break a small 
 rod of lead, while a rod of iron, of the same size, would 
 resist all our power. It is because the cohesion is so 
 strong in iron that this metal is so well adapted to use in 
 making carriages, in building bridges, and in many other 
 arts which you can easily mention, where great strength 
 is needed. 
 
 If cohesion is strong enough to bind the molecules of a 
 body firmly together, the body is a solid ; but if it is very 
 feeble indeed, the body is a liquid.
 
 22 NATURAL PHILOSOPHY. 
 
 Are particles of different kinds of matter held 
 together? There is also an attraction between particles 
 of different kinds of matter. When, for example, one 
 writes upon the blackboard, he leaves fine particles of the 
 crayon clinging to the surface of the board. Particles of 
 water cling to the hand that is withdrawn from a bath in 
 water; and it may be that particles of soil, clinging to the 
 hand unpleasantly, made the bath necessary in the first 
 place. 
 
 In all such cases we notice that there is an attraction 
 between particles of different kinds of matter. Attraction 
 between particles of unlike kinds is called adhesion. 
 
 By what experiment may we illustrate it ? A 
 very prettv experiment is shown in Fig. 2. It illustrates 
 Fi ?- 2 - the adhesion between water and 
 
 brass. A round plate of brass, 
 having a handle fastened to its 
 center, is laid flat upon the sur- 
 face of water, and then slowly 
 and gently lifted. The water un- 
 der it is also lifted a little, as the 
 picture shows it. 
 
 You can use a plate of wood or 
 of glass in the same way. 
 In what curious way may the adhesion between 
 solids and liquids be shown ? If you will take two 
 pieces of glass and put them side by side no farther apart 
 than the thickness of a sheet of paper, and will then bring 
 their lower edges carefully in contact with the surface of some 
 colored water, you will see that fluid suddenly spring up 
 an inch or two between the plates arid remain standing at 
 that height. In fact, it will stay up between the plates even 
 if you lift them quite away from the water. It must be
 
 ATTRACTION. 23 
 
 the attraction between the water and the glass which lifts 
 the fluid and holds it up between the plates. 
 
 What is the effect when the plates are not paral- 
 lel ? Still more curious is the effect if you will put the' 
 plates so that their edges will be nearer together at one 
 side than at the other. The water jumps up as before, 
 but its upper edge, instead of being horizontal as it was 
 in the other experiment, will be in the form of a beautiful 
 curve. The liquid rises highest where the plates are near- 
 est_togethe-. 
 
 Suppose small tubes be used instead of plates. 
 When small glass tubes are used instead of plates, the fluid 
 will rise still higher just twice as high as between plates 
 whose distance apart is equal to the diameter of the tube 
 used. 
 
 It has also been proved that the liquid will rise highest 
 in the smallest tube. It will rise two times as high in a 
 tube whose diameter is only one Jtalf as great as that of 
 another. 
 
 What is the law? The law is this: the height to 
 which the fluid rises is inversely as the "diameters of 
 the different tubes." If, for example, one tube is three 
 times the diameter of another, water will rise in it only 
 one third as high. 
 
 What other examples of this action ? Water soaks 
 upward through parous soils, and by this means they are 
 kept moist and fertile. The oil is lifted through the lamp- 
 wick to supply the flame above. Ink spreads through 
 blotting-paper when only one corner of it touches the drop. 
 All these and many other familiar tilings that might be 
 named, are caused by the same influence which lifts water 
 in small tubes or between glass plates. It is an attraction 
 between solid and liquid bodies.
 
 2-J- NATURAL PHILOSOPHY. 
 
 This attraction between solid and liquid bodies is very 
 generally called capillary attraction, but it is really nothing 
 different from adhesion. 
 
 Give examples of the action of gravitation. A 
 
 stone dropped from the hand falls swiftly to the ground, 
 because there is an attraction between the earth and the 
 stone. An apple bends its stem because the same kind of 
 attraction is pulling it toward the earth, and when the 
 fruit ripens and the stem has grown weaker, the same 
 force causes the apple to break away and fall. This at- 
 traction is called gravitation. It is the attraction which 
 acts upon all bodies and through all distances. 
 
 Give examples of pressure caused by gravita- 
 tion. Gravitation not only causes a body to fall if left 
 without support, it also causes one body to press upon 
 another on which it rests. A stone press :s heavily upon 
 the ground because gravitation is pulling it downward. 
 All things upon the earth are held there, and exert their 
 pressure, because gravitation is acting upon them. Some 
 are held with much more force than others, as we may 
 easily learn by trying to lift them. A pail of water hangs 
 heavily upon the arm because gravitation is pulling it 
 down. 
 
 What is weight ? It is easier to lift a block of wood 
 than a stone of the same size, because gravitation is pulling 
 the stone down with more power. To say that the stone 
 is heavier than the wood means just the same as to say 
 that the attraction of the earth upon the stone is stronger 
 than upon the wood. Indeed, the wcV^jofji^Jwi^is 
 Qnljjhe measure of the attraction whjclTthe earth exerts 
 upon it. 
 
 How do we tell whether two bodies have equal
 
 ATTRACTION. 25 
 
 weights ? A pair of scales enable us to tell whether 
 bodies have equal weights. If we put one body in each 
 scale-pan and the two are balanced, we know that gravita- 
 tion is pulling one down just as much as the other; in 
 other words, the two bodies are equal in weight. 
 
 What are sets of weights ? A piece of metal upon 
 which the earth exerts a certain amount of attraction may 
 be called an ounce weight j then another upon- which the 
 attraction is twice as great is called a two-ounce weight ; 
 and if upon a third the attraction is sixteen times as great 
 as upon the first, it would be a sixteen-ounce weight, or a 
 pound avoirdupois. Several such pieces of metal, made 
 with care to represent the various units of weight, form 
 what is called a " set of weights," to be used in weighing 
 the various articles in trade. 
 
 In what direction does the earth attract 
 bodies ? The earth attracts all bodies toward its 
 center. From whatever point a ball is dropped, it will 
 fall in a straight line toward the center of the earth. 
 
 This direction is always perpendicular to the surface 
 of still water. You can easily examine this fact yourself 
 by fastening a string to some heavy body and then hold- 
 ing or hanging it over a vessel of water, as you see it in 
 the picture. (Fig. 3, p. 26.) The string shows the direc- 
 tion of the force of gravity * exactly, and it is easy to see 
 that it is perpendicular to the surface of the water. 
 
 Such a cord and ball is called a plumb-line : builders 
 use it to find out whether their walls are vertical. 
 
 Does gravity always cause motion downward ? 
 While the earth's attraction is forever downward, yet 
 it does sometimes produce motion upward. For ex- 
 
 * The earth's attraction is sometimes called gravity.
 
 NATURAL PHILOSOPHY. 
 Fig. 8. 
 
 ample, it lifts the higher pcale-pan of a balance by pull- 
 ing the other downward at the same time, with greater 
 force. 
 
 In the same way gravitation causes the upward motion 
 of smoke by pulling the heavier air down under it, thus 
 pushing it upward. 
 
 Why does a cork rise in water? One more illus- 
 tration will be enough. A cork at the bottom of a vessel 
 of water quickly rises to the surface. Now it does not rise 
 because it is light, as many people will say it does. The
 
 ATTRACTION. 27 
 
 fact is that gravitation pulls both the cork and the water 
 downward, but it pulls the water with the greatest force. 
 The water must go down under the cork, and in doing so 
 must push the cork upward. 
 
 Is the earth attracted by small bodies ? To say 
 that the earth attracts an apple is not more true than to 
 say that the apple attracts the earth. The truth is simply 
 that they attract each other. The earth attracts every 
 body great and small, and every one attracts the earth in 
 return. Every leaf and every rain-drop or snow-flake that 
 falls to the ground attracts the earth just as truly as it is 
 attracted by it. 
 
 Is the earth moved by this attraction ? The earth 
 attracting the rain-drop, makes it fall toward the ground ; 
 the rain-drop attracts the earth in return : can we suppose 
 that the great earth moves up to meet it ? We have seen 
 thousands of rain-drops fall, but who ever saw the earth 
 go up to meet them ! And yet perhaps it does, for we 
 could not see it if it did. The attraction would make 
 the drops go as many times farther than it would the 
 earth, as the earth is times heavier than the drops, and it 
 would not be possible to see the motion through so small 
 a distance. 
 
 Is gravitation confined to the earth? But this 
 force is not confined to the earth and the bodies near its 
 surface : the sun and all the other bodies in the heavens 
 attract each other. It is exerted by every body of matter 
 upon every other in the universe. Grains of sand are 
 held by it in their places on the sea-shore, and it keeps 
 the sea itself from rising out of its bed. It is at the same 
 time acting upon the earth itself and upon all the other 
 heavenly bodies to keep them in their orbits. 
 
 On -what does the strength of this force de-
 
 28 NATURAL PHILOSOPHY. 
 
 pend? The strength of gravitation depends upon two 
 things : first, upon the quantity of matter in the body 
 exerting it ; and second, upon the distance through which 
 it acts. 
 
 If the quantity of matter is doubled the attraction will 
 be doubled also. Or, in general terms, the attraction is 
 in proportion to the quantity of matter exertiny it. 
 
 But it' the distance be doubled the attraction will be 
 only one fourth as strong. At three times the distance 
 the force is only one ninth as strong. In general terms 
 we say, the attraction is inversely as the square of the dis- 
 tance between the bodies. 
 
 What is the center of gravity of a body ? Boys 
 are sometimes very fond of balancing books or ball- 
 clubs, or even long poles, upon the end of a finger. They 
 often become very skillful in doing this, without knowing 
 that every time they do it they are trying an experiment 
 in natural philosophy. The fact which the experiment 
 illustrates is this : there is a point in every body wfcich if 
 supported the whole body will be at rest. This point is 
 called the center of gravity. The ball-club has a center 
 of gravity, and if the finger can be kept exactly under that 
 point the club will not fall. 
 
 Illustrate by using a ruler. Or, to study this sub- 
 ject further, let a ruler be balanced across your finger. 
 There will be just as much of the weight of the ruler on 
 one side of the finger as on the other, and a point exactly 
 over the finger and in the middle of the ruler is the center 
 of the weight of the ruler, or, as we have already named it, 
 the center of gravity. 
 
 Every body has a center of gravity, and when this point 
 is supported the whole body will be at rest. 
 
 Where is the center of gravity of a body? The
 
 ATTRACTION. 29 
 
 center of gravity is not always in the center of the body. 
 Suppose one end of your ruler to be loaded with lead : you 
 would then have to put your finger nearer to the heavier 
 end in order to balance it ; the center of gravity would 
 be nearer to the loaded end. 
 
 When two boys are playing at seesaw the support of 
 the board must be under the center of gravity, but if the 
 boys are not of the same weight the support, as every one 
 knows, must be nearer to the heaviest boy. The center of 
 gravity of the board and boys together is nearer to the 
 end where the large boy sits. 
 
 In Fig. 4 the center of gravity in each body is at G. 
 
 Fig. 4 
 
 "What is the line of direction ? Now imagine a 
 vertical line drawn through the center of gravity, as shown 
 by the vertical dotted lines in Fig. 4. This line ^ will 
 show the direction in which the body would fall if it 
 were left without support, and it is called the line of direc- 
 tion. 
 
 Where may we place the support for the center 
 of gravity? We may support the center of gravity by 
 placing the support at any point in the line of direction. 
 It may be placed at the center of gravity, or at some
 
 30 NATURAL PHILOSOPHY. 
 
 point above it, or at some point below it. Fig. 5 shows 
 a disk of metal supported in these three ways. 
 
 Fig. 5. 
 
 Describe three kinds of equilibrium. When all 
 
 parts of a body are balanced it is said to be in equilibrium. 
 Now when the support is at the center of gravity, as 
 shown in the middle disk of the figure, the body is said 
 to be in indifferent equilibrium, because it will rest as 
 well in one position as another. 
 
 When the support is placed above the center of gravity, 
 as in the disk at the right hand, the body is said to be in 
 stable equilibrium, because it will not rest as well in any 
 other position. 
 
 When the support is placed exactly below the center of 
 gravity, as in the disk at the left hand, the body is said to 
 t>e in unstable equilibrium, because the slightest force will 
 push it over. 
 
 That a body may stand, where must the line of 
 direction pass? If the line of direction passes through 
 any point in the base on which the body is placed, the 
 body will stand, but if this line passes outside of the base,
 
 ATTRACTION. 3 [ 
 
 the body must fall. The leaning cylinder in Fig. 4r does 
 not fall, because the line of direction passes through the 
 base, and hence the center of gravity is supported ; but if 
 it should lean a little more this line would pass outside 
 the base, and the cylinder would tip over. A table stands 
 very firm because it is not easy to tip it so far that the 
 line of direction would pass outside the base. 
 
 Carriages may lean considerably to one side without 
 overturning (Fig. 6), but an accident is sure to happen if 
 
 Fig. & 
 
 /hey lean so far as to throw the line of direction beyond 
 the lower side of the wheel. 
 
 Upon what does the stability of a body de- 
 pend ? Now some bodies stand more firmly than others, 
 and in looking for the reason we find that the stability 
 of a body depends upon two things. The first of these is, 
 the height of the center of gravity above the base. 
 
 A wagon loaded with hay overturns easily, while if 
 loaded with stone it would pass the same spot in the road 
 with perfect safety. The center of gravity of the load of 
 hay is so much higher, that to lean a little throws the line
 
 32 NATURAL PHILOSOPHY. 
 
 of direction beyond the wheel. The higher the center of 
 gravity of any body is, the more unstable will it be. 
 
 What else influences the stability of a body ? 
 The size of the base is the second thing that influences 
 the stability of a body. It will of course be more difficult 
 to tip the line of direction beyond a large base than be- 
 yond a small one. A narrow boat overturns more easily 
 than a wide one, or, to mention an example which you 
 may see at any time, a thick book will stand upon its end 
 more firmly than a thin one of the same height. 
 
 We see, then, that the lower the center of gravity and 
 the broader the base, the firmer will the body stand. 
 
 Mention illustrations of these principles. Illus- 
 trations of tliese principles of center of gravity are among 
 the most common affairs of life. Indeed, we unconsciously 
 apply them in almost every motion and position of our 
 own bodies. 
 
 When standing, the base upon which the body rests is 
 the space between the feet ; the center of gravity must be 
 
 Fig. 7- 
 
 ^=LV = 
 
 kept over this base or the person will fall. In carrying a 
 pail of water we unconsciously lean to the other side, and
 
 ATTRACTION. 33 
 
 if the load is very heavy we at the same time stretch ont 
 the opposite arm (Fig. 7). The pack-peddler leans for- 
 ward, for it he did not the heavy load would throw the 
 center of gravity behind his feet and he would tumble 
 backward. 
 
 What illustration does the showman furnish ? 
 The showman oifers a gold coin to the boy who will stand 
 with his heels pressed against the wall of a room and then, 
 pick it from the floor in front of him without falling. He 
 is perfectly safe in making the offer. For no one can 
 stoop without falling, unless when he throws his head 
 forward he can, at the same time, throw some other part 
 of his body backward far enough to keep his center of 
 gravity over his feet. He can not do this with his heels 
 pressed against a wall. 
 
 Why is a child so long learning to walk? When 
 we think how narrow the base is on which a child must 
 stand, being just the space on the floor between its little 
 feet, and then how high is the center of gravity of his 
 body, we need not wonder that he is so long a time in 
 learning to walk. The many falls and bruises which the 
 little one gets mark his failures in the art of supporting 
 the center of gravity always over the base. 
 
 How may we try the experiment ? Let one who 
 has forgotten how hard it was for him to learn to walk 
 refresh his memory by trying to walk on stilts. Skill in 
 this, like that of the child in walking, needs only the 
 power to keep the center of gravity of the body, every 
 moment, over some point in the base.
 
 LIQUIDS. 
 
 How do liquids differ from solids ? The molecules 
 of every solid substance are held together so that the 
 body will keep whatever form you may choose to give it; 
 but in water the molecules are held together with such 
 feeble force that they can move among themselves with 
 the greatest ea"se, and you can not give it any shape but 
 that of the vessel which holds it. 
 
 Water and other substances, in which cohesion is so 
 slight that the molecules move freely among themselves, 
 are called liquids. 
 
 Is there any cohesion in liquids ? Still the cohe- 
 sion in a liquid is strong enough to be detected. Look 
 
 again at Fig. 2, and notice 
 that the water would not be 
 lifted under the disk, as it 
 is there shown to be, unless 
 the particles of water cling 
 to each other. This shows 
 cohesion among them. The 
 drop of dew collected upon 
 a leaf (Fig. 8) shows cohe- 
 sion in water, for what 
 else could hold the parts of 
 the drop together ? 
 
 How can we judge of its strength ? To get a bet-
 
 LIQUIDS. 35 
 
 ter idea of the force of cohesion in water, we may watch 
 it dripping from some support. A drop grows larger 
 while clinging to its support, until at last it breaks away. 
 The weight of the drop just at the moment when it breaks 
 away is just enough to pull the molecules of water apart, 
 and measure* the cohesion in the liquid. The liquid in 
 which there is the greatest cohesion will give the largest 
 drops. 
 
 Is -water compressible ? A famous experiment was 
 made at Florence about a hundred years ago to find out 
 whether water could be compressed. A hollow globe of 
 gold was tilled with water, and then the opening sealed so 
 very tight that no water could pass it. An enormous 
 pressure was then put upon the globe, when, to the sur- 
 prise of all, the water oozed through the pores of the 
 metal. This experiment seemed to prove that water was 
 not compressible. But more careful experiments have 
 since shown that water is compressible. It is in so 
 slight a degree that the Florentine experiment was too 
 rude to show it at all. It needs a pressure of 15 Ibs. 
 upon every square inch of the surface of the vessel in 
 which the water is held to compress the fluid .0000503 of 
 its bulk. 
 
 Is water elastic ? The instant that the pressure is 
 removed from the compressed water it springs back to its 
 former bulk, and this proves it to be elastic. 
 
 What is more remarkable, it springs back with exactly 
 as much force as was exerted to compress it. * When com- 
 pressed only .0000503 of its bulk it will spring back with 
 a force of 15 Ibs. to a square ine\ Now when a body 
 springing back restores all the force that compressed it,
 
 NATURAL PHILOSOPHY. 
 
 Fig. 9. 
 
 it is paid to be perfectly elastic. Water and other liquids 
 are perfectly elastic. 
 
 What shows the downward pressure of water ? 
 
 The downward pressure of water is shown by its w r eight. 
 To lift a pailful of water you must overcome its down- 
 ward pressure. This may tax your strength severely, 
 because, if the pail holds one cubic foot of the liquid, you 
 must lift a weight of G2 Ibs. ; a cubic foot of water 
 weighs 62| Ibs. 
 
 Does water exert pressure upward? To learn 
 whether water presses upward as well as downward the 
 following experiment (Fig. 9) may be tried. A plate of 
 
 metal is hung from the 
 end of a string, which is 
 passed through a glass 
 tube open at both ends. 
 By means of this string 
 the plate of metal may be 
 held tightly against the 
 lower end of the tube. 
 Now if this end of the 
 tube is pushed down into 
 a vessel of water, the string 
 may be dropped and the 
 plate of metal will still 
 stay up against the glass. 
 By a moment's thought 
 you see that it must be 
 the water that holds the 
 heavy metal up, and that to do this it must exert an 
 upward pressure. 
 Does water exert pressure sidewise ? The same
 
 LIQUIDS. 
 
 37 
 
 experiment shows that water exerts a pressure sidewise, 
 for you may find that the water is gradually pushed side- 
 wise between the plate and the end of the tube, slowly 
 filling the tube with water. 
 
 In -what direction does -water exert pressure ? 
 In fact, water and other liquids, when at rest, exert press- 
 ure in all directions. And another fact should be remem- 
 bered ; it is, that the pressure at any point is equal in all 
 directions. If, for example, there is at any point a down- 
 ward pressure of 10 Ibs. there will be, at the same time, 
 a pressure of 10 Ibs. upward and sidewise, and indeed in. 
 every possible direction. 
 
 The pressure of water in several Fi? - 10 - 
 
 directions at once is very well 
 shown in Fig. 10. 
 
 Why is the surface of wa- 
 ter at rest always level ? It 
 is because water presses equally 
 in all directions that a body of 
 water can not be quiet unless its 
 upper surface is level. 
 Let us explain this more fully. 
 The molecules of water move so 
 easily that if the pressure in one 
 direction is never so little more 
 than in another, the liquid will 
 
 move. Now if the downward pressures at all points are 
 equal, all the other pressures must be equal too, and the 
 water will not move, but the downward pressures will not 
 be equal at all points unless the surface of the water is 
 level, and for this reason the water can not rest unless its 
 surface is level. 
 
 The wind may cover the surface of the sea with ripples 
 
 233560
 
 38 NATURAL PHILOSOPHY. 
 
 or lash it into billows ; but let the wind be hushed, and the 
 ripples or billows will gradually sink into a surface 
 smoother than that of the most polished mirror, just 
 because the pressure in all directions can not be made 
 equal without. 
 
 Will the shape of the vessel make any differ- 
 ence? No matter how irregular the form of a vessel 
 may be, all parts of the surface of the water in it roust be 
 at the same height, or in other words level. The vessel 
 shown in Fig. 11 has a very irregular shape. There is 
 
 Fig. 11. 
 
 first the large vase at the left hand, then the horizontal 
 tube, and finally the tubes reaching upward from the 
 last; yet it is all one vessel, because the water can pass 
 freely from one part to another. 
 
 If water is poured into the vase it will rise just as fast 
 in the tubes, and will at last stand at the same height in 
 all parts, as the picture shows. 
 
 How are cities supplied with water? It is on
 
 LIQUIDS. 39 
 
 this principle that many cities are supplied with water. 
 Water from the streams of the country around is led into 
 a reservoir where its surface will be higher than the city. 
 A large pipe is then laid under-ground, reaching from the 
 reservoir down to the city, and branches from it are laid 
 under the streets. From these main pipes -a branch goes 
 into each house which is to receive the water, and 
 reaches up to the room where the water is to be drawn. 
 Now the water will rise in these pipes as high as the 
 surface of that in the reservoir, if they will allow it to do 
 so, and, of course, if one be opened anywhere below that 
 level the water will flow from it. 
 
 How are fountains produced ? If the pipe which 
 is bringing water from a reservoir does not rise as high 
 as the reservoir, the water will spout upward in the form 
 of a fountain. In Fig. 11 one of the tubes is shorter 
 than the others, but the water rises almost as high as it 
 does in them : being thrown into the air instead of rising 
 in a pipe, we call it a fountain. 
 
 Upon what does the pressure of -water on the 
 bottom of the vessel which holds it depend? 
 Suppose the bottoms of two vessels are the same in size, 
 but that one vessel is twice as high as the other. When 
 both are filled with water it is found that there will be 
 just twice as much pressure on the bottom of the highest. 
 If one is three times as high as the other, the pressure on 
 its bottom will be three times as great. 
 
 The pressure upon the bottom of a vessel * of water is 
 always just in proportion to the height of the water. 
 
 Does not the shape of the vessel make a differ- 
 ence ? We may take vessels of very different shapes, but 
 if they are filled with water to the same height, arid if their 
 bottoms are of the same size, the pressure on the bottom
 
 40 NATURAL PHILOSOPHY. 
 
 will be the same in all. Suppose, for example, that each 
 vessel has a bottom whose surface is ten square inches : 
 one of them may be just as large at the top as at the 
 bottom, another may be larger at the top, and another 
 smaller ; but when they are filled to the same height with 
 water the pressure upon the bottoms will be alike in 
 all. 
 
 The pressure upon a bottom of given size depends 
 I , entirely upon the height of the water above it. 
 % How may a little water exert very great press- 
 ure ? We may now notice a curious fact, which seems 
 at first to be impossible. A very small quantity of water 
 may exert an enormous pressure. 
 
 Fig. 12 shows how this may be proved. In the first 
 place, a very tight cask is filled with water and a tall tube 
 is afterward screwed into the top. By filling this tube 
 with water the cask, unless uncommonly strong, will be 
 broken asunder. The very small quantity of water in 
 the tube, no more than a child could lift, exerts a pressure 
 strong enough to break the staves of the cask. 
 
 How can this be explained ? Suppose the end of 
 the tube is -^ of a square inch, and that the tube is high 
 enough to hold a pound of water. The pressure on -gL- of 
 an inch would be one pound, and on a whole inch it 
 w r ould be 50 Ibs. And since water presses equally in all 
 directions there would be 50 Ibs. pressure on every square 
 inch of the inside surface of the cask. Such a pressure is 
 more than the cask can bear. 
 
 Would any other equal pressure have the same 
 effect? Any other pressure equal to the weight of the 
 column of water in the tube would have the same effect. 
 The pressure of your hand, or of a pound-weight of metal, 
 might take the place of the pound of water in the tube ;
 
 41
 
 42 NATURAL PHILOSOPHY. 
 
 the pressure, exerted in any way, would be transmitted 
 equally in all directions and break the cask. 
 
 Show how a light weight may balance a heavy 
 one. Now suppose two cylinders, one just twice as large 
 as the other, to be joined together by a tube at their bot- 
 toms (Fig. 13), and let there be a piston fitting each cylin- 
 Fig. is- der exactly, and carrying a table 
 
 as the picture shows them. Now 
 if a one-pound weight be put upon 
 the small table it will balance a 
 two-pound weight upon the other. 
 If one cylinder were one hun- 
 dred times larger than the other, 
 then one pound on the small table 
 would balance one hundred pounds 
 on the large one. 
 What machine acts on this principle ? The hy- 
 drostatic press is made to act on this principle. The 
 piston in the small cylinder is pushed down by hand, or 
 perhaps by a steam-engine, while any thing to be pressed 
 is put between the large table and a solid pressure-plate 
 built above it. 
 
 This machine is used for pressing hay and cotton into 
 bales, for testing the strength of ropes, and, in a word, it 
 is preferred to any other machine whenever a great press- 
 ure is to be exerted.
 
 GASES. 
 
 How do gases differ from liquids ? In water and 
 in other liquids there is a slight degree of cohesion, but in 
 air and other gjises there is no cohesion at all. The mole- 
 cules of air are trying to get just as far away from each 
 other as possible at all times ; and this is true also of all 
 bodies in the form of air, or, as they are called, gases. 
 
 Air is the most common of all gases, and on this account 
 it is used to illustrate the properties of this class of bodies. 
 
 Is the air expansible ? An easy and pretty experi- 
 ment will teach us whether air can be expanded. Take a 
 small vial having in it a little colored water, and fasten 
 into its neck an air-tight cork, through which a small tube 
 just reaches into the bottle. This tube should be several 
 inches long. If the vial be held bottom upward the col- 
 ored water will riot run into the tube, but if the lips be 
 applied to the lower end of the tube, and the air be drawn 
 out, the colored water will quickly run down. This shows 
 that the air above the water in the vial expands to push 
 the water out. 
 
 In what other way is the air of the vial ex- 
 panded ? If, instead of taking the air out of the tube, 
 you gently warm the vial, you will see the colored liquid 
 move out of the vial and down the tube. In this experi- 
 ment the air is expanded by heat.
 
 44: NATURAL PHILOSOPHY. 
 
 Boys sometimes amuse themselves by bursting balloons 
 or bladders tilled with air by warming them. They thus 
 illustrate the expansibility of air, for the air when heated 
 tries to till more room than it did when cold, and in trying 
 
 f\s. 14. to get larger bursts the balloon with a loud re- 
 port like a gun. 
 
 Is air compressible ? If we take a cylinder 
 with a piston fitting it air-tight (Fig. 14), we may 
 easily push the piston down some distance into 
 the cylinder. No air gets out, but the piston, 
 while going down, crowds the air along before it 
 until the cylinder may be less than half full. By 
 greater force than can be given by the hand 
 alone the piston may be crowded down until the 
 cylinder may be less than a hundredth or a thous- 
 andth part full. 
 
 Is air elastic? Compressed air will spring 
 back to its original bulk when the pressure is 
 taken away, and this shows that it is elastic. 
 
 It is also found that air after being compressed 
 will spring back with just as much force as was 
 put upon it ; this shows it to be perfectly elastic. 
 
 Does air have weight ? A thin globe made 
 of glass or metal is weighed when full of air 
 (Fig. 15). The air is then taken out of it by 
 means of an air-pump, soon to be described, and 
 the empty globe is weighed. The globe weighs more 
 when full of air than when empty, and this proves that 
 air has weight. 
 
 If the globe will hold 100 cubic inches of air, it will
 
 CASES. 
 
 4:5 
 
 weigh about 31 grains less when empty, and this shows 
 
 that 100 cubic inches weigh about 31 grains. 
 
 Does gravitation act upon air ? The weight of air, 
 
 like the weight of wood or Fig. is 
 
 iron, is caused by the at- 
 traction of gravitation. 
 
 Gravitation acts upon the 
 
 invisible air in just the 
 
 same way that it does upon 
 
 water or upon oil, only its 
 
 action is not so strong. 
 Try this experiment : into 
 
 a tall glass jar (Fig. 16) or 
 
 even a goblet put first some 
 
 water, and then pour in 
 
 some oil ; the oil will lie on top of the water. Afterward, 
 
 if you can have some mercury you will be able to pour it 
 Fig. 16. into the jar carefully without 
 
 disturbing the other liquids. 
 The mercury will go to the 
 bottom and form a layer un- 
 der the others. 
 
 Now there are four sub- 
 stances in the jar, arranged 
 in layers. There is first mer- 
 curj r , then water, then oil, and 
 then air. And they are in 
 this order because gravita- 
 tion is strongest on mercury, 
 weaker on water, weaker yet 
 on oil, and weakest on air. 
 
 For a similar reason the water of the sea is above the 
 
 rocks and then the atmosphere above the water ; but if grav-
 
 4t> NATURAL PHILOSOPHY. 
 
 itation did not act upon air at all, the atmosphere would 
 leave the earth entirely and fly off into space beyond. 
 
 In -what directions does air exert pressure ? The 
 pressure of the air may be shown in a very simple way. 
 Cork one end of a lamp-chimney, and stretch a piece of 
 caoutchouc over the other. Put a piece of pipe-stem 
 tightly through the cork, and the apparatus is finished. 
 .Now with the lips at the pipe-stem, take the air out of the 
 chimney and you will see the caoutchouc pushed into it. 
 There is nothing outside to push it into the tube but the 
 air, and so the experiment shows the pressure of the air. 
 
 Now hold the tube upward or downward, or in any 
 direction whatever, and the caoutchouc will be pressed in 
 as before ; and hence we see that the air presses in all 
 directions. 
 
 It is also found by experiment that the pressure of air 
 in all directions is equal. In this respect air and water 
 are alike. 
 
 The finest illustrations of the pressure of the air may 
 be given by means of the air-pump. 
 
 Describe the air-pump. In this instrument there is 
 a cylinder (C, Fig. 17), with a tube leading from the bot- 
 tom of it. The other end of this tube is bent upward so 
 as to pass through a horizontal plate of metal, P. At the 
 end of this tube, in the cylinder, there is a little door or 
 valve, as it is called, which opens upward, and will open 
 for air to go up, but will shut when the air tries to go 
 down again. In the cylinder there is a piston, and in this 
 there is another valve which opens upward. The plate, 
 P, is so smooth that a glass vessel, R, open at the bottom, 
 will stand upon it and fit so closely that no air can pass 
 between them. This vessel, or any other from which air 
 is to be taken, is called a receiver.
 
 GASES. 
 
 Explain the action of the pump. When the piston 
 is lifted the air in the receiver, R, will expand, and a part 
 
 Fig. IT. 
 
 of it will go through the valve at the bottom of the cylin- 
 der. When the piston is pushed down again, the air in 
 the cylinder will push its way through the valve in the 
 piston. When the piston is lifted again, the air above it 
 is lifted out of the instrument entirely, while another part 
 of the air in the receiver comes through the valve into the 
 cylinder. And in this way every upward motion of the 
 piston pumps a part of the air out from the receiver. 
 
 The air can be so nearly pumped out, or exhausted, as 
 it is usually called, that there will not be enough left to 
 lift the very delicate valves of the instrument. 
 
 How -will the receiver show the pressure of the 
 air ? After the air has been pumped out of the receiver, 
 it will be found impossible to lift it away from the pump- 
 plate. The outside air presses so heavily upon it that if 
 the receiver is lifted the pump will rise with it.
 
 8 NATURAL PHILOSOPHY. 
 
 How may the pressure be shown by the Magde- 
 burg cups? Fig. IS shows the Magdeburg cups. These 
 Fig. i& cups are made of metal, and their edges 
 
 are so smooth that they will fit each 
 other air tight. The lower one may be 
 screwed upon the pump-plate, the other 
 then placed upon it and the air taken 
 out. They may then be removed from 
 the pump without letting the air get into 
 them, and when this is done, the pressure 
 of the outside air will hold them together 
 with great force. The illustrious Otto 
 de Guericke of Magdeburg, who invented 
 the air-pump, and to whom we also owe 
 the invention of these cups, made a pair so 
 large that it needed the strength of four horses to pull 
 them apart. 
 
 How may the pressure be shown by the fountain 
 in vacuo ? The pressure of air is shown by a still more 
 beautiful experiment, represented in Fig. 19. A tall 
 glass receiver, R, made air-tight, has a tube passing 
 through the bottom. The lower end of this tube may 
 be screwed upon the plate of the air-pump : the other 
 end reaches some distance into the vessel. After the air 
 is exhausted from the receivers, if the lower end of the 
 tube is placed in water and the valve opened, an elegant 
 fountain will be thrown up inside by the pressure of the 
 air upon the water outside. 
 
 How may the upward pressure of air be shown ? 
 We do not need an air-pump to show the upward 
 pressure of air. Just take a common bottle and fill it to 
 the brim with water ; then place a piece of paper over its 
 mouth, and while you hold the paper with one hand, turn
 
 GASES. 49 
 
 the bottle bottom upward with the other. You may now 
 let go of the paper and the water will not run out of the 
 
 Fig. 19. 
 
 bottle. The water and the paper are both held up by the 
 upward pressure of the air. If the neck of the bottle is 
 very small, the paper need not be used. 
 
 How may the downward pressure be easily 
 shown ? Take a tall bottle with a wide mouth, and sink 
 it in a vessel of water, and when full of the liquid lift it 
 gradually with its bottom upward until its neck only is 
 covered. The bottle will still be full of water. The 
 pressure of the air on the water in the vessel pushes the 
 liquid up into the bottle and holds it there. 
 
 It is the pressure of air which also drives water through
 
 50 
 
 NATURAL PHILOSOPHY. 
 
 a glass tube or a straw when, one end being in the liquid, 
 the lips are applied to the other end. All that the lips 
 do is to take the air out of the tube above the water. 
 
 How high a column of water will the pressure 
 of the air sustain ? The water will be sustained to a 
 
 height of about 34 feet 
 by this pressure of the air. 
 How high a column 
 of mercury? A col- 
 umn of mercury about 
 30 inches high will weigh 
 as much as a column of 
 water 34 feet high if they 
 are of the same size. 
 Therefore, a column of 
 mercury only 30 inches 
 high will be supported 
 by the pressure of the air 
 Can we prove this 
 by experiment ? The 
 pictures show how this 
 can be easily proved by 
 experiment. A glass tube 
 more than 30 inches 
 long is taken. One 
 end of it is closed, the 
 other open. It is first 
 filled with mercury, and 
 then the open end is 
 shut with the finger, 
 while the tube is turned, closed end upward, as shown in 
 Fig. 20. The open end is then put into a dish of mer- 
 cury nnd the finger taken away. The mercury will no 
 

 
 GASES. 
 
 51 
 
 longer quite fill the tube, but will sink so as to leave the 
 upper part empty, as shown in Fig. 21. The air-press- 
 ure supports this column of mercury, and the height 
 measures about 30 inches. 
 
 How much pressure does the atmosphere exert 
 upon a square inch ? A column of mercury one inch 
 square and thirty inches high will weigh 15 Ibs. To 
 balance 15 Ibs. of mercury in the tube, or to keep it from 
 F! s- 21. falling out, would need 
 
 another pressure of 15 
 Ibs., and since the air 
 does this we know that it 
 must be exerting a press- 
 ure of 15 pounds to the 
 square inch. 
 
 Air is lighter than the 
 lightest down, but reach- 
 ing above us to a height 
 of many miles, the quan- 
 tity in all must be im- 
 mense. It covers every 
 thing upon the earth, and 
 presses upon each square 
 inch of the surface of 
 every thing with a force 
 of 15 Ibs. The total 
 pressure upon each oiie 
 of our bodies is said to be 
 about 20,000 Ibs. We 
 are able to bear this great pressure without feeling it, 
 because it is so exactly equal in all directions, and because 
 every point on the body, bearing its own share, divides 
 the labor so justly that no point is burdened.
 
 52 NATURAL PHILOSOPHY. 
 
 Is the pressure of the atmosphere always alike ? 
 
 By watching the column of mercury in the tube, Fig. 
 21, it will be found to be higher at some times than at 
 others. This must be because the pressure of the atmos- 
 phere is not always the same. It is sometimes a little 
 more than 15 Ibs. to the inch and sometimes a little 
 less. 
 
 How does the pressure vary with the weather ? 
 In bright, clear weather the atmosphere is heavier than 
 when storms or clouds prevail. Hence the column of 
 mercury will be higher in fair weather than in foul 
 weather; and by watching the changes in the height of 
 the mercury column \ve may judge something of what the 
 character of the weather is to be. 
 
 How does the pressure vary with the height ? 
 At the level of the sea the column of mercury stands 30 
 inches high. A gentleman by the name of Pascal, in 
 France, with a tube and cistern of mercury, traveled up a 
 mountain-side to find what effect, if any, would be produced 
 upon the column. As he climbed the mountain higher 
 and higher, he found that the mercury sank lower and 
 lower in the tube. We learn from his experiment that the 
 pressure of the atmosphere is less as the height is greater. 
 
 May we not know this without experiment ? 
 Indeed, no experiment is needed to prove this, for it is 
 very clear that when going up we leave a part of the 
 atmosphere below us, and there being then less above us, 
 the pressure exerted mu^t be less. 
 
 What is the Barometer? Now the barometer is an 
 instrument which shows the changes in the pressure of 
 the atmosphere. It consists of a tube with a cistern of 
 mercury, like that shown in Fig. 21, placed in a frame- 
 work of wood or metal which protects it from injury. A
 
 GASES. 
 
 53 
 
 scale placed behind the tube shows the height of the 
 mercury column. 
 
 How does this instrument foretell changes in 
 the weather ? By the motion of the mercury up or 
 down we may judge something of the future character of 
 the weather. If we see the mercury rising, we may expect 
 fair weather : if the mercury falls, we may expect foul 
 weather. 
 
 How is it used to find the height of mountains ? 
 From the sea-level to the top of Mont Blanc is a height 
 of about 15,000 feet. In going to 
 the summit, the mercury column 
 falls about 15 inches. From this, 
 and other observations like this, 
 it appears that the mercury sinks 
 about one inch for every thousand 
 feet the barometer is taken upward. 
 To get the height of a mountain, 
 then, we may count 1,000 feet for 
 every inch through which the mer- 
 cury falls in being carried to the 
 summit. 
 
 This rule will not give the exact 
 height, owing to things which we 
 will not now try to explain, but 
 when all things are taken into 
 account in making the calculations, 
 the height of a mountain may be 
 calculated by the barometer per- 
 haps more easily and exactly than | 
 by any other method. 
 
 Describe the lifting-pump. In this very common 
 and useful instrument the pressure of the atmosphere ia 
 
 I c
 
 NATURAL PHILOSOPHY. 
 
 made to lift water from a well or cistern. Its parts and 
 their action are very w r ell shown in Fig. 22. It consists 
 of two cylinders, one above the other, the upper one, 6 Y , 
 being often much larger than the other, c, which reaches 
 down into the well. Where these cylinders join each 
 other there is a little door, or valve, which opens upward, 
 and will allow water to go up, but will not let it go 
 down again. In the upper cylinder there is a piston 
 which may be lifted or pushed down by the handle of 
 the purnp. In this piston there is a valve, or, it may be, 
 more than one, which opens upward. 
 
 Explain the action of the pump. By the first 
 strokes of the piston the air is taken out of the cylinders, 
 and then the pressure of the atmos- 
 phere upon the water in the well 
 pushes the water up into the pump 
 just as it will push water up into a 
 straw or other tube when the air is 
 drawn out at the top by applying 
 the lips. After the air has been all 
 taken out, the water will till the 
 pump full to the spout, and then 
 every time the piston is raised it will 
 lift a portion of water out at the 
 spout, while more is pushed in at the 
 bottom by the air to take its place. 
 
 Describe the force-pump. Fig. 
 23 shows a kind of pump which is 
 g_ used for throwing water to greater 
 
 heights. It is called t\\Q force-pump. 
 
 The spout is at the bo; torn of the 
 upper cylinder instead of near the top of it, as in the lift- 
 ing-pump, and instead of having any valve in the piston
 
 GASES. 
 
 there is one opening into the spout. In other respects it 
 is like the lifting-pump. 
 
 Explain its action. When the piston is lifted the 
 atmosphere pushes water up into the upper cylinder. 
 When the piston is pushed down the water is pushed 
 through the valve into the spout. This spout may reach 
 even to the top of a house, and the water will go higher 
 by each stroke of the piston until it reaches the top and 
 runs over. A jet of water would be thrown out by each 
 downward stroke of the piston, but if a steady stream, is 
 wanted, the spout leads into an air-chamber where the 
 water condenses the air. This condensed air exerts a 
 steady pressure on the water in the chamber and throws 
 it out in a steady stream. 
 
 The fire-engine is a form of force-pump. In the steam 
 fire-engine the piston is moved by the power of steam. 
 
 What is the siphon? In Fig. 24, the tub* from 
 which the water appears to be running is a siphon. The 
 instrument is used to pass a 
 liquid from one vessel to an- 
 other. The siphon is never 
 any thing more than a bent 
 tube, one arm being longer 
 than the other. 
 
 How is it used? When 
 the siphon is to be used, it 
 must first be tilled with water, 
 and then the end of the long 
 arm is closed with the finger, 
 while the short arm is put into 
 the liquid in the vessel. The 
 moment the finger is removed, the liquid will begin to 
 flow up the short arm, over the bend and out at the end
 
 56 NATURAL PHILOSOPHY. 
 
 of the long arm ; nor will it stop running until the end 
 of the short arm is uncovered, or until the liquid is as 
 high in the second vessel as in the one from which it runs. 
 
 Explain its action. It often at first seems a mystery 
 why the liquid should run upward through the short arm 
 and out from the other ; but we shall see that the forces 
 that push it in that direction are stronger than those that 
 push the other way. 
 
 To push it out through the long arm there is, first, 
 the weight of the liquid in that arm, and, second, the 
 pressure of the air on the water in the vessel. 
 
 To keep it in, there is, first, the weight of the water in 
 the short arm, and, second, the pressure of the air upward 
 against the water at the end of the long arm. 
 
 Now the first tvvo forces are stronger than the last two, 
 because the weight of the water in the long arm is greater 
 than of that in the short arm ; and the water runs in 
 the direction of the greater force.
 
 MOTION. 
 
 Can bodies move themselves? Animals can 
 move from place to place, because they have the power 
 of will, and their bodies must obey it ; but that a book or 
 a block of stone should of its own accord move out of its 
 place, we pronounce to be impossible. Such bodies of 
 matter can move only as they are either pushed or pulled. 
 The ship, for example, is pushed along by wind : a train 
 of cars is pulled along by a steam-engine. A stone falls 
 to the ground because it is pulled down by the attraction 
 of the earth, and the smoke rises because it is pushed up- 
 ward by the heavier air. 
 
 A body at rest would rest forever if it were neither 
 pushed nor pulled by some force outside of itself. 
 
 Can bodies stop themselves ? A moving ship will 
 not stop suddenly when the sails are taken down. The 
 water finally stops it ; no one ever thinks of saying that 
 tho ship stops itself. A train of cars moves along some dis- 
 tance after the steam has been cut off: it is finally stopped 
 by the friction of the wheels as they rub upon their axles 
 and upon the track, together with other obstacles which 
 it meets ; it does not stop itself. A horse and his rider 
 are moving along together ; let the horse suddenly stop, 
 and his rider is plunged over his head ; the man can not 
 stop himself. 
 
 We learn from these observations that a. body in motion 
 
 r
 
 58 NATURAL PHILOSOPHY. 
 
 would move forever if it were not stopped by some force 
 outside of itself. 
 
 Can a body change the direction of its mo- 
 tion ? When a ball is struck with a bat, it flies in the 
 direction of the blow, and in no other, unless turned 
 aside by some other force. 
 
 The same thing appears to be true of all other motions, 
 for who ever saw a moving body suddenly of its own ac- 
 cord start off in another direction ! A body in motion 
 would move forever in a straight line unless turned aside 
 by some force outside itself. 
 
 What is the first law of motion ? We may now 
 state, in very few words, what we have thus far learned 
 about motion, as follows : 
 
 A body at rest would rest forever, or if in motion would 
 move forever in a straight line, unless kept from doing so 
 by some force outside of itself. 
 
 This principle is called the first law of motion. 
 
 Why then does not a stone move in a straight 
 line when thrown from the hand? When a stone 
 is thrown in a horizontal direction, we find that instead 
 of going along in that direction, it very soon flies lower 
 and lower, until at length it strikes the ground. It would 
 move in a straight line if the attraction of the earth did 
 not pull it to the ground. 
 
 Are other motions caused by more than one 
 force? If the wind blows while the rain falls we see 
 the drops coming obliquely down to the ground. Gravi- 
 tation alone would bring them vertically through the air, 
 but the wind at the same time pushes them sidewise ; 
 their motion is due to these two forces. 
 
 Now, the more examples of motion we examine, the 
 more certain we become that the motions of bodies are
 
 MOTION. 50 
 
 generally caused by two or more forces acting upon them 
 at the same time. 
 
 Does each force produce as much effect as if it 
 acted alone ? Here is an experiment that any one can 
 try for himself while studying this subject. Put a ball at 
 one corner of a table. You may snap it with the fingers of 
 one hand and make it roll along the side of the table, and 
 if you afterward snap it with the fingers of the other 
 hand you may make it roll across the end; but if you 
 skillfully snap it with the fingers of both hands at once, it 
 will follow neither the side nor the end, but you will see 
 it dari obliquely across to the opposite corner. 
 
 If one hand would roll the ball the whole length of the 
 table in one second, and if the other, would roll it across 
 the end in one second, then, when both hands were used 
 at once, the ball will roll to the opposite corner in exactly 
 one second. But to get to this opposite corner, the ball 
 must go the whole length and the whole width of the table 
 both at once; so that each force causes just as much mo- 
 tion in the second of time as if it were acting alone. 
 
 Give another example. The swift motion of a can- 
 non-ball is caused by the explosion of gunpowder, but 
 gravitation is at the same time pulling the ball down 
 toward the ground. If the ball is shot in a horizontal 
 direction it will strike the ground at the 'same time it 
 would if dropped from the mouth of the gun. The force 
 of gravitation pulls the ball downward through the same 
 distance while it is moving horizontally as it would in the 
 same time if falling vertically. 
 
 Suppose the ball shot directly upward. A ball 
 will fall about 16 feet in the first second after it starts. 
 Now suppose a ball shot directly upward, and that the 
 force of the powder would be strong enough to send it up 

 
 00 NATURAL PHILOSOPHY. 
 
 100 feet in the first second : the ball will only rise 84 feet. 
 The attraction of gravitation which would make it fall 16 
 feet if it were not for the powder, will cut off just 16 feet 
 from its ascent. 
 
 What is the second law of motion? From such 
 facts as these we infer that : 
 
 A force will cause the same amount of motion, and in 
 1he same direction, whether the body it acts upon be at rest 
 or already in motion. 
 
 This principle is called the second law of motion. 
 
 What is meant by action and reaction ? He who 
 strikes the table with his hand gets a blow from the table 
 in return, as he very well knows by the pain it occasions 
 when the blow is hetivy. The hand acts upon the table 
 and the table reacts upon the hand. Whenever two 
 bodies act upon each other, the effect of one of them is 
 called action, that of the other is called reaction. 
 
 A bullet may perhaps fracture a stone against which it 
 is fired, but the bullet will be flattened, showing that the 
 stone has returned the blow. The bullet acts upon the 
 stone and the stone reacts upon the bullet. 
 
 Are action and reaction in the same direction ? 
 When a book lies upon the table it presses downward, but 
 the table is at the same time pressing upward to keep the 
 book from going to the floor. Action and reaction must 
 always be in opposite directions. 
 
 Which is the stronger ? Take the case of the book 
 on the table : the action of the book downward, or its 
 pressure, is just equal to its weight. If the table should 
 react with a force (/reater than the weight of the book, it 
 would throw the book upward ; if with less force, the 
 book would be able to break through it : it can be 
 neither greater nor less, because the book is at rest. In
 
 MOTION. 61 
 
 every other case as well as this, action and reaction are 
 equal. 
 
 What is the third law of motion? From what has 
 just now been said we may gather this general statement: 
 
 Every action must be followed ly an opposite and 
 equal reaction. 
 
 This principle is called the third law of motion. 
 
 What is an impulsive force? When a bullet is 
 shot from a gun, the force of the gunpowder acts upon it 
 only for a single moment when it starts. A force which 
 acts for a moment only is called an impulsive force. 
 
 Other examples are common enough. When a ball is 
 hit with a bat, the force of the blow is spent upon the ball 
 in an instant. And at the moment when a stone leaves 
 the hand that throws it, the force of the hand is spent. 
 Both of these are impulsive forces. 
 
 What do we notice about the motion caused by 
 impulsive forces ? We notice that the motions produced 
 by these impulsive forces are all alike in one thing at 
 least: the velocity is greatest at the beginning. The 
 speed of the bullet is greatest at the moment when it 
 leaves the gun, and grows less and less until it is stopped 
 entirely. And in the case of the ball struck with a bat, 
 and of the stone thrown from the hand, motion is swiftest 
 at the beginning and gradually grows slower. 
 
 Why do these motions grow slower ? It would 
 not be so if it were not for the resistance of the air. The 
 resistance of the air which hinders the snow-flakes so that 
 they can not fall like rain-drops or hail-stones, also hinders 
 the motion of every thing else. 
 
 Cut a leaf of paper into pieces an inch in length and 
 half as wide ; toss them upward into the quiet air of the 
 room aniL^vvatch their slow and curious motions to the 
 
 and wa
 
 32 NATURAL PHriX)SOPHY. 
 
 floor : if it were not for the air which hinders them they 
 would fall like bullets. 
 
 The air hinders the motion of heavy bodies, and the 
 faster they move the more it will affect it. Even the 
 motion of cannon-balls is rapidly lessened by the resist- 
 ance of the air. 
 
 Now if there were no air nor other resistances the mo- 
 tion caused by an impulsive force would not grow less. 
 The moving body would pass over equal distances in equal 
 times', or in other words, its motion would be uniform. 
 
 What is a constant force ? The force of the hand 
 which tosses a stone upward into the air is an impulsive 
 force, but gravitation which brings it down again is not. 
 Never for a single instant does gravitation cease to act 
 upon the falling stone, and on this account it is called a 
 constant force. A constant force is one whose action is 
 all the time alike. 
 
 What do we notice about the motion it pro- 
 duces? The motion of a falling body is swifter and 
 swifter the farther it falls. This is true not only of mo- 
 tion caused by gravitation, but of motion caused by any 
 constant force whatever ; the velocity increases while the 
 force is acting. 
 
 How do the air and other resistances affect 
 this motion? In this case also the resistance of the air 
 hinders the motion, and it hinders it more and more as the 
 velocity is greater. In fact, the motion of a body falling 
 from a great height may become so swift that the resistance 
 of the air will be as strong as the force of gravitation itself, 
 and after that moment the motion will be uniform. 
 
 It is just so with a train of cars. The power of the 
 steam starts it and for a little time makes it go faster and 
 faster, but the motion soon becomes uniform because the 
 
 i) been vi so
 
 MOTION. 63 
 
 many resistances which the train meets soon equals the 
 power of the steam. 
 
 The motion ot a sail-boat increases at first, luit very 
 soon the resistance of the water becomes so strong that it 
 needs the whole force of the wind to overcome it, and after 
 that the boat sails on at a uniform rate. 
 
 In -what kind of a path will an arrow go ? If an 
 arrow is shot from the bow directly upward, it will go up in 
 a straight line and its motion back again will also be in a 
 straight line. If, however, the arrow be fired in any other 
 direction its path will be a curve instead of being straight. 
 
 Why will the path of the arrow be curved? It 
 is easy to see that there are two forces acting on the arrow. 
 There is, first, the force of the bow which sends it forward, 
 and then, second, the force of gravitation which pnlls it 
 toward the ground. The bow would send it in a straight 
 line, but gravitation is all the time pulling it down out of 
 that line. The arrow obeys both of these forces at once, 
 going forward and downward at the same time, its direc- 
 tion changing a little all the time. For this reason the 
 path of the arrow is curved. 
 
 In any case, if a body moves in a curved path it is being 
 acted on by two forces, and one of these, at least, must 
 constantly act. 
 
 Give another example. We may make an easy ex- 
 periment to illustrate this statement more fully. Tie a 
 string to the stem of an apple and make the apple swing 
 around the hand in a circle. You can feel the apple 
 pulling as if struggling to get away from the hand, and 
 should you let go your hold of the string the apple would 
 dart off in a straight line in just whatever direction it 
 happened at the moment to be going. 
 
 What two forces make the apple move in the
 
 64 NATURAL PHILOSOPHY. 
 
 circle? There is one force, we notice, which is trying to 
 move the apple in a straight line, and the string is 
 another force which is pulling it out of that line every 
 moment, 
 
 By these two forces acting together the apple is moved 
 in a circle. 
 
 What are the centrifugal and centripetal forces ? 
 Now one of the two forces by which curved motion is pro- 
 duced has been called the centrifugal force, and the other 
 the centripetal force. The one which would send the 
 body away in a straight line is the centrifugal force ; that 
 which pulls it out of the straight line is the centripetal 
 force. 
 
 What familiar examples of the action of these 
 forces? The stone in a sling, at the moment when it is 
 set at liberty, darts off in a line as straight as the path of 
 an arrow or a bullet : but before it is set at liberty, the 
 sling-cord pulls it out of that line and keeps it moving in 
 a circle. 
 
 A wet mop, made to turn swiftly on its handle as an 
 axis, throws the water in all directions and soon dries 
 itself. It is the centrifugal force which sends the water 
 away. And this illustrates what we sometimes see among 
 animals. Sheep, for example, in wet weather throw the 
 water off themselves by shaking their fleeces in a kind of 
 half rot^ary motion. Water-dogs on coming to land dry 
 themselves in the same way. 
 
 "A loaded stage-coach running south and turning sud- 
 denly to the east or west, strews its passengers on the 
 south side of the road. A man on horseback when turn- 
 ing a corner leans much toward the corner in order to 
 overcome the centrifugal force which would throw him 
 away from it."
 
 MOTION. 65 
 
 A carriage-wheel turning swiftly often throws the dirt 
 in straight lines from its circumference. In the same 
 way, were it not for the attraction of gravitation, all 
 bodies on the face of the earth would be thrown out into 
 the heavens by the centrifugal force due to the rotation 
 of the eartli upon its axis. 
 
 The earth is moving in an orbit which is almost a cir- 
 cle, the diameter of which is about 190,000,000 miles, and 
 it is going at the rate of about 68,000 miles an hour. At 
 every moment during this wonderful journey the earth is 
 struggling to fly away in a straight line, but the powerful 
 attraction of the sun is the strong arm which constantly 
 pulls it out of this line into the graceful curve through 
 which it flies.
 
 VIBRATIONS. 
 
 Describe the pendulum. In Fig. 25 we notice a 
 ball B hung from a fixed point A by means of a cord. 
 This ball represents a pendulum : 
 any body hung from a fixed point, 
 under which it may swing from 
 side to side, backward and for- 
 ward, is a pendulum. 
 
 If such a bull were pulled aside 
 and then dropped !t would swing 
 for a long time. You can easily 
 try the experiment by hanging an 
 '- apple in the same way and mak- 
 ing it swing. 
 
 What is meant by vibra- 
 tion and amplitude ? If the pendulum (Fi>. 25) h 
 lifted to C it will swing to D, a point almost as far on the 
 other side, and then return. It will keep on moving back 
 and forth in tin's arc until the resistance of the air finally 
 stops it at B, the place from which it first started. Its 
 motifni from one end of its arc tj the other is called a 
 vibration, and the distance from one end of its arc to the 
 other is called its amplitude. 
 
 Does the time of one vibration depend upon 
 amplitude? The distance through which the pendulum 
 swings makes very little difference in the time it takes to
 
 VIBRATIONS 
 
 67 
 
 pass through it ; in other words, a pendulum will swing 
 through a long arc just as quickly as through a short one. 
 The reason that the long journey is made in the same 
 time as the short one is this : the longer the arc the steeper 
 are its ends, and on this account the swifter the pendulum 
 will fall. 
 
 Does the time of one vibration depend upon the 
 weight of the ball ? It is another curious property of 
 the pendulum that whether it be made of lead or of wood 
 or of other material, it will make its vibration in the 
 same time. Its weight, and we may add, the material of 
 which it is made, makes no difference in the time of one 
 vibration. 
 
 Does the time of one vibration depend upon the 
 size of the pendulum ? Nor does the size of the pen- 
 dulum make any difference in the time of one vibration. 
 Of course the resistance of the air will be 
 more on a large ball than on a small one, 
 and on that account a large pendulum 
 will not continue to vibrate as long as a 
 small one, but they will swing from one 
 end of the arc to the other in the same 
 time, while the motion does continue, no 
 matter how much they may differ in size. 
 
 Upon what does the time of one 
 vibration depend? But if we take 
 pendulums of different lengths, as shown 
 in Fig. 2G, we shall find the longest one 
 vibrating slowest. In all cases the long- 
 est pendulum needs the longest time to 
 make one vibration. The time of one 
 vibration depends altogether upon the 
 length of the pendulum. 
 
 Fig. 26.
 
 jgg NATURAL PHILOSOPHY. 
 
 What is the law? If the pendulum P (Fig. 26) is 
 just/bwr times as long as another, P', we shall tind by 
 trying the experiment that it takes just twice the time it 
 does the other to make a vibration. We notice that : 
 
 The time of one miration is in proportif/i to the (square 
 root of the Length of the pendulum. 
 
 If then one pendulum is 9 times the length of another, 
 it will take it 3 times as long to vibrate once. 
 
 The length of a pendulum that' will vibrate in one 
 second is about 39.1 inches: a pendulum \ of that length 
 would vibrate in a second according to the law, since 
 the square root of \ is \. 
 
 How is the pendulum used to measure time ? 
 Now if we know the time it takes a pendulum to make 
 one vibration, we may measure any length of time by 
 simply counting the number made. The resistance of air 
 would however soon stop the swinging, and, even if it did 
 not, the counting of the vibrations would be a tedious 
 task. Ingenious men have overcome these difficulties by 
 inventing the clock, by which people are everywhere able 
 to measure time. 
 
 Briefly describe its action. In this instrument a 
 weight or a spring keeps a set of wheels in motion, and 
 these wheels keep the pendulum vibrating and at the 
 same time register the number of vibrations it makes, by 
 making an index or hand point to the divisions of a grad- 
 uated circle. 
 
 Explain its action more fully. In looking at the 
 interior of a common clock, which is the best and per- 
 haps the only way any one can clearly learn its action, 
 we find that a pendulum is so connected with a toothed 
 wheel that at the end of every two vibrations it allows 
 one tooth to escape. If the pendulum vibrates twice a
 
 VIBRATIONS. 69 
 
 second it allows one tooth to escape at the end of each 
 second, and if there are sixty teeth on the wheel it will 
 turn around just once in sixty seconds or a minute. To 
 the axis of this wheel thg second-hand of the clock is 
 fixed. 
 
 This wheel is connected with another that turns once 
 around in an hour, and to the axis of this one the minute- 
 hand is fastened. 
 
 There is still another wheel in the set which can turn 
 once around in twelve hours, and to tho axis of this the 
 hour-hand is fastened. 
 
 How does a watch differ from a clock ? A watch 
 differs from a clock in having a vibrating wheel, called 
 the balance-wheel, instead of a pendulum. The vibration 
 of the balance-wheel allows one tooth of a wheel to pass 
 just as the pendulum does in the clock, and the number 
 of beats is recorded in the same way. 
 
 What are chronometers ? Time-keepers of the 
 most wonderful perfection have been made for the pur- 
 pose of telling the longitude of a ship at sea, and for other 
 purposes where great accuracy is required. They are 
 called chronometers. 
 
 Concerning their perfect action Arnott says: After 
 months spent in a passage from South America to Asia, 
 my pocket chronometer, with others on board, announced 
 one morning that a certain point of land was then bearing 
 east from the ship at a distance of fifty miles ; and in an 
 hour afterwards, when a mist had cleared away, the 
 looker-out on the mast gave the joyous call, " Land ahead," 
 verifying the report of the chronometers almost to a mile 
 after a voyage of thousands. 
 
 The method of using a watch to tell the longitude of a 
 place on the earth may be found explained in astronomy.
 
 TO NATURAL PHILOSOPHY. 
 
 What other use may be made of the pendulum ? 
 
 The pendulum lias been used to determine the shape of 
 the earth. 
 Describe the experiment with a vibrating cord. 
 
 Let a small cord fastened at one end pass over two 
 
 Fig. 27. 
 
 bridges upon which it rests and be stretched by a heavy 
 weight hung at the other end (Fig. 27). Then if a violin 
 bow be drawn across the cord, or if a person, taking hold 
 of its middle point, pull it aside and let it go again, it will 
 swing back and forth so swiftly that its motions can not he
 
 VIBRATIONS. 71 
 
 counted, and it will look like a gauzy spindle, as the picture 
 represents it. 
 
 What is meant by vibration and amplitude ? 
 The continued motion of the cord back and forth is called 
 vibratifm. But when we speak of a vibration, or a single 
 vibration, we mean the {notion from one aide to the other 
 and lack again to the starting-point. 
 
 The distance from one side to the other, that is, the 
 distance through which any point of the cord travels, is 
 called its amplitude of vibration. 
 
 Can the number of vibrations be counted ? The 
 motion of the cord is so very rapid that all we. can see 
 when it vibrates is a gauze-like swelling of its middle 
 parts ; and yet it is possible to find out exactly how 
 many vibrations it makes in a second. 
 
 If the cord, like those of a violin or piano, gives a sound 
 when it vibrates, the number of vibrations may be regis- 
 tered by the syren : for the number will be the same as 
 the number of air-puffs which escape from that instru- 
 ment when it makes a sound of the same pitch as that 
 made by the cord, and the number of air-puffs is registered 
 by the " hands " upon the upper part of the instrument, as 
 seen in Fig. 31. 
 
 If the vibrations of the cord do not produce sound, yet 
 the number made in one second may be very exactly 
 shown by the aid of electricity. We will not attempt to 
 describe the instrument now. See Text-look of Phi- 
 losophy, p. 151. 
 
 Does the rapidity of vibration depend upon the 
 length of the cord ? When two cords are taken, one 
 twice as long as the other, but alike in every other re- 
 spect, it is found by experiment that the long one will 
 vibrate onlv one half as fast as the other. In this cafe
 
 72 NATURAL PHILOSOPHY. 
 
 the number of vibrations in a second is inversely as the 
 length of the cord. 
 
 This is also true of all other cases. If one string is, for 
 example, one tenth as long as anothej, it will vibrate ten 
 times as fast. 
 
 Does the rapidity of vibration depend upon the 
 weight of the cord ? The wire-wound string of a 
 violin is much heavier than one which is not wound, 
 and we iind that it vibrates more slowly. The heavier 
 the cord the slower the vibration. This is always true. 
 
 If, to be more particular, we take one cord four times 
 as heavy as another, in all things else they beii:g alike, it 
 will vibrate only one half as fast. In all cases it will be 
 true as it is in this one, that the number of vibrations in a 
 second is inversely as the square root of the weight of the 
 cord. If, for another example, we suppose one cord to 
 weigh 16 times as much as another of the same length, it 
 will vibrate only \ as fast. 
 
 What is the third thing on which the rapidity 
 of vibration depends ? The rapidity of vibration de- 
 pends also on the weight or force by which the string is 
 stretched. This weight or force is called the tension of 
 the cord. If, for example, the weight which stretches the 
 cord over the bridges in Fig. 27 is 56 Ibs. the tension of 
 the cord is said to be 56 Ibs. 
 
 Xow we find that, when other things are equal, the cord 
 will vibrate faster as the tension is made greater. If the 
 tensions of three cords are as the numbers 1, 4, 9, the 
 number of vibrations a second will be as 1. 2, 3. But these 
 last numbers are the square roots of the first, in their 
 order, and this teaches us that the number of vibrations a 
 tsecond is directly as the square root of the tension of th<i 
 cord.
 
 VIBRATIONS. 
 
 73 
 
 To what are these principles applied? In the 
 
 manufacture of musical instruments in which the sounds 
 are made by vibrating cords or wires, such as the guitar 
 and the piano, these principles become important. The 
 cords must each be of the right length and weight and 
 tension, or else they will fail to give the correct tones for 
 music. 
 
 Describe the vibration of a wire fixed at one 
 end. When a wire, or even a slender rod of wood, is 
 fastened at one end in a vise, and its other end is drawn 
 
 Fig.
 
 74 NATURAL PHILOSOPHY. 
 
 aside, it will spring back and forth so swiftly as to appear 
 to be flattened out into a gauze-like fan (Fig. 28). 
 
 On -what does the rapidity of its vibration de- 
 pend ? If the rod is shortened it will vibrate faster ; in 
 a word, the number of vibrations in a second will depend 
 on the length of the wire or rod. The law seems to be 
 different from that which applies to cord?. It states that 
 the number of vibrations in a second is inversely as tlte. 
 square of the length of the rod. According to this law, 
 if one rod 36 inches long makes 1 vibration a second, 
 
 Fig. 29. 
 
 mother one only 12 inches long will make 9 in the same 
 time ; being ^ as long it vibrates 9 times as fast.
 
 VIBRATIONS. Y5 
 
 How may the vibrations .of a bell be shown ? 
 In Fig. ^9 we are shown a hellhaped glass vessel with a 
 little pendulum-ball hanging beside it. By drawing a 
 violin- bow across the edge of this bell we make the glass 
 vibrate, and we shall know that the vibrations are made 
 because the little prndulum-ball will fly back and forth 
 with a violent clatter. The edge of the glass, springing 
 back and forth, puts the ball in motion. 
 
 This is one of the many cases of vibration in which the 
 motion is too delicate to be seen, and the existence of 
 which would not be known if some way had not been 
 discovered by which to make the vibrations show them- 
 selves. Cases of such invisible vibrations are very com- 
 mon. In fact, almost every solid body we can see around 
 
 Fig. 30. 
 
 us is already or else may be put into a state of invisible 
 vibration.
 
 76 NATURAL PHILOSOPHY. 
 
 How may the vibrations of a brass plate be 
 shown? If a plate of brass is fastened at its center, and 
 a violin-bow be drawn across its edge (Fig. 30), it will 
 vibrate. Its vibrations may be felt by gently touching 
 it with the finger, but by sprinkling fine sand all over 
 the plate its vibrations will be shown in a curious and 
 much more satisfactory way. The picture represents 
 the beautiful effect which will be produced. The sand 
 will dance about, and finally collect into straight lines 
 and curves, sometimes in one form and sometimes in 
 another, changing with every change in the point to 
 which the bow is applied or upon which the finger is 
 laid. The reason for this curious arrangement is, that 
 some parts of the plate vibrate more than others, and 
 the sand gathers itself upon those points where there is 
 the least motion. 
 
 How may the vibration of water be shown ? 
 
 Let the glass vessel (Fig. 29) be almost filled with water, 
 and the bow then drawn across its edge. The fiuid will 
 be thrown into violent commotion. Hosts of little wave- 
 lets will be thrown up and down in quick succession upon 
 its surface, the water being thrown into vibration by the 
 vibration of the glass. 
 
 By skillfully drawing the bow these wavelets may be 
 brought into four and sometimes into six beautiful groups 
 separated from each other by portions of water which 
 seem to be at rest. Not many effects so fine can be so 
 easily produced. 
 
 How may water-waves be made on a larger 
 scale ? By tossing pebbles into the water of a quiet lake 
 or pond we may cause circular waves which spread away 
 farther and farther from the place where the pebble was
 
 VIBRATIONS. 77 
 
 dropped. By watcbing the motion of the water in these 
 waves we may easily study the character of such vibra- 
 tions. 
 
 In what direction does the water really move ? 
 While looking at the growing circles of water started by 
 the pebble it is most natural to think that the water is 
 moving just as it appears to be outward in all directions ; 
 and yet it is easy to show that it is not. 
 
 Little sticks and straws upon its surface would be car- 
 ried along outward too, if such were the motion of the 
 water, hut we tind that all the sticks and straws will do 
 is simply to rise and fall, which shows that the real mo- 
 tion of the water is up and down only, and not outward 
 from the pebble as it appears to be. 
 
 How is the motion in the billows of the sea ? 
 The waves of the sea are of the same nature. The force 
 of the wind, however, drives the water along at the same 
 time that it is vibrating up and down. But the billows 
 roll long after the wind has ceased to blow, and even 
 they, at such times, are not able to carry along the bits 
 of wood and other light bodies that may be floating upon 
 them. Each wave will seem to roll out from under all 
 such bodies, and let them down into the furrow to be 
 lifted again by the next ; and this shows that the real 
 motion of the water is simply a motion up and down. 
 
 Does the air vibrate ? The air is so elastic that it 
 yields to every force, even the very slightest, and then 
 afterward springs back again. On this account it is in a 
 state of vibration all the time. We can not stir a hand 
 without causing the air to vibrate. It is made to tremble 
 by every breath we exhale, and it quivers at every motion 
 of our lips.
 
 SOUND. 
 
 How is the sound of a piano-wire produced ? If 
 
 we look careliilly at a piano-wire while it is giving its 
 sound we can often see that it is in motion ; its appear- 
 ance will be much like that shown in Fig. 27. And even 
 if its delicate motions can not be seen, they may often be 
 felt by placing the finger very gently upon the wire, or 
 heard as a violent clatter if a knife-blade be used instead 
 of the finger. The fact is that the sound of the wire is 
 produced by these rapid vibrations. 
 
 Are other sounds produced in the same -way ? 
 By laying the finger gently upon the head of a drum a 
 tremulous motion will be felt whenever the sound of the 
 drum is heard. And in Fig. 29, while the motion of the 
 little ball shows that the glass beside it is vibrating, a 
 ringing sound is at the same time heard, and whenever a 
 bell of any kind is ringing it may be shown that it is also 
 vibrating. A body always vibrates when it emits a 
 sound ; all sounds are produced by vibrations. 
 
 Do all vibrations produce sound? When a cord is 
 long and not too tightly stretched (Fig. 27), its vibrations 
 may be easily seen, but no sound will be heard. 
 
 A sudden motion of the hand puts the air in vibration, 
 but gives no sound. 
 
 These and many other illustrations which might be 
 given teach us that all vibrations do not produce sound.
 
 SOUND. 79 
 
 What is the reason that some vibrations do not 
 produce sound ? If we make the string, Fig. 27, vi- 
 brate faster and faster, by increasing the weight which 
 stretches it, we shall very soon be able to hear a sound 
 which it gives, and then if the weight be made less again 
 the sound becomes inaudible. This shows that when 
 the sound is not heard the vibrations of the cord are too 
 slow to produce it. Some vibrations do not produce 
 sound because they are too slow. The slowest vibrations 
 that may produce sound are at the rate of 16 a second. 
 
 Is there another reason why vibrations do not 
 produce sound ? A string or wire may be made so short 
 and tense that its vibrations can not be heard. It then 
 vibrates so very fast that no sound is made by its motion. 
 Some vibrations are too fast to produce sound. The most 
 rapid vibrations that can be heard are made at the rate of 
 38,000 a second. 
 
 What are sound- vibrations ? All vibrations be 
 tweeii the limits of 16 a second and 83,000 a second 
 may be called sound-vibrations. No others ought to 
 be so called because, if any are either slower than 16 a 
 second or swifter than 38,000 a second they can not be 
 heard. 
 
 Is this true for all persons? There are persons, 
 however, to whose ears these limits will not apply. Some 
 ears are more sensitive than others ; they can hear sounds 
 that others can not. The squeak of a bat is made by very 
 rapid vibrations, and it is said that there are persons who 
 can not hear it at all. The same thing may be said of the 
 chirping of a cricket ; two persons sitting in the presence of 
 one of these animals may have ears of such different power 
 that while one is annoyed by the shrillness of the cricket's 
 voice, the other may not know that there is any noise
 
 80 XATCJRAL PHILOSOPHY. 
 
 produced. The limits named must, therefore, not be 
 taken as absolute. 
 
 Will sound- vibrations pass through air ? We could 
 not hear the sound of a distant bell if the air did not bring 
 its vibrations to our ears. But we may describe more 
 carefully the way in which the sound comes to us. In 
 the first place, the ringing bell is itself vibrating, as we 
 know, and by these vibrations the air which is in contact 
 with the metal is made to vibrate also. This air causes 
 another portion beyond it to vibrate, and that in turn 
 gives motion to another portion still farther away, until 
 finally the air in contact with the ear is made to vibrate 
 and then we hear the sound. Yery quickly after the 
 hammer strikes the bell, all the air between it and the ear 
 is put in motion, but not until this motion has traveled, 
 step by step, from the bell to the ear, can the sound be 
 heard. 
 
 In this way all sounds made in the air travel from 
 place to place. In speaking, our lips intrust their mes- 
 sages to the air and the air carries them to the ears of 
 our friend and delivers them with the most perfect accu- 
 racy. 
 
 Will sound-vibrations pass through -water ? Let 
 two stones be struck together under water and the sound 
 of the blow will be distinctly heard above the surface. 
 Or if the head is plunged beneath the surface while the 
 blow is struck, the sound will be heard more distinctly 
 than in air; its loudness will be likely to surprise the 
 hearer. These things show that sound-vibrations pass 
 through water with great facility. 
 
 Will sound-vibrations pass through solid bodies ? 
 To show that sound-vibrations also pass through wood,
 
 SOUND. 81 
 
 let the ear be placed upon one end of a long table while 
 a pin is drawn across the other end. One who tries this 
 experiment for the first time may be astonished to hear a 
 sound so loud as the scratch of the pin will be. The 
 sound-vibrations in this case travel through the wood of 
 which the table is made. 
 
 The power of solids to convey sounds is also illustrated 
 when boys at the station, waiting for a train of cars, put 
 their ears upon the iron track to learn whether it is com- 
 ing near. The sound of the train can be heard much 
 farther through the solid iron than through the air. 
 
 " The American Indians understand that solid bodies, 
 even the earth itself, convey sound with great facility. 
 When these wild and artful people suspect that enemies 
 are approaching they apply their ears close to the ground 
 in order to discover the noises made by the footsteps of 
 their foes when too far off for the sound to be conveyed 
 through the air." 
 
 Can sound- vibrations pass through a vacuum ? 
 We have seen that sounds can pass through solids, liquids, 
 and gases, but we must remember that they can not pass 
 unless one or another of these forms of matter is present. 
 
 This may be shown by experiment. A bell may be 
 placed in the receiver of an air-pnmp, and be so fixed 
 that while there it may be struck. Before the air is 
 exhausted its ringing may be loud and clear, but long 
 before the air is all taken out it will be almost too 
 feeble to be heard at all. Sound can not pass through a 
 vacuum. 
 
 What is the velocity of sound in different sub- 
 stances ? Sound travels faster in some substances than 
 in others. By very careful experiments it has been 
 found that in air whose temperature is 61? F, sound goes
 
 82 NATURAL PHILOSOPHY. 
 
 at the rate of 1,118 feet a second. It would travel faster 
 through warmer air and more slowly in that which is 
 colder. 
 
 In water, sound travels faster than in air ; its velocity 
 is about 4,714 feet a second. 
 
 In solid bodies the velocity of sound is still greater. 
 In pine wood, for example, it travels at the rapid rate of 
 10,900 feet a second. 
 
 Do all kinds of sound go with the same velocity ? 
 In air all sounds travel with the same rapidity. We 
 may, for example, while listening to the music of a dis- 
 tant band, notice that the heavy sound of the drum and 
 the shrill notes of the fife, made at the same moment, 
 reach the ear together, and this shows that they must 
 come with equal swiftness. 
 
 The same thing is true of all sounds whatever. Loud 
 sounds and low sounds, the sweetest and the harshest 
 alike, travel through the air at the rate of 1,118 feet a 
 second. The overpowering roar of the thunder can not 
 outrun the delicate song of the sparrow. In any medium 
 all sounds travel with equal velocity. 
 
 How can we measure distances by sound ? If 
 sound goes through air at the rate of 1,118 feet each 
 second, it will go twice that distance in two seconds and 
 ten times that distance in ten seconds ; so that we need 
 only to multiply 1,118 feet by the number of seconds to 
 tell how far the sound has come. 
 
 A man is chopping wood in the distance, and 3-011 wish 
 to know how far he is away : watch the motions of his axe 
 and listen to the sound of the blows. You will hear the 
 sound while the axe is over his head. The sound is made 
 when the blow is struck,- and it takes as long for the 
 sound to come to you as it does for the axe to be lifted.
 
 SOUND. & v 
 
 If it is one second between the time you see the blow and 
 the time you hear the sound, then the woodman must be 
 1,118 feet away. 
 
 Give another example. In a similar way you may 
 tell how far oft' a flash of lightning occurs. The sound 
 of thunder is made when the lightning is seen, but it is not 
 often heard until several moments afterward. If you can 
 count the seconds between the flash and the roar you may 
 multiply 1,118 feet by the number, and this will give the 
 number of feet to the place where the lightning occurred. 
 
 Upon what does the loudness of sound depend ? 
 
 The harder one strikes a piano-key the louder the sound 
 of the note will be, but all that the heavier blow does is 
 to make the piano-wire vibrate through greater distances. 
 In all cases the loudest sounds are caused by vibrations 
 through the greatest distances. 
 
 Now the distance through which the vibrating particles 
 swing back and forth is called the amplitude of vibration, 
 and hence we may say that the louduess of sound depends 
 upon the amplitude of the vibrations which produce it. 
 
 The harder one strikes a bell the louder it rings, because 
 the heavier blow causes the particles of the bell to vibrate 
 through greater distances, or, in other words, to have a 
 greater amplitude of vibration. 
 
 What effect does distance have upon the loud- 
 ness of sound ? The deafening roar of a cataract when 
 heard through a distance of several miles becomes a gentle 
 murmur, and the deep-toned thunder, if made too far 
 away, can not be heard at all. 
 
 A powerful human voice may be heard at a distance of 
 about 700 feet ; at greater than that distance it is gener- 
 ally too feeble to be heard.
 
 84 NATURAL PHILOSOPHY. 
 
 Sounds, however loud when near at hand, die away in 
 the distance until they become inaudible. 
 
 "Why is this ? This is because the amplitude of the 
 vibrations of the air grows less and less as the distance is 
 greater and greater, until at last it is too little to affect 
 the ear. 
 
 How can we illustrate this? When a pebble is 
 dropped into quiet water, the height of the circling waves 
 is greatest near the center where the pebble is dropped, 
 and grows less and less as they spread outward, until at 
 last we can not see the waves at all. So when a bell is 
 struck, the amplitude of the air-waves near to the bell is 
 greatest, and it grows less and less as the waves spread 
 outward in all directions from the bell, until at last it is 
 so little that the sound can not be heard. 
 
 Which will convey sound farthest, solids, liquids, 
 or gases? In solid bodies the vibrations diminish slowly, 
 and on this account sounds may be heard through them 
 at long distances. In liquids the vibrations diminish more 
 rapidly than in solids, so that through them sounds can 
 not be heard as far. In gases the vibrations die out still 
 faster, and hence sounds can not be heard as far in them 
 as in either of the other forms of matter. 
 
 We now see why the sound of a coming train of cars 
 can be heard sooner by placing the ear to the iron track. 
 The solid iron conveys the sound to the station, but in the 
 air the vibrations die away before they get there. 
 
 Can sound be heard farther in air at some times 
 than at others ? When the air is dense, as in a cold still 
 winter morning, or damp, as it often is in a still June 
 evening, we all know how much more distinctly distant 
 sounds can be heard than at other times. The rattling of 
 carriages, the laughing of children, the songs of birdc, and
 
 SOUND. 85 
 
 a thousand other sounds, many of which at other times 
 would not be noticed, are heard with surprising clearness 
 in still damp summer evenings. 
 
 What is the law ? But whether a sound can be heard 
 through a distance great or small, the rapidity with which 
 it dies away is governed by the following law : 
 
 The intensity or loudness of sound is inversely as the 
 square of the distance from its source. 
 
 We understand by this law that at twice the distance 
 sound will be only one fourth as loud ; or at oue half the 
 distance it will be four times as loud. 
 
 Will a tube convey sound ? If we put our lips to 
 one end of a long tube of metal, or even of pasteboard, 
 while a friend puts his ear at the other end, we tind that 
 even a gentle whisper will be very distinctly heard. 
 
 A gentleman in the city of Paris tried the experiment 
 with one of the water-pipes of that city, and found that a 
 word spoken in a very low tone was heard by a friend 
 three fourths of a mile away, while, as we have already 
 learned, a powerful voice in the free air would not be 
 heard more than one fifth of that distance. 
 
 Why can sounds be heard farther in tubes ? The 
 reason that sounds can be heard so much farther in tubes 
 than in free air is that the tube keeps the vibrations from 
 spreading in all directions outward as they do in the 
 atmosphere. They are all collected in the tube and sent 
 forward together, and they will not die away as fast as 
 when allowed to scatter in all directions in the air. 
 
 What are such tubes called? Such tubes are called 
 speaking tubes. They are often placed in dwellings so that 
 the mistress may talk with her servants even while they 
 are in different and distant rooms. They are also placed
 
 86 NATURAL PHILOSOPHY. 
 
 in large hotels for a similar purpose, and in large manu- 
 facturing establishments they are very common and a 
 great convenience. 
 
 What is an echo? "Who, after loudly tittering some 
 word or sentence, lias not at some time heard the sound of 
 his voice coming back to him from a distant wood, or 
 perhaps from the wall of a distant building? A part of 
 the last word spoken, if not the whole of it, and sometimes 
 more words than the last one, may be heard as distinctly 
 as if some mimic were shouting back what he had heard. 
 Such sounds are called echoes. 
 
 How is an echo produced ? When we throw a ball 
 perpendicularly against the floor it bounds directly back 
 to the hand. In the same way when the sound of our 
 voice strikes perpendicularly against a distant wall, it 
 bounds back to us and we catch it in our ears, and thus 
 hear the word which we uttered, as an echo. 
 
 Or, if we throw a ball obliquely against the floor it 
 bounds away obliquely from us: we can not catch it, but 
 it may be caught by another person standing in the right 
 place. In much the same way if a sound strikes obliquely 
 against a distant wall it will be thrown obliquely away 
 from it, and a person standing in its path will hear the 
 sound as if it came to him obliquely from the wall. It 
 is in this way that we hear the echoes of sounds which are 
 not made by ourselves. 
 
 What is reflection of sound? When sound is 
 thrown back from a surface against which it strikes, it is 
 said to be reflected* 
 
 Mention some remarkable echoes. An echo in 
 Woodstock Park repeats seventeen syllables distinctly. 
 Another near Milan repeats a single syllable thirty times:
 
 SOUND. 87 
 
 in this case the sound is thrown from one reflecting sur- 
 face to another many times, and each reflection sends an 
 eel 10 to the hearer. 
 
 " Placing himself close to the upper part of the wall of 
 the London Coliseum, a circular building 130 feet in 
 diameter, Mr. Wheatstone found a word pronounced to 
 be repeated a great many times. A single exclamation 
 sounded like a peal of laughter, while the tearing of a 
 piece of paper was like the patter of hail. " 
 
 MUSICAL SOUNDS. 
 
 What -was Galileo's experiment ? Galileo passed 
 the back of his knife-blade quickly over the rough edge of 
 a coin and produced a musical sound. You may repeat his 
 experiment easily, or in the absence of a coin with ridges 
 upon its edge, you may use a piece of one of the heavier 
 strings of a violin or piano, which are wound with fine 
 wire; or, indeed, if there is nothing better at hand, the 
 edge of a small file may be used. If you pass the blade 
 slowly over one of these rough surfaces a series of unpleas- 
 ant taps will be heard, but puss it swiftly and you will 
 produce a shrill musical sound. 
 
 How may a note be produced by a slate-pencil ? 
 '' Every schoolboy knows how to produce a note with 
 his slate-pencil. Holding it vertically and somewhat 
 loosely between the fingers, on moving it over the slate 
 a succession of taps is heard. By pressure these taps can 
 be caused to follow each other so quickly as to produce a 
 continuous sound. We must not call it musical, because 
 this term is associated with pleasure, and the sound of the 
 pencil is not pleasant." But it is a continuous tone, and 
 yet we know that it is made up of separate taps of the 
 pencil.
 
 NATURAL PHILOSOPHY 
 
 Fig. 81. 
 
 May other separate sounds be made to cause a 
 musical sound ? Any sound whatever, repeated fast 
 enough, will produce a continuous sound or a tone. To 
 illustrate this curious fact still further, suppose a stiff card 
 is held against the edge of a cog-wheel turning slowly ; 
 the card will strike each cog that comes around and every 
 blow- will be heard separately, but when the wheel turns 
 swiftly the same blows, made faster, produce a continuous 
 and shrill musical sound. 
 
 If a watch could tick fast enough, say 80 or 100 times 
 a second, no one would be able to count the ticks, or even 
 to hear them separately, for they would produce a steady 
 musical sound. 
 
 Could the blows of a hammer upon an anvil be made fast 
 enough and with regularity, they would produce a clear 
 and powerful musical tone. 
 
 Even puffs of air when made 
 
 ,^.- -- y..^^^ * n quick succession result in 
 music. This interesting fact is 
 
 wll^V ( ^<l ; l'bt 8nown by means of the siren a 
 I \__^ / Wjx very ingenious instrument which, 
 
 ^Jii .......;. .J'W we must now describe. 
 
 Describe the siren. In Fig. 
 31 this instrument is represented. 
 A pipe, P, enters a wind-chest, 
 W. The top of this chest is 
 pierced with a circular row of 
 holes. A disk of metal lies very 
 near to the top of the wind-chest 
 and is pierced with holes exact- 
 ly corresponding with those in 
 the chest itself. The disk is so 
 fixed that it may be made to revolve very swiftly, and
 
 SOUND. 89 
 
 the pointers, like the hands of a clock, seen at the top of 
 the picture, are to tell the number of turns the disk makes 
 in a second. 
 
 Now suppose the holes in the disk are exactly over 
 those in the top of the chest. Then if a blast of air is 
 forced through the pipe, P, into the chest, it can escape 
 through the holes in steady streams ; but let the disk be 
 turned just a little so that its holes are not directly over 
 the others : by this means the holes in the chest will be 
 closed so that the streams of air can not escape. As the 
 disk revolves the holes in the chest will be rapidly opened 
 and shut, and the streams of air will, in this way, be cut 
 up into puffs. 
 
 When the disk turns slowly the separate puffs may be 
 heard, but when it goes rapidly these same puffs produce 
 a musical tone of great purity. 
 
 What is necessary, then, to produce a musical 
 sound ? All that seems necessary to produce a musical 
 tone out of any sound whatever is that the separate pulses 
 be made with sufficient regularity and swiftness. 
 
 What is meant by the pitch of musical sounds ? 
 
 To distinguish musical sounds as being high or low, the 
 term pitch is used. 
 
 Some sounds, like the gravest notes of an organ, 
 are so very low that we can scarcely hear them, while 
 others, like the highest notes of the piano, are so very- 
 high that they are very far above the reach of any human 
 voice. Between the highest and the lowest there may be 
 a multitude of musical notes whose only difference is a 
 difference in pitch. 
 
 Upon -what does pitch depend ? If we take several 
 strings all alike, except in length, we shall find that the
 
 90 NATURAL PHILOSOPHY. 
 
 shortest string always gives the highest sound. And now 
 if we remember that the shortest string always vibrates 
 the fastest, we see that the highest tone is the one that is 
 made by the most rapid vibrations. 
 
 This is true in all cases. The pitch of musical sound 
 depends entirely upon the rapidity of vibration. 
 
 Upon what does the rate of vibration of a string 
 depend? The rapidity of vibration of a string or wire, 
 as we learned while studying the subject (p. 71), de- 
 pends upon the length of the string, its tension, and its 
 weiy/tt. 
 
 How then may we vary the pitch of the sounds 
 of strings? If we change either the length of the vibra- 
 ting string, or its tension, or its weight, we change the 
 pitch of the sound it produces. 
 
 To shorten the string makes the tone higher; to increase 
 the tension also makes the tone higher ; but to increase 
 the weight of the string makes the tone lower. 
 
 How is ihe pitch of the sounds obtained i i the 
 piano ? In the piano the sounds are made by the vibra- 
 tion of wires. These wires are of different lengths and 
 also of different weights; the shortest and the lightest 
 giving the highest tone. Now one end of each wire is 
 wound around a screw, and by turning the screw in one 
 direction the wire is tightened and the pitch raised, but 
 by turning it the other way the wire is loosened and the 
 pitch lowered. 
 
 How is the pitch obtained in a violin ? The violin 
 has four strings, each having a weight different from tho 
 others, the lightest string giving the highest note. Each 
 string is wound upon a " key " by which it may be tight- 
 ened or slackened, the tighter string giving the higher 
 note. They are all of the same length, but the player
 
 SOUND. 91 
 
 changes the length of the vibrating part of eacli at pleas- 
 ure by pressing a finger upon it at any desired point. 
 
 By having the right difference in the weight, the ten- 
 sion, and the length of the strings, the correct pitch of the 
 notes is obtained. 
 
 How is the sound of the organ produced ? In the 
 organ the musical sounds are caused by vibrations of col- 
 umns of air. A large bellows is kept full of air ; the organ- 
 ist, by pressing the keys of the organ, opens the way from 
 this bellows into one or more of the many pipes of the 
 instrument, and jets of air going into the pipes at the 
 bottom, make the air throughout their whole lengths 
 vibrate, and these vibrations produce the musical sounds. 
 
 Upon what does the pitch of organ-tones de- 
 pend? The pitch of the tone in an organ depends on 
 the length of the organ-pipe. The shorter the pipe the 
 higher the pitch will be. 
 
 The lowest note used in music is made by an organ- 
 pipe, open at the top, whose length is 32 feet. If the 
 pipe is closed at the top, a note of the same pitch will be 
 made by a pipe 16 feet in length. The highest notes are 
 made by pipes which are only a few inches long.
 
 LIGHT. 
 
 What are luminous bodies ? Luminous bodies are 
 those which produce light. The sun is a luminous body 
 because it shines by light which itself produces. The 
 flame of a candle, and a red-hot iron ball, are also lumin- 
 ous bodies for the same reason. 
 
 What are non-luminous bodies? Non-luminous 
 bodies are those that do not produce light. If they shine 
 at all it is because they first receive light from some other 
 source and then throw it off again. A piece of rock or 
 of wood, a flower, a cloud, all these are non-luminous 
 bodies. The rnoon is also non-luminous, for while it shines 
 with a steady and bright light, yet it produces none itself. 
 What we call the moonlight is light which goes from the 
 sun to the moon first, and is then thrown from the moon 
 to us. 
 
 What are transparent and opaque bodies ? Some 
 bodies, like glass and air, allow light to pass freely through 
 them ; all such are called transparent bodies. Others, 
 like iron and wood, forbid the passage of light through 
 them ; such are called opaque bodies. 
 
 Is any substance perfectly transparent ? We do 
 not mean that a transparent body will allow all the light 
 that falls upon it to go through. The air, for example, 
 which is one of the most transparent of all things, does not 
 let all the sunlight come through it. One can often look
 
 LIGHT. 93 
 
 directly at the sun, rising in the morning or setting in the 
 evening, without doing any injury to the eye, but if the 
 air did not shut out a large portion of the sun's light a 
 single glance would blind him. Perhaps no substance 
 exists which i& perfectly transparent. 
 
 Is any substance perfectly opaque ? On the other 
 hand, small quantities of light will pass through wood 
 and even through gold, for we all know that it is easy to 
 see through a thin shaving of almost any kind of woo^l, 
 and, if a piece of gold-leaf is at hand, we may hold it be- 
 tween the eye and a window and see that light comes 
 through it : this light is green. 
 
 What are rays, beams, and pencils of light ? A 
 ray is a single line of light. Several parallel rays together 
 form a beam of light. 
 
 The ray of light is quite too delicate a thing to be 
 obtained in practice ; the smallest line of light which it 
 is possible to use is made of many rays ; it is a beam. 
 
 A pencil of light is a collection of rays that are not 
 parallel. 
 
 In what direction does light go from a luminous 
 body ? If a lamp is suddenly lighted in the center of a 
 dark room, every part of the room will be instantly made 
 light. This shows that the light goes from the flame in 
 all directions. In the same way light is given from a 
 glowing coal, from a red-hot iron, from the sun, indeed, 
 from all luminous bodies, in every possible direction at 
 once. 
 
 Does it move in straight lines ? In order to show 
 by experiment whether the path of light is straight or 
 crooked we need a darkened room. It is. easy to darken 
 a room by closing the shutters, and at the same time, if
 
 94 NATURAL PHILOSOPHY. 
 
 need be, hanging shawls or blankets over the windows. 
 It' a small hole is made in one of the shutters to let the 
 sunlight through, the path of the sunbeam in the room 
 may be distinctly seen. If the air is sprinkled with chalk- 
 dust the beam is peculiarly bright and beautiful, and no 
 artist could possibly draw a line so absolutely straight as 
 it is seen to be. 
 
 We often see such beams coming through our half 
 closed window-shutters and streaking the dusty air of our 
 rooms with bars of light, beautiful illustrations of the 
 fact that light always moves in straight lines. 
 
 What other illustrations of this fact ? When the 
 sun is sinking behind clouds in the western sky, it often 
 presents an appearance which is very well shown in the 
 picture (Fig. 32), and which is sometimes described by 
 saying that the sun "draws water.'' Is it possible that 
 any one ever believed the streaks he saw were really 
 streams of water being pulled up by the sun ? We know 
 now that they are beams of li_>ht from the sun, which is 
 shining through openings in the clouds. They are made 
 in just the same way as are the beams of light seen in our 
 rooms when the sun shines through openings in the win- 
 dow-shutter. 
 
 How are shadows made? It is upon the same 
 principle that shadows are made. To illustrate, just put 
 a book at a convenient distance in front of a lamp-flame. 
 Now some of the rays of light fall upon the book, and 
 can not go farther, but others just graze the edges of the 
 book and pass in slraiyht lines onward, so that behind the 
 book and reaching to the opposite wall is a space from 
 which the light is shut out. This apace which is deprived 
 of light ia the shadow of the book. 
 
 We are apt to call the black spot on the wall the shadow,
 
 LIGHT. 
 
 95 
 
 but the shadow really reaches from the book to the wall ; 
 it is all the space behind the book from which the light is 
 shut out, and the black spot on the wall is only one end 
 of it. 
 
 Fig. 32. 
 
 Of what two parts is a shadow composed ? If 
 
 we examine the end of the true shadow of the book as it 
 appears upon the wall, we may notice that there is a dark 
 middle part, and then a border all around this, which is 
 much lighter. One can hardly fail to see these two parts 
 by examining the shadows made by objects around the 
 evening lamp. There is in every shadow a dark middle 
 part surrounded by another lighter portion. The dark 
 middle part is called thevmbrd, and the lighter portion is 
 called the penumbra. These parts reach throughout the 
 whole length of every shadow. 
 
 How are they formed? A very easy experiment 
 will explain how these two parts of a shadow are pio-
 
 96 NATURAL PHILOSOPHY. 
 
 duced. Place two lamp-flames on a table near each other, 
 and hold a lead pencil or a narrow strip of wood between 
 the flames and a sheet of paper, at some distance from 
 them. Two distinct shadows will be seen, one cast by 
 each light. ISTow move the pencil nearer to the paper, 
 the shadows will approach each other, until at last they 
 overlap and form one, in which the umbra and the penum- 
 bra may be seen with surprising clearness. The umbra 
 gets no light from either flame, but it is easy to see that 
 every part of the penumbra is getting light from one or 
 the other. On this account the umbra is darker than the 
 penumbra. 
 
 Is it so with a single flame ? Now in the shadow 
 cast when a single flame is used, the outer parts are getting 
 light from one ed<je or another of the flame, while the mid- 
 dle part is getting no light at all from any portion of the 
 flame. For this reason the umbra is darker than the 
 penumbra. 
 
 What is the velocity of light ? The light from the 
 flash of a gun in the distance comes to us so very quickly 
 that it is impossible to measure the brief time it takes. 
 So swiftly does lij.ht travel, that, could it move in curved 
 lines, it would go around the world more than seven times 
 in a second ! Its rate of motion or velocity is about 
 190,000 miles a second. 
 
 It will, of course, be interesting to know how a velocity 
 so very great could be measured. It was first done by 
 the Danish astronomer, Komer, about two hundred years 
 ago. 
 
 How was the velocity of light found ? This gen- 
 tleman, by observing the eclipse of one of Jupiter's satel- 
 lites, found out how long it takes light to go across the
 
 LIGHT. 97 
 
 orbit of the earth, and he then divided that distance by 
 the number of seconds, and so found out how far the light 
 could go in one second. 
 
 Explain the operation more fully. The time when 
 an eclipse of Jupit r's moon is to begin is exactly known, 
 as it would be seen when the earth is in that part of its 
 orbit which is nearest to Jupiter. Now if the earth is in 
 the opposite part of the orbit, the eclipse will not be seen 
 to begin at the moment at which it is predicted, but 16 
 minutes and 36 seconds, or 996 seconds afterward. It is 
 seen by light which comes from it, and the reason it begins 
 late is that the light has so much farther to come. In fact 
 it takes the light 996 seconds to go across the orbit of the 
 earth. The distance across is about 190,000,000 miles, 
 and 190 millions divided by 996 gives about 190,000 for 
 the number of miles that light can travel in one second 
 of time. 
 
 How does distance affect the intensity of light ? 
 
 The nearer a body is to the source of light, the brighter 
 or more intense will be the light that falls upon it. 
 
 Let us study this subject more closely by experiment. 
 We may take a sheet of paper, or, what is a little better, 
 a square of stiff card board, and hold it halfway between 
 a lamp flame and the wall of a room. If we measure the 
 shadow on the wall, we find it to be just twice as long and 
 twice as wide as the card-board, and hence its surface is 
 just four times as large as that of the board. 
 
 Now if the light that falls upon the card-board could go 
 to the wall it would cover the whole surface of the shadow, 
 and bo spread over a surface four times as large as that on 
 Avhich it does fall. Being spread over four times as much 
 surface it can be only one fourth as bright. 
 5
 
 93 NATURAL PHILOSOPHY. 
 
 We see from this experiment that at 2 times the 
 distance from the flame the light is only one quarter as 
 intense. If the distance is taken 3 instead of 2, the 
 intensity would be only one ninth. The law is this: 
 
 The intensity of light varies inversely as the square of 
 the distance from its source. 
 
 The light of the stars is dim, but if any one of these 
 glimmering points could be brought as near to us as the 
 sun, it would be more dazzling than that body is to us. 
 Or, if the sun could be carried out as far as the stars, it 
 would be no brighter than they. 
 
 How can we see objects ? A luminous body is seen 
 .by means of light which comes directly trom it into our 
 eyes. It is the light which comes from a red-bot iron ball 
 which enables us to see it. 
 
 We see non-luminous bodies also by means of light which 
 comes from them: it is impossible to see any object that 
 does not send light into the eye. 
 
 How can this be true of non-luminous bodies ? 
 Non-luminous bodies have no light of their own to send to 
 the eye, but they receive light from luminous bodies, and 
 throw it off again into the air. This light which they 
 throw off may enter our eyes, and by this means the body 
 becomes visible. 
 
 All non-luminous bodies are invisible in a very cloudy 
 night, because no light from the sun or moon or stars falls 
 upon them, and they can send none to the eye. All 
 objects in our room disappear instantly when the lamp is 
 extinguished, because no light then falls upon them, and 
 we therefore get none from them. 
 
 What is reflected light? This light which is thrown 
 off from non-luminous bodies is called r< fleeted light.
 
 LIGHT. 99 
 
 What experiment can illustrate reflection of 
 light? Let abeam of sunlight into a darkened room 
 through a hole A, Fig. 33, in the shutter of a window, and 
 make it fall upon a looking-glass. This beam will bound 
 off, or be reflected from the glass, and go up to the ceiling, 
 where it will form a bright spot C. Let the air of the 
 room be sprinkled with dust, and the beams of light will 
 
 Fig. 33. 
 
 be shown as clearly as they are represented in the picture, 
 which is a very good picture of the experiment. 
 What are important parts to notice here ? It is 
 
 very necessary to understand clearly the use of certain 
 terms which are applied to certain things represented in 
 this figure. There is first the beam AB, which falls upon 
 the reflecting surface, and is called the incident beam; 
 and then the beam BC, which is thrown from the reflect- 
 ing surface, and is called the reflected beam. The point 
 B is called the point of incidence. 
 
 And now, if we suppose a perpendicular BD to be
 
 100 NATURAL PHILOSOPHY. 
 
 erected, we have two angles made, one, ABD, the angle 
 between the incident beam and the perpendicular, is the 
 angle of incidence ; the other, CBD, the angle between 
 the reflected beam and the perpendicular, is the angle of 
 reflection. 
 
 What is the law of reflection? ]STow these two 
 angles are exactly equal in this experiment : they are 
 always so. The law of reflection states that the angle of 
 reflection must be equal to the angle of incidence. 
 
 What are mirrors ? Some bodies reflect light much 
 better than others. Yery little is reflected from the sur- 
 face of rough iron, for example, while a great deal is 
 thrown from the surface of new tin. Those which reflect 
 light most freely are called mirrors. 
 
 A common looking-glass is the most familiar example 
 of a mirror, but other forms are almost as common. The 
 inside surface of a bright silver spoon is a good mirror, 
 and the outside surface also, because from both these sur- 
 faces light is very freely reflected. 
 
 What are three forms of mirror ? There are three 
 forms of mirrors which need especial notice : 
 
 First, the plane mirror, one whose surface is like that 
 of the looking-glass, plane or flat. 
 
 Second, the concave mirror, one whose surface is like 
 that of the inside of the silver spoon, hollowed or concave. 
 
 Third, the convex mirror, one whose surface is like the 
 outside of the silver spoon, rounded or convex. 
 
 What effect do plane mirrors cause? If light 
 from any object falls upon a plane mirror, and after re- 
 flection enters our eyes, we see an image of the object 
 (Fig. 34). It appears to be behind the mirror, and just as 
 far behind it as the object itself is in front (Fig, 35).
 
 LIGHT. 
 
 Fig. 34. 
 
 101 
 
 
 Give examples. Our own image seen in a looking- 
 glass is the most familiar example of this effect, but look- 
 ing down into the water of a well or of a lake will show 
 the image just as perfectly. The surface of quiet water 
 is indeed a very perfect plane mirror, and forms images 
 of all objects above it with wonderful clearness. How 
 beautiful are the pictures of the mansions and shrubbery 
 along the bank of a river, or near the shores of a lake, as 
 we see them presented in the still water between us and 
 them 1 
 
 How are images formed ? The way in which this
 
 102 NATURAL PHILOSOPHY. 
 
 curious effect is produced may be understood by studying 
 Fig. 35, which represents a boy looking at the image of a 
 candle in a looking-glass. 
 
 Fig. 35. 
 
 Rays of light are shown going from the tip of the can- 
 dle-flame to the mirror, and being reflected from it into 
 the eye of the boy. These rays appear to have come from 
 a point behind the mirror, and this point is the image of 
 the tip of the flame from which the rays first started. 
 
 Now every point on the whole candle will send rays to 
 the mirror to be thrown back into the same eye, and thus 
 form an image of every point, or in other words an image 
 of the candle itself. 
 
 And so, when a person sees his own image in a mirror, 
 he may think of its being formed in the same way. He 
 may think of rays of light going from every point on his 
 person to the mirror, and thence, being reflected, coming 
 into his eyes, and then he may think of his eyes tracing 
 these rays right back, in the direction from which they 
 last came, to points behind the mirror; these points make 
 up the image which he sees.
 
 LIGHT. 
 
 103 
 
 How may more than one image of an object be 
 made at once ? By using two looking-glasses several 
 images of one object may be seen at once. 
 
 By placing two looking-glasses parallel to each other 
 and near together, and then putting the eye at one end, 
 while a ball or other object is put between them at the 
 other, a large number of images of the ball may be seen. 
 The picture, Tig. 36, shows how the light must be reflected 
 
 back and forth from one mirror to the other to make so 
 many images. 
 
 How may just three be formed ? By holding two 
 mirrors at right angles (Fig. 37), any object placed be- 
 tween them will give three images.
 
 104 
 
 NATURAL PHILOSOPHY. 
 Fig. 37. 
 
 Fig. 88. 
 
 How may five be obtained ? But if instead of bein^, 
 at right angles the mirrors 
 are inclined, as in Fig. 38, 
 making an angle of CO , just 
 live images will be formed. 
 Whatis the effect of 
 a ccncave mirror upon a 
 team of light ? Fig. 39 
 shows the effect of a con- 
 cave mirror on parallel rays. 
 The rajs, after reflection, 
 will no longer be parallel ; 
 they will converge and cross 
 each other at the point F. 
 
 This point is called the focvs of the mirror. Any point 
 where rays, after reflection, cross each other, is a focus ; 
 but when the mirror is held perpendicular to the path
 
 LIGHT. 
 
 105 
 the 
 
 of the beam, as in the picture, the focus is called 
 pr Inc tpal focus. 
 
 What is the effect of the concave mirror on di- 
 verging rays ? Let a candle-ilame be held some distance 
 
 Fig. 39. 
 
 before a concave mirror (Fig. 40), and the light reflected 
 will make a bright spot upon the finger or any thing else 
 
 Fig. 40. 
 
 held at the point F. We see that while the rays from the 
 candle are diverging, those which are thrown from the 
 mirror are converging. 
 
 What effect does a concave mirror always pro- 
 duce ? In both these pictures we notice that the rays, 
 after reflection, are brought nearer together than they 
 were before, or, in other words, that they are collected by 
 reflection. A concave mirror always makes the rays go 
 on nearer together after reflection than they did before. 
 '
 
 106 NATURAL PHILOSOPHY. 
 
 Describe the image formed by a concave mirror 
 when an object is placed very near. By holding 
 one of these instruments very near to the face, a person 
 can hardly recognize himself in the immense image which 
 he sees. The picture (Fig. 41) represents a concave mir- 
 ror, M, with a young man standing very near to it ; his 
 
 Fit:. 41. 
 
 face ii supposed to be between the focns and the mirror. 
 The image formed is much larger than the object, and 
 seems to be almost upon the surface of the mirror, instead 
 of being as far behind it as we are accustomed to see it in 
 a common looking-glass. 
 
 .The concave mirror always forms an image larger than 
 the object if the object is very near to it. 
 
 Suppose the object i^ moved away from the 
 mirror? If a candle is put a little farther away from 
 the mirror than the focus, a still more curious effect will 
 be produced. The image will not appear behind the 
 mirror at all ; it wil] be in front of it and farther away 
 than the object itself, and, curiously enough, always bot- 
 tom upward, or inverted. 
 
 The picture (Fig. 42) represents the experiment. The
 
 LIGHT. 107 
 
 image could not be seen if a screen were not, put in just 
 the right place to receive it, because no light would come 
 
 Fig. 42. 
 
 from it to our eyes. The screen receives the light which 
 forms the image and reflects it to the eye. 
 
 Now let the object be moved still farther away. 
 If the candle is carried etiH farther away from the 
 mirror, the image will move up toward it and grow smaller. 
 At length the object and the image will be at equal dis- 
 tances from the mirror; they will then be of equal size, 
 but as the candle is carried out farther and farther the 
 image will go nearer and nearer to the mirror, growing 
 smaller and smaller all the time, until finally it appears 
 only as a bright spot at the focus.
 
 108 
 
 NATURAL PHILOSOPHY. 
 
 The concave mirror is the only form that can produce 
 images in front of it, or that are inverted. 
 
 Describe the image by a convex mirror. Fig. 43 
 shows the small image which i.s formed when a person 
 
 Fig. <. 
 
 looks into a convex mirror. It is always, as we see it 
 here represented, behind the surface of the mirror, erect, 
 and smaller than the object. 
 
 i r f- 44. How is light reflected 
 
 from rough surfaces ? 
 
 When a beam of light 
 falls upon a rough sur- 
 face it is reflected irregu- 
 larly. This effect is shown 
 in Fig. 4i. The result is 
 that the reflected light 
 is scattered in every di- 
 rection. 
 
 Do the rays obey the law of reflection ? When 
 we say that the reflection is irregular, we do not mean 
 that the rays are not thrown off according to the law of 
 reflection. That law is never transgressed. Every ray
 
 LIGHT. 109 
 
 must be thrown in such a direction that the angle of reflec- 
 tion is equal to the angle of incidence. But on a rourh 
 surface, like that seen in Fig. 44, the reflecting points are 
 not in regular order, and for this reason the reflected 
 rays are not. 
 
 Do transparent bodies reflect light? Transpar- 
 ent bodies do reflect light : we know this from the 
 simple fact that we are able to see them ; for we have 
 learned before, that an object is visible only by the light 
 which is thrown from it into our eyes. Glass and water 
 are transparent bodies, and yet these bodies are able to 
 reflect a great deal of light. 
 
 How do they differ from opaque bodies ? An 
 opaque body does not allow light to pass through it ; a 
 transparent body allows the light to pass through it 
 freely. For instance, we can not look through a piece 
 of sheet-iron ; it stops the light and reflects it, but we see 
 distinctly through glass, because it can reflect only a part 
 of all the light that falls upon it and allows the rest to 
 come through. 
 
 Can this be shown by experiment ? Water is as 
 transparent as glass, and we can easily show that water 
 does not reflect all the light that falls upon its surface. 
 The picture (Fig. 45) tells us at once how such tin experi- 
 ment is to be made. A glass vessel iilled with water is 
 placed so that a beam of sunlight coining into a dark room 
 through a hole in a shutter can fall upon the surface of 
 the fluid. The beam of light can be seen in its passage 
 through the water. 
 
 All transparent bodies allow light to go through them 
 in this way, but it is not always possible to see the path of 
 the rays.
 
 110 
 
 NATURAL PHILOSOPHY, 
 Fig. 45. 
 
 What else may be noticed in the experiment? 
 
 If the air in the room is a little dusty, the path of light 
 from the window to the water, as well as through the 
 water itself, can be seen, and then it will be noticed that 
 the beam seems to be broken just at the point where it 
 enters the liquid. This is also shown in the picture. 
 The beam is perfectly straight while in the air, arid per- 
 fectly straight while in the water also, but it is bent just 
 where it passes from one into the other. 
 
 Is this always the case ? This bending of the raya 
 of light in passing from one transparent substance into 
 another generally takes place. 
 
 The light that comes through the windows into our 
 houses is bent twice, once when it passes from the air
 
 LIGHT. in 
 
 outside into the glass, and again when it passes from the 
 glass into the air in the room. 
 
 What is this bending of the rays called? This 
 bending' of the rays of light in passing from one substance 
 into another is called refraction. 
 
 What terms must now be understood ? To ex- 
 plain certain terms used in connection with the subject of 
 refraction we must first suppose a line to be drawn per- 
 pendicular to the surface where refraction occurs, and to 
 be extended both ways from it, or into both substances. 
 In Fig. 4fi, for instance, we must suppose a line perpen- 
 dicular to the water at the Fig. 46. 
 point I, where the light H I 
 enters, and to reach up into 
 the air and down into the 
 water. Then we have, 
 
 The incident beam that 
 which falls upon the sub- 
 stance into which it is to 
 pass, R I: 
 
 The refracted learn that 
 which passes through the 
 second substance, I S : 
 
 The angle of incidence -the angle between the incident 
 beam and the perpendicular : 
 
 The angle of refraction the angle between the refract- 
 ed beam and the perpendicular. 
 
 On what does the amount of refraction depend ? 
 Some substances bend the rays more than others. The 
 amount of bending depends upon the difference in the 
 density of the two substances. If two substances could 
 have exactly the same density, light would pass from one 
 to the other without being bent at all, but if one is more
 
 112 NATURAL PHILOSOPHY. 
 
 dense than the other, the bending will occur. For in- 
 stance, water is more dense than air, so that light which 
 passes either from air into water or from water into air, 
 will be retracted. 
 
 What is the law of refraction ? If the rays are 
 passing from one medium into a denser one the angle of 
 refraction will be smaller than the angle of incidence, but 
 if passing into one which is less dense the angle of refrac- 
 tion will be larger than the angle of incidence. 
 
 If, for illustration, the beam R I (Fig. 46) is the incident 
 beam going from air down into water, it will be bent so 
 as to go in the direction I S. Now it h easy to see that 
 the angle S I P, in the water, or the angle of refraction, is 
 smaller than the angle in the air, or the angle of inci- 
 dence. 
 
 But suppose S I represents a beam going from the 
 water up into the air above, it will be bent so as to go in 
 the direction of I R. In this case the angle in the air is 
 the angle of refraction : the angle of incidence is in the 
 water. The light is passing into a less dense substance 
 and the angle of refraction is larger than the angle of 
 incidence. 
 
 How does a straight stick appear when partly 
 plunged into water? If a straight stick is plunged a 
 part of its length into clear water it will look as if it were 
 broken or bent just at the surface of the water. This 
 effect ij caused by refraction, and it is very common. 
 Boatmen arc especially familiar with it, since their oars 
 always look bent at the point where they enter the water. 
 (Fig. 47.) 
 
 Why does the stick appear to be bent ? Of course 
 the oar or stick is not bent, and it looks to be so only be- 
 cause the light that cornea wit of the water from it is bent
 
 113 
 
 Fig. 48. 
 
 when it enters the air, and the part that is in the water 
 will appear to be in the direction in which the light from 
 it enters the eve. All this may be understood if you 
 carefully examine Fig. 4, where the true place of the 
 
 stick in water is shown 
 b} r dotted lines, and two 
 rays that come from 
 the lower end of it are 
 shown bent at the sur- 
 face of the water, so 
 as to enter the eye, 
 which traces them back 
 in straight lines, so that 
 they seem to come from 
 a point above the ere 
 from which they started.
 
 114 NATURAL PHILOSOPHY. 
 
 What is another curious effect of refraction ? 
 
 Water does not appear to be as deep as it really is on 
 account of the retraction of the light which comes from 
 the bottom into the air. The young man represented in 
 Fig. 50 sees the bottom of the water lifted far toward the 
 surface. The rays of light from the point O, for example, 
 are refracted at the surface rig. 49. 
 
 and by this means enter 
 the eye as if they came 
 from O' ; and so the bot- 
 tom at seems to be 
 at O'. 
 
 What are lenses ? 
 
 The instruments used to 
 refract light are called len- 
 ses ; they are transparent bodies having either one or two 
 curved surfaces. 
 
 Fig. 4:9 is a picture of one kind of bns. Looking at 
 the side of the instrument it is circular ; looking at the 
 edge of it, it is found to be thicker through the middle ; 
 both sides are convex. This is called the double convex 
 lens. 
 
 Sometimes both sides of the lens are concave and the 
 middle is thinner than the edge ; in this case it is called a 
 double concave lens. 
 
 How many kinds are there ? There are six differ- 
 ent forms of lens. They are shown in section by Fig. 51. 
 They are named as follows : 
 
 1 The double convex. 4 The double concave. 
 
 2 " piano convex. 5 " piano concave. 
 
 3 " meniscus. G " concavo convex. 
 The first three act upon light in the same way : on
 
 115 
 
 tins account only the first or double convex lens will be 
 particularly noticed. 
 
 The last three are alike also in their effect, and we need 
 give close attention only to the action of the double 
 concave. 
 
 What is the effect of the convex lens on parallel
 
 HO NATURAL PHILOSOPHY. 
 
 rays ? If a double convex lens is held in the path of a 
 sunbeam, especially in a darkened room, the rays will not 
 come out of it parallel : they will be so bent as to all 
 come to one point, as we are shown by Fig. 52. 
 
 Fig. 52. 
 
 The point F (Fig. 52), where the parallel rays are col- 
 lected, is called the principal focus of the lens. 
 What effect is produced on diverging rays ? We 
 
 may suppose the rays to go froin the focus to the lens : 
 these rays (F A,*Fig. 52) are diverging and, as the figure 
 represents them, they will be parallel after going through 
 the lens. 
 
 But diverging rays are not always made parallel ; indeed, 
 they will not be unless they start from the focus. If they 
 start from a point between the focus and the lens they 
 
 Fig. 58.
 
 LIGHT. 
 
 117 
 
 will diverge after going through, but the divergence will 
 l>e less than before. On the other hand, if the rays start 
 from a point farther than the focus from the lens, they 
 will be converging after refraction. This case is beauti- 
 fully shown in Fig. 53. 
 
 Does the concave lens have the same effect ? 
 The concave lens has exactly the opposite effect. The 
 rays after passing through a concave lens are separated 
 instead of being collected. 
 
 Fig. 54. 
 
 This effect is well shown in Fig. 54, which represents 
 a double concave lens refracting parallel rays of light. 
 
 Fig. 55. 
 
 They are supposed to enter the lens on the side F, parallel 
 to each other, but on coming out on the other side they
 
 US NATURAL PHILOSOPHY. 
 
 are diverging. All the concave lenses have the effect to 
 separate rays by retraction. 
 
 Describe the image formed by a convex lens. 
 Most perfect and very beautiful images are formed by the 
 use of convex lenses. If one of these instruments is held 
 at a little distance from any object, a flower, for example, 
 it will form an image which may be caught upon a screen 
 placed in the right spot. This image will be on the other 
 side of the lens from the object, and inverted (Fig. 55). 
 
 Explain the production of the image. Fig. 56 
 and Fig. 57 will help us to understand how this image is 
 
 Fig. 56. 
 
 formed. They show a lens with a small arrow, a J, near 
 to it. Two rays of light are seen going from a through 
 the lens, and after refraction meeting again at A. This 
 point, A, is the image of the point, , from which the rays 
 started. Two other rays are seen going from 5 through 
 the lens and being refracted to the point 13. This point 
 B is the image of the other end of the arrow. Every 
 point between a and b in the arrow will send off rays of 
 light which, after going through the lens, will be brought 
 together again at corresponding points between A and B, 
 and all together they make up the whole image A B. 
 When will the image be larger than the object ?
 
 LIGHT. 
 
 119 
 
 Tf the image is farther than the object from the lens 
 (see Fig. 56), it will be larger than the object. 
 
 When will it be smaller ? But if the image is nearer 
 to the lens than the object is (see Fig. 57), it will be 
 smaller. 
 
 Fig. 57. 
 
 Whichever is farthest from the lens will be the 
 largest. 
 
 Will the image ever be on the same side of the 
 lens as the object? In the cases thus far examined we 
 suppose the object to be outside or beyond the focus of 
 the lens. If the object is put between the lens and its 
 focus, the image will be seen on the same side as the 
 object. It will be erect, and very much larger than the 
 object. In Fig. 58 tins case is shown. A small insect is 
 between the focus F, and the lens, and a person looking 
 
 Fig. 5S.
 
 120 NATURAL PHILOSOPHY. 
 
 through the lens, ins'ead of seeing the little creature a 5, 
 will behold its magnified image, A B. 
 
 What is the effect of concave lenses ? Concave 
 lenses have just the opposite effect ; they form an image 
 always smaller than the object. In Fig. 5i) we can see 
 
 Fig. 59. 
 
 how this is done. The light from the vase A B, after 
 going through the concave lens, seems, to the eye, to have 
 come from the smaller image a I. 
 
 Is all light of the same color ? The light which the 
 sun sheds upon all things so freely, is said to be white 
 light, but yet all light is not white. The light of some 
 stars, for example, is as red as a flame of fire, while 
 others shed upon us a delicate light as green as that of an 
 emerald. 
 
 Why are bodies of different colors ? We can see 
 an object only by means of the light that ia reflected by 
 it. Now, if the light which it reflects is red, then the 
 color of the body is itself red. If a body reflects blue 
 light, the body ha- a bine color; in every case the color 
 of a body is the color of the light which that body reflects.
 
 LIGHT. 121 
 
 The meadows are green because the vegetation throws 
 green light to our eyes. 
 
 What curious experiment -will illustrate this ? 
 
 " Fill a spirit-lamp with alcohol in which a large quantity 
 of salt has been dissolved ; on being lit it will be found to 
 burn with a livid yellow flame." Let a room be lighted 
 entirely by one or two of such lamps. "It should, if pos- 
 sible, be hung with pictures in water and oil colors, and 
 the persons present ought to wear nothing but the bright- 
 est colors, and the table be ornamented with the gayest 
 of flowers." Let the lamps be brought into this darkened 
 room, and an astonishing appearance will be presented. 
 " The furniture and every other object which the room 
 contains will reflect but a single color. The brightest 
 purple, the purest lilac, the liveliest green will be con- 
 verted into a monotonous yellow. The same change will 
 take place in the countenances of those present: every one 
 will laugh at the appearance of his neighbor's face without 
 thinking that he is just as great a subject of laughter to 
 them." 
 
 Nothing can, better than this experiment, show that 
 bodies will seem to be of the color which they can reflect. 
 When they receive only yellow rays, they can themselves 
 be of no other color. And if any of them are not able to 
 reflect yellow light, these will appear black. 
 
 Then why, in the sunlight, are not all bodies 
 white ? All bodies in the sunlight are receiving only 
 white light, and if white light was like that of any other 
 color they would all be white. The white light must 
 contain all other colors which bodies reflect. These bodies 
 receive all these color3 alike, and then each one makes 
 choice of the color which it will reflect. A rose gets white 
 light from the sun, and then from among all the colors it 
 6 -
 
 122 NATURAL PHILOSOPHY. 
 
 contains, it reflects the red only. A violet reflects blue 
 instead of red or any other, while the leaves of a tree re- 
 flect only the green rays of the white light which the sun 
 sheds upon them. 
 
 By -what instrument can sunlight be separated 
 into its colors? The instrument used in the arts to 
 decompose light is called a. prism. It is generally nothing 
 
 Fig. 60. 
 
 more than a triangular piece of glass, but it may be made 
 of many other substances. Fig. 60 represents the prism, 
 and Fig. 61 shows how this instrument is often mounted 
 upon a stand to be convenient for use. 
 
 Describe the experiment with the prism. Let a 
 prism be held in a beam of sunlight as it enters a dark- 
 ened room ; the rays which come through the prism will 
 strike the wall or ceiling of the room, or upon a screen, 
 and form there a patch of beautifully colored light (Kig. 
 62). All the colors of the rainbow will be seen ; and 
 what is still more beautiful, if dust be sprinkled into the 
 air of the room, these colors will be seen reaching all the
 
 LIGHT. 
 
 123 
 
 way from the prism to the wall. Rays of purest bine, of 
 most delicate violet, of the brightest yellow, with others 
 
 Fig. 61. 
 
 
 of different colors, will be seen spread out like a fan from 
 the prism through the dusty air 
 
 This arrangement of colors formed from the sunlight 
 which passes through a prism is called the solar spectrum. 
 
 What are the colors in the solar spectrum? 
 There are seven colors in the solar spectrum. They are 
 arranged in the following order : red, orange, yellow, 
 green, blue, indigo, and violet. 
 
 These are the colors of which sunlight is composed, and 
 the colors of all bodies in the world are produced by the 
 mixture of two or more of these in different proportions. 
 
 Will the seven colors produce white light ? If
 
 '124 NATURAL PHILOSOPHY. 
 
 the colors formed by a prism are made to pass through 
 a double convex lens (Fig. 63) they will be brought 
 together again and the spot of light upon the wall will be 
 white. The prism decomposes the white light and brings 
 
 Fig. C2. 
 
 out the colors : the convex lens combines the colors and 
 makes white light again. Here, then, is a double proof 
 
 Fig. 63. 
 
 that white light is made up of the seven colors of the 
 spectrum.
 
 LIGHT. 1 95 
 
 How is the rainbow formed ? In a shower of rain 
 each drop of water is able to decompose the sunlight and 
 give the different colors of the spectrum. This it will do 
 if the sun is shining brightly at the time the drop is fall- 
 ing, and you will remember that all the rainbows you 
 ever saw were seen while the sun was shining. When 
 the sun is behind you, and a shower is falling in front of 
 you, the rays which pass through each drop are decom- 
 posed and the colors come out in such a direction that 
 some of them enter your eye. Some drops send the red 
 color to the eye : others in a different place send orange 
 and others still send yellow; another set gives blue, an- 
 other indigo, and finally another violet. And these 
 seven colors are so arranged as to form the beautiful 
 " bow of promise." 
 
 OPTICAL INSTRUMENTS. 
 
 What does Fig. 58 represent? By turning back 
 to Fig. 58, we see that a convex lens when held between 
 the eye and a little insect will help us to see a very large 
 image instead of the little creature itself. Try it yourself 
 by taking grandmother's spectacles, if you have no other 
 lens, and hold one of the glasses just at the right place, 
 which you can find by moving it back and forth between 
 your eye and the page of the book. The letters will look 
 much larger than they really are. 
 
 A double convex lens used in this way is called a 
 simple microscope. 
 
 This little instrument, by making every little thing 
 look larger than it is, becomes a very pleasant, and at the 
 same time a very useful, instrument to every body. Most 
 people use it for viewing tine engravings and in look-
 
 126 
 
 NATURAL PHILOSOPHY. 
 
 Fig. 64. 
 
 ing at photographs. The watch-maker uses it to examine 
 the minute parts of his work, and the jeweler uses it also 
 for the same purpose. 
 
 What is the compound microscope? The com- 
 pound microscope is an instrument by which to see the 
 images of objects which are so very small that the eye 
 alone may not see them at all. 
 
 Describe it. It contains more than one lens; in its 
 simplest form it has two. We can understand it best by 
 studying Fig. 04. Let us begin at the bottom, and notice 
 first a concave mirror. "We see the rays 
 of sunlight which fall on this mirror are 
 thrown upward and brought together at 
 a. Now the little object to be magni- 
 fied is placed at this point and the 
 bright light which goes up from it must 
 pass through the lens Z>, which is very 
 near to it. This lens would magnify the 
 object, but not enough, and BO the light 
 after going through it is made to go 
 through another larger lens B, and then 
 into the eye of the person. The little 
 thing at the point a is made to look 
 large enough to fill all the space between 
 C and D. 
 
 What are the lenses called ? The 
 lens 1) is called the object-glass, and the 
 other, near the eye, is called the eye-piece. 
 The eye-piece is often made of two lenses 
 and the object-glass sometimes of as 
 many as eight. 
 
 How much will this instrument magnify? By 
 this instrument we are able to make the diameter of the
 
 LIGHT. 
 
 12' 
 
 image 2,000 times greater than the real diameter of the 
 object, and in that case the surface of the image would be 
 4,000,000 times as large as that of the object examined ! 
 ''Under such a power a hair would appear about six 
 inches thick, a fine needle would look like a street-postj 
 and a grain of sand like a mass of rock." Such power is 
 only necessary in examining the very smallest objects. 
 All common preparations are best examined with a power 
 which makes the diameter appear to be only 500 or 600 
 times larger than it really is. 
 
 What has the microscope revealed ? This instru- 
 ment has made known a world of little things around us 
 which no human eye could ever see without its help. 
 Little animals, and little plants, so very small that thou- 
 sands of them together would not be as large as the small- 
 est particle of dust you ever saw, are almost everywhere 
 in the soil and water and other substances around us. 
 
 What is a telescope ? The telescope is an instru- 
 ment by which we are able to examine objects which are 
 so far away that the eye alone can not see them distinctly. 
 It contains lenses or mirrors by which the images of dis- 
 tant objects are made near to the eye. 
 
 Describe one kind. We can describe one kind of
 
 128 
 
 NATURAL PHILOSOPHY. 
 
 telescope best by means of the foregoing diagram, Fig C5. 
 A large convex lens is in one end of a tube and a smaller 
 one is at the other end. This small one is the eye-yluw, 
 and can be moved back and forth so as to be fixed at just 
 the right distance from the other. The light from a 
 distant body coming through the large lens forms an 
 image at a J, and then a person looking through the eye- 
 glass sees this image magnified at A' B'. 
 
 Fig. 06. 
 
 The tube containing these glasses is mounted in some 
 way to allow it to be pointed toward any object in the
 
 LIGHT. 12 ;) 
 
 heavens. The picture, Fig. 60, shows a small one. One 
 of the largest of this kind of telescope in the world is at 
 Harvard College. Its object-glass is about eighteen inches 
 in diameter. 
 
 What has the telescope revealed ? The telescope 
 has made known a great many things about the sun and 
 moon and stars. It has shown that the moon is covered 
 with mountains and valleys ; and that the sun has im- 
 mense black spots on its surface that looks to us so bright. 
 It shows that there are hosts of stars in the sky, which 
 could never have been seen without its help, some of them 
 being so far away that light, travelling fast enough to go 
 around the world about seven times a second, would need 
 many hundreds of years to come from them to us.
 
 HEAT. 
 
 What is the chief source of heat ? From the sun 
 more heat is received than from all other sources together. 
 It is more than 90,000,000 of miles from the earth, and 
 yet there comes out through that vast distance a constant 
 flood of heat which, if withdrawn for a single year, would 
 leave the whole earth in a degree of cold which even the 
 Arctic regions never had. 
 
 What is a source of artificial heat ? Combustion 
 is the chief source of artificial heat. Wood, coal, or other 
 fuel burning in our stoves or furnaces warms our dwell- 
 ings, cooks our food, and makes the steam by which our 
 machinery is driven. Next to the sun, combustion is cer- 
 tainly the most important source of heat. 
 
 How is the heat in combustion produced? If 
 you shut the draught of an " air-tight" stove the iire will 
 go out; or, if you put a lighted candle under the receiver 
 of an air-pump, it will die away when the air is exhausted. 
 We learn from such experiments that no fuel can burn 
 without air. Unless air can pass over the hot fuel in the 
 stove there can be no fire. 
 
 Now the air is made up of two parts which the chemist 
 calls oxygen and nitrogen, and it is the oxygen of the air 
 passing over the fuel which causes the combustion. The 
 oxygen unites itself to the carbon and other materials of
 
 HEAT. 131 
 
 which the fuel consists, and to this action the heat of the 
 fire is due. 
 
 What is another source of heat ? Mechanical ac- 
 tion, such as rubbing or pounding, will produce heat. 
 Let the fingers be pressed down upon the table, and then 
 smartly rubbed back and forth : the heat caused by this 
 friction will be quickly felt. Or if a small cord or a 
 thread is swiftly drawn through the hand which holds it 
 tightly, the hand will be cruelly burned. If two pieces 
 of wood are rubbed upon each other briskly enough they 
 may be set on fire ; in this way savage people are said to 
 have kindled their fires : more civilized people now do it 
 more easily by tim ply rubbing the end of a match. 
 
 Blows also produce heat, as any one may easily prove 
 by pounding a bullet with a hammer, fur he can soon 
 make it too hot to be comfortably held in the hand. 
 
 Does heat pass from one body to another ? From 
 every heated body rays of heat are continually going 
 away. This is almost too familiar to need illustration, for 
 the stove gives its warmth to all other objects in the room, 
 and a red-hot cannon-ball will part with its heat so rapidly 
 as to very soon get dark and finally cold. 
 
 But there is this curious fact to add to what has just 
 been said : no body is at any time so cold that it is not 
 giving off heat to every other around it. Heat is con- 
 stantly passing away from every body, no matter how cold 
 it may already be, and what is given off by one is being 
 received by others in its neighborhood, so that it is true 
 that even a block of ice is giving heat to a red-hot stove, 
 if placed in its vicinity. 
 
 Then -why do hot bodies grow colder ? Now if 
 the ice and the stove in this illustration should each give
 
 132 NATURAL PHILOSOPHY. 
 
 off just as much heat as it gets from the other back again, 
 the ice would not melt nor the stove grow cold. But the 
 hot stove is giving off much more than it gets, and on this 
 account it becomes gradually colder if the tire is not kept 
 np, while, at the same time, the ice gets much more than 
 it gives, and is of course melted by it. 
 
 A body gets warmer only when it is getting heat from 
 others faster than it is giving heat to them : it gets colder 
 only when it gives heat faster than it gets it. 
 
 How does heat get from one body to another ? 
 Heat travels outward from a hot body in waves something 
 like the motion of water-waves when a pebble is thrown 
 into a pond or lake. As the pebble puts the water in 
 motion, so the hot body gives motion to the substance 
 which fills the space around it ; and as the waves of 
 water spread outward in all directions from the pebble, so 
 the waves of heat spread in all directions from their 
 source. These waves of heat warm every body against 
 which they strike. 
 
 With -what velocity do these -waves travel ? 
 And they go from one body to another so very swiftly 
 that one can not measure the small instant of time they 
 take to pass through any common distance. They start 
 from a hot stove and at the same instant they seem to 
 strike the face of a person in the most distant corner of the 
 room. Indeed, their velocity is so great that they would 
 be able to go quite around the world as many as seven 
 times in a single second ! The velocity of heat is like 
 that of light : together they come to us from the sun, a 
 distance of more than 90,000,000 miles, in about 8 min- 
 utes. This would be at the rate of about 190,000 miles a 
 second. 
 
 What name is given to heat sent off by bodies in
 
 HEAT. 133 
 
 this way ? The heat which travels in this way is called 
 radiant heat. Its peculiarities are, when briefly stated, 
 first, it goes in straight lines ; second, it goes in all possi- 
 ble directions from its source ; and third, it moves with 
 very great velocity. 
 
 This mode of transferring heat from place to place is 
 called radiation. 
 
 Is there another mode ? Heat does not always 
 travel in this way. If you take one of grandmother's knit- 
 ting needles and hold one end of it in a lamp-flame you will 
 feel the other very soon getting warm. The heat enters the 
 metal at one end and travels, step by step, from one parti- 
 cle to another until at length it reaches the fingers. In this 
 way it is carried from one part of a body to another, or it 
 maybe from one body to another, if they touch each other. 
 This mode of transferring heat is called conduction. 
 
 Do all solids conduct heat alike? If we take 
 two wires of equal size and length, one being of copper 
 and the other of iron, and place one end of each in a 
 flame, we shall find that the heat travels through, the cop- 
 per to the other end quicker than through the iron. 
 Copper conducts heat better than iron does. A rod of 
 glass may be melted within an inch of the fingers that 
 hold it without burning them, and a splinter of wood may 
 be held in the same way while it burns to ashes. 
 
 We thus learn that each solid has a rate of its own at 
 which it may conduct heat. Liquids scarcely conduct it. 
 at all, and gases in a degree still less. 
 
 Bodies that conduct heat freely are called good con- 
 ductors, but those that do not are called poor conductors 
 or non-conductors. 
 
 Are liquids and gases conductors of heat ? 
 Water is BO very poer a conductor of heat that if you put
 
 134 
 
 NATURAL PHILOSOPHY. 
 
 ice at the bottom of a glass vessel and then apply heat to 
 the water above it, Fig. 67, you may make the water boil 
 
 Fig. 67. 
 
 Fig
 
 135 
 
 without melting the ice ; the heat w r ill not travel down- 
 ward through the water to the ice. Other liquids, except 
 mercury., are like water in being very poor conductors of 
 heat. 
 
 Gases are still poorer Fi ?- 69 - 
 
 conductors than liquids. 
 
 What is one effect 
 caused by heat? Let 
 
 us learn by experiment 
 what effect heat pro- 
 duces : 
 
 1st. In solids. A ball 
 of iron or of brass is taken 
 just large enough to pass 
 easily through a ring of 
 the same material. The 
 ball is then heated by a 
 lamp, after which it will 
 be too large to go through 
 the ring. It will rest 
 upon the ring (Fig. 68) 
 until it gets cold again, 
 when it once more passes 
 easily as at first. We j 
 see that heat makes this -Sjjj 
 ball larger. And it 
 the same effect upon ~ 
 other solids. 
 
 2d. Liquids. A glass bulb with a long open stem is 
 -used. The bulb is filled with water and the stem partly 
 filled, after which, if the bulb is plunged into hot water 
 (Fig. 69), the water in the stem will be seen slowly rising,
 
 136 
 
 NATURAL PHILOSOPHY. 
 
 Fijr. 70. 
 
 until perhaps it will run over the top. The water grows 
 larger as it gets warmer. We see that heat expands this 
 liquid : it does the same thing for others. 
 
 3d. Gases. Fig. 70 shows an experiment with air. 
 The glass bulb with its long open 
 stem is used for this also. The 
 little black spot near the end of 
 the tube represents a little drop 
 of ink which has been put into 
 the tube and which will be held 
 there by the walls of glass. Now 
 when the warm hands take hold 
 of the bulb the drop will run up 
 still higher in the tube. The air 
 below it pushes the ink up be- 
 cause it wants more room. The 
 heat of the hand makes the air 
 larger than it was. Heat also 
 expands all other gases. 
 
 "We learn from these experi- 
 ments that the general effect of 
 heat is to expand all bodies to 
 which it is applied. 
 
 What facts illustrate the 
 expansion of solids ? An iron 
 gate which opens and shuts easily 
 in cold weather, will stick, in 
 a warm day, owing to the heat which expands it. Pipes 
 of cast-iron for conveying hot water are longer when full 
 than when empty. It is said that an ignorant man once 
 tried to warm a large manufactory by steam. He laid 
 one large iron pipe from the boiler to the farther end of 
 the building, and then passed branches from this through
 
 HEAT. 137 
 
 holes into the several rooms. The very first time he 
 filled the pipes with steam, the expansion of the main 
 pipe tore it away from all its brandies ! 
 
 The rails of a railroad-track are longer in summer than 
 in winter. 
 
 What facts illustrate the expansion of liquids ? 
 A kettle nearly full of cold water will be quite full when 
 the water is heated : the water will run over long before 
 it boils. 
 
 Twenty gallons of alcohol in midwinter will become 
 about twenty-one gallons in midsummer. Hence cunning 
 dealers try to make purchases in winter and sales in 
 summer, that the heat of summer may add to their profits. 
 
 Does heat always expand water? At all tem- 
 peratures above 39 water will be expanded by applying 
 heat, but at temperatures below 39 water will be con- 
 tracted by applying heat. At 39 a given weight of water 
 is as small as it can be ; heat it or cool it as you will, and 
 it will be expanded. 
 
 Are all liquids like water in this respect ? To 
 show how different are the effects of a change of tempera- 
 ture in water and other liquids, the following experiment is 
 made. Three glass globes with long necks are placed in 
 a large dish nearly filled with ice-cold water (Fig. 71). 
 
 Suppo'se the water cooled to 32? At 32 the 
 water freezes. The expansion at this moment is greater 
 than at any moment before, so that the ice is larger than 
 the water from which it is made. 
 
 On this account pitchers and water-pipes are often 
 broken by the water freezing in them. 
 
 Ice being larger must also be lighter than the water 
 from which it is made. Were it not for this fact our 
 ponds and rivers would never be covered with a blanket
 
 138 NATURAL PHILOSOPHY. 
 
 of ice as now they are in the winter. The ice, instead, 
 would sink to the bottom as fast as formed. There could 
 be no skating then, you notice ; but that would not be the 
 
 Fig. 7L 
 
 saddest of the story, for there would soon be no human 
 beings to enjoy that or any other sport. As it now is, the 
 ice stays on top and keeps the water from freezing to any 
 great depth ; but if it should sink, it would go on forming 
 until the whole body of water would become ice from 
 bottom to top, and then the atmosphere would get colder 
 and colder, until neither plants nor animals could live 
 at all. 
 
 What facts illustrate the expansion of air ? The 
 snapping of wood in the tire is caused by the expansion 
 of air. The air in the pores of the wood, suddenly heated, 
 expands and bursts the wood witli a sharp report, 
 
 The first balloons that were made were filled with hot 
 air, and they went up toward and even above the clouds,
 
 HEAT. 139 
 
 because the air they contained was expanded and made 
 lighter than the cold air so light that it could rise and 
 carry the balloon up with it. Only toy-balloons are now 
 iil led with hot air ; those by which men are carried to 
 the clouds are tilled with common illuminating gas, or 
 sometimes with hydrogen. 
 
 Which is most expanded by heat, solids, liquids, 
 or gases ? A little addition of heat expands a gas very 
 much ; the same applied to a liquid would cause an in- 
 crease in size difficult to see, and if it were applied to 
 a solid, would not change its size enough to be noticed at 
 all. 
 
 Do all solids and liquids expand equally? Instead 
 of all solids expanding equally, each one has a certain 
 rate of its own. Brass for example, will expand faster 
 than iron. Each liquid also has a rate of its own. 
 
 Do all gases expand equally ? Gases all expand 
 alike by heat. Air, oxygen, and all other gases, heated 
 alike, will expand equally. 
 
 What is a second effect of heat ? If iron is heated 
 it will go on expanding long after it has become red-hot, 
 until finally it melts. The solid is then changed into a 
 liquid ; this is a second effect of heat. It is called lique- 
 faction. 
 
 Do all solids melt at the same temperature ? A 
 few examples which all have noticed will show that solids 
 melt at very different temperatures. The warmth of the 
 hand will melt ice, but not wax. Sulphur will melt on a 
 hot stove ; it needs a temperature of 230, but iron does 
 not melt until it is heated to about 3,000. Each sub- 
 stance has a certain temperature at which it melts. This 
 temperature is called the melting-point.
 
 140 
 
 NATURAL PHILOSOPHY. 
 
 What is a third effect of heat ? If water is heated 
 it will go on expanding until its temperature is 212 ; at 
 this temperature it bolls. The liquid is then changed into 
 a gas ; this is a third effect of heat. It is called vaporisation. 
 Do all liquids boil at the same temperature ? 
 Water boils in an open iron vessel at the temperature of 
 212, but alcohol will boil at 173, and ether only needs 
 to be heated to 95. These illustrations show that each 
 liquid has its own degree of heat at which it boils : this 
 temperature is called the boiling-point. 
 
 Describe the experiment in which, by cooling 
 the vessel, water is made to boil. A very curious 
 experiment is represented in Fig. 72. In the first place a 
 Fi C . 72. glass bulb with long open stem 
 
 is partly filled with water. This 
 water is then boiled for some time 
 until the steam has driven all 
 the air out of the vessel. While 
 the water is still boiling, the 
 stem is tightly corked and the 
 heat taken away at the same 
 moment. All this is done to 
 get rid of the air and leave 
 nothing but water and steam in 
 the vessel. The bulb is then 
 turned upward as in the picture. 
 After thisjww cold water upon 
 the bulb, and the water inside 
 will boil vigorously. Stop pour- 
 ing cold water and the boiling will cease, but as. often as 
 the cold is applied the boiling will begin. This may be 
 kept up until the vessel is cold enough to be held in the 
 hand without inconvenience.
 
 HEAT. 141 
 
 . 
 
 What does the cold water do ? Now all that the 
 cold water does is to cool the steam that is in the bulb 
 and condense it into water. By this means the pressure 
 of the steam is taken off from the water inside. 
 
 Describe another experiment in which water is 
 made to boil without fire. The pressure may be taken 
 from the surface of the water in another way. The water 
 is put into a flask, to the -top of which is fastened a long 
 tube. This arrangement is shown in Fig. 73. The 
 
 Fig. 73. 
 
 water is then heated until it begins to boil. The flask 
 is then taken from the fire and its tube is fastened to the 
 plate of the air-pump. On working the pump, the air 
 and steam are taken from the flask, and the water, which 
 by this time is much below 212 C \ begins to boil violently. 
 What do these experiments teach? We learn 
 from these experiments that by taking pressure away from
 
 142 NATURAL PHILOSOPHY. 
 
 the surface of water, boiling will go on at a lower temper- 
 ature. 
 
 In the open air water boils at 212 C , but the pressure of 
 the atmosphere is 151bs. upon each square inch of surface. 
 If it were not for this pressure water would boil at a tem- 
 perature much lower than 212, indeed at a temperature 
 not much above that of a hot summer day. 
 
 Water boils at a low temperature on top of high mount- 
 ains. In fact, at places very high above the level of the 
 sea, boiling water is not hot enough to cook meat, or even 
 to boil eggs, because the pressure of the atmosphere is so 
 much less than at the sea-level. 
 
 Suppose the pressure is increased ? On the other 
 hand, if water is heated under a greater pressure than that 
 of the atmosphere, the boiling-point will be higher than 
 212. In a word, the boiling-point is mixed by increasing 
 the pressure and lowered by lessening it. 
 
 Does steam exert pressure ? Steam often lifts the 
 lid of a kettle in order to make its escape, and when 
 confined in a boiler, it sometimes bursts the stoutest 
 bands of iron, killing people and destroying buildings by 
 the force of the explosion. Such facts show that steam 
 can exert a great pressure when confined. 
 
 How can it be used to move machinery ? If 
 there is a piston moving freely inside of a cylinder, and 
 then if steam is let into the cylinder, first at one end and 
 then at the other, its expansive force or pressure will 
 knock the piston back and forth from one end to the 
 other with great rapidity and power. The piston may 
 move a crank which turns a wheel, and then by bands or 
 cogs this wheel may turn other wheels. In this way 
 steam is made to move machinery. This is the principle 
 of the xteam-engi'ne, by which ships are driven over the
 
 HEAT. 143 
 
 sea and railroad trains across the continent, and by 
 which so much of all the machinery in the world is 
 moved. 
 
 Can water be heated above 212 ? If water is 
 slowly heated its temperature will rise until the liquid 
 boils at 212 : after that the water grows no hotter. The 
 tire may be quickened and the boiling will be more vio- 
 lent, but the water will not become any hotter. This is 
 always true when the water is heated in open vessels 
 such as are generally used. 
 
 What becomes of the heat added to the boiling 
 water? Now the tire gives heat to the boiling water 
 all the time, but, as we see, does not make it any hotter. 
 All the heat that goes into the water is then used in 
 changing the wafer into steam. 
 
 Can -we get this heat back again ? All this heat 
 which has been used to change the water into steam, will 
 be given up when the steam changes back into water. 
 This is the reason that a plate grows hot so very quickly 
 when held in the steam that issues from the spout of the 
 tea-kettle. 
 
 Is this principle ever applied? Buildings are 
 sometimes warmed by steam. From a large steam-boiler 
 cast-iron pipes are laid to the many rooms to be wanned, 
 and the steam is forced through their entire length. The 
 steam is condensed in going through the cold pipes, and 
 gives up to them the heat which it took from the fire. 
 They very soon become very warm, and warm the air 
 which is in contact with them. 
 
 Do we know the temperature of bodies by feel- 
 ing them? Let ns take three vessels of water, one almost 
 as cold as ice, another just warmer than the ha^d, and a
 
 144 NATURAL PHILOSOPHY. 
 
 third as hot as the hand can bear. Let one hand be held 
 in the first vessel of cold water and the other in the vessel 
 of hot water for a while, and then let both be plunged 
 into the vessel of warm water. It will be found that to 
 one hand the water is cold, to the other it is hot, at the 
 same time. Of course, by the feeling we could not tell 
 whether it is really hot or cold. 
 
 Give another example. An oil-cloth and a carpet, 
 where they lie together upon the floor, are of the same 
 temperature, but the oil-cloth will feel cold and the carpet 
 warm to the hand at the same time. 
 
 The reason of this is that the oil-cloth is a better con- 
 ductor of heat than the carpet, and takes the heat of the 
 hand away faster, so that the hand grows cold quicker 
 when upon it. 
 
 By what instrument can we find the tempera- 
 ture of bodies ? The instruments by which to measure 
 temperature are called thermometers. The common ther- 
 mometer contains mercury, which expands when heated, 
 and contracts when cooled, and by these changes of 
 volume shows the temperature. 
 
 Describe the common thermometer. The ther- 
 mometer is very common, and a single look at it would 
 be better than any description of it can be. However, it 
 ma} 7 be described as a glass tube, with a bulb at one end, 
 while the other end is shut air-tight, containing mercury 
 which fills the bulb and part of the stern, and having a 
 scale behind the stem to show the height of the fluid. 
 Fig. 74 shows two forms of this instrument. 
 
 How is it used? By putting the bulb into water or 
 any othei substance the height of the mercury in the stem 
 will show how hot it is. Put the bulb in water, for in- 
 stance, and if the mercury rises in the stem up to the
 
 HEAT. 
 
 145 
 
 place marked 90 on the scale, then the 
 temperature of the water is 90. 
 
 How is Fahrenheit's thermometer 
 graduated ? The place where the 'mer- 
 cury stands when the bulb is immersed in 
 boiling water is marked 212 on the scale ; 
 where the mercury stands when the bulb is 
 immersed in freezing water is marked 32 : 
 the space between these is divided into 180 
 equal parts, called degrees, and divisions of 
 the same size are marked off on the scale 
 both above and below these points. 
 
 How is the centigrade thermometer 
 graduated ? In the centigrade thermom- 
 eter, the place where the mercury stands 
 when the bulb is placed in boiling water is 
 marked 100 ; where it stands when the 
 bulb is in freezing water is called 0, and 
 the distance between is divided into 100 
 equal parts or degrees.
 
 s 
 
 MAGNETISM. 
 
 What is a loadstone ? Several hundred jears ago 
 pieces of a certain kind of iron ore were found in the 
 earth which had the power to attract bits of iron. They 
 would lift needles or small nails or other bits of iron 
 which they touched, and hold them suspended in the air 
 as if they were cemented to the stone with glue. The 
 ore of iron which has this wonderful power is called the 
 loadstone. It is found sometimes in this country, but not 
 in such abundance as in Sweden and Norway, and in some 
 parts of Asia. A good specimen may be bought for a few 
 cents, and it is an interesting and instructive toy. It is 
 often called the natural magnet. 
 
 What is a magnet ? If a loadstone is rolled in iron 
 filings it will attract them and hold them clinging to its 
 surface, but when rolled in filings of brass or copper not 
 one of these will it pick up. It has a curious preference 
 for iron. Any body that will attract iron in preference 
 to other metals is called a magnet. 
 
 What are artificial magnets ? A bar of steel may 
 be made a magnet by simply rubbing it upon the load- 
 stone. In this way the blade of a penknife may be given 
 the power to pick up bits of iron, such as small needles, 
 tacks, or iron filings. One blade can get this power also 
 from another which has been already made magnetic; and 
 what is a little singular, perhaps, is that the one that gives
 
 MAGNETISM. 147 
 
 this power to another is none the weaker for it; it is 
 even stronger than before. All such pieces of steel are 
 magnets. 
 
 Iron also may be made magnetic, but it will not stay so 
 unless it be first hardened more than usual. 
 
 In what two shapes are magnets made ? ArtiV 
 ficial magnets are made in two forms. They are generally 
 either a straight bar of steel or else a bar bent into the 
 form of a horse-shoe. The first is called the bar magnet, 
 and the second is called the horse-slioe magnet. 
 
 What is an armature ? A horse-shoe magnet gener- 
 ally has with it a bar of iron to reach across from one end 
 of the magnet to the other. Such a bar is called the 
 armature, 
 
 In what part of a magnet is the attraction 
 strongest ? By rolling a bar magnet in a bed of iron 
 filings and then lifting it, the filings may be seen clinging 
 to the ends of the bar in Fig . 75. 
 
 curious tufts (Fig. 75), while 
 along the middle few or none 
 will be found. 
 
 This experiment shows the power of the magnet to be 
 much greater at the ends than elsewhere. The ends of 
 the magnet, or the points at which the force is strongest, 
 are called the poles. 
 
 , Fig. '76 represents a horse-shoe magnet with an arma- 
 ture across the poles holding up a heavy weight. 
 
 What is the magnetic needle ? A slender bar mag- 
 net balanced upon a pivot (Fig. 77) is called a magnetic 
 needle. 
 
 Lett to itself, such a needle will always be found point- 
 ing toward the north and south. It will not rest in any 
 other position. If you push it out of this direction it will
 
 148 
 
 NATURAL PHILOSOPHY. 
 
 Fig. 76. 
 
 swing back again the moment you let go of it, and after 
 vibrating from one side to the other for a time, it will at 
 last come to rest again with the same end to the north as 
 before. 
 
 The end that points toward the 
 north is called t\\Q north pole : the 
 other end is called the south pole. 
 What use is made of the 
 magnetic needle ? What is 
 called the mariner's compass is a 
 magnetic needle placed over a dial 
 on which are marked north, south, 
 east, west, and many other direc- 
 tions, or as they are called, " points 
 of the compass." This little in- 
 strument is placed where it will 
 be every moment in view of the 
 man who guides the ship, and tells 
 him every moment in what direc- 
 tion the ship is going. 
 
 No matter how dark the night 
 or how rough the sea may be, the 
 faithful needle, pointing always so 
 nearly north and south, guides the 
 storm-tossed searm.n safely to his 
 port. 
 
 What is a dipping needle ? If a magnetic needle 
 is hung in a way to let its poles move up and down it will 
 not rest in a horizontal position. The picture (Fig. 78) 
 shows what direction it will take : the north pole will be 
 lower than the other. A needle fixed in this way is 
 called a dipping needle. 
 
 Is the dip of the needle everywhere alike ? The
 
 MAGNETISM. 
 
 149 
 
 dip of the needle is not the same in all places on the earth. 
 In the most northern regions the needle is most oblique, 
 
 Fig. 77, 
 
 that is to say, the dip is 
 greatest. Just at the north 
 pole the needle would point 
 its north pole downward to 
 the ground. As the needle 
 is carried farther to the south 
 the north pole rises until 
 when at the equator the nee- 
 dle would be horizontal, or 
 have no dip at all, and then 
 when carried farther into 
 the southern hemisphere the south pole would dip instead 
 of the north pole. 
 
 Fig. 78 
 
 How will one magnet act upon another ?
 
 150 NATURAL PHILOSOPHY. 
 
 Let one magnet be brought near to another ; suppose, 
 for instance, that you hold one in the hand and point its 
 north pole toward the south pole of the magnetic needle. 
 They will quickly come together : if in reach of eacli 
 other they can not be kept apart. These unlike poles at- 
 tract each other. 
 
 Next point the north pole toward the north pole of the 
 needle, and it will swing quickly away, so that it will 
 be almost impossible to make the two magnets touch each 
 other. These poles of the same name repel each other. 
 
 State the law of attraction and repulsion ? It 
 will always be the case as in the experiments just de- 
 scribed, that poles of unlike names attract each oilier, 
 while poles of the same name repel each other. 
 
 Describe the experiment to illustrate induction. 
 
 Hold a strong magnet in a vertical position and touch 
 the lower pole with the end of a much smaller bar of 
 iron : the magnet will hold it h'rmly in the air. Another 
 smaller bar may be hung from the lower end of the first, 
 and another yet from it. The first bar of iron receives its 
 magnetism from the magnet, and then the second from 
 the first and the third from the second. This power of a 
 magnet to impart magnetism to other bars of iron or steel 
 is called induction. 
 
 Will induction occur when the magnet does not 
 touch the iron ? We may cover the end of the magnet 
 with paper so that the bar of iron can not touch the polo, 
 and vet find that it becomes a magnet by induction as 
 before. 
 
 Or we may show this same thing by another and more 
 curious experiment. Let a horse-shoe magnet be placed 
 poles upward, and lay across its ends a piece of stiff card-
 
 MAGNETISM. 151 
 
 board or a piece of glass. Sprinkle iron filings upon the 
 cardboard and at- the same time gently tap it with the 
 linger. The filings will then be seen to collect in clusters 
 around the poles of the magnet and to arrange themselves 
 in strange curves from pole to pole. 
 
 Now in this experiment each little filing becomes a 
 magnet by induction through the card or glass, and then 
 each pole of one attracts the opposite pole of its neighbor, 
 so that they cling to each other in curves and clusters
 
 ELECTRICITY BY FRCITION. 
 
 Describe the experiment with the glass rod. 
 
 Fig. 79 shows the results of an easy and amusing ex- 
 periment. A glass rod, or perhaps a stick of sealing-wax, 
 must be rubbed briskly with a flannel cloth tor a few 
 
 Fig. TO. 
 
 moments, and then held near to pieces of some light sub- 
 stance, such as bits of cotton or balls of pith taken from 
 the elder-bush or corn-stalk. These light bodies will 
 quickly jump upward against the rod, and then, as if dis- 
 appointed with their visit, as quickly jump away again.
 
 ELECTRICITY BY FRICTION. l.-j3 
 
 What does this experiment show? We see by 
 
 this experiment that rubbing glass with flannel gives to 
 the glass a power which it did not have before the power 
 to attract and to repel light substances. 
 
 What is this power called? This new power 
 aroused in the glass is called electricity. In this case the 
 electricity is produced by friction. 
 
 Does friction always produce electricity? 
 Whenever substances are rubbed together electricity is 
 evolved. Arid yet if an iron rod is used in place of the 
 glass in the experiment (Fig. 79), the pith balls will not 
 stir from the table, because the electricity flies along the 
 surface of the iron and away through the hand as fast as 
 it is produced. 
 
 What are conductors and non-conductors? All 
 bodies which will allow electricity to pass over their sur- 
 faces freely are called conductors of electricity. Iron is a 
 good conductor, and so are other metals and many com- 
 mon substances besides. 
 
 Bodies which, like glass, will not allow electricity to pass 
 freely over them are called non-conductors. Besides glass 
 many other common substances are non-conductors. Air 
 is one of the most perfect of them all. And among others 
 it is well to mention India-rubber, sealing-wax, and silk. 
 
 What is an electroscope ? An instrument to detect 
 the presence of electricity in any body is called an electro- 
 scope. Fig. 80 will give a good idea of one of these instru- 
 ments. It is only a little ball of pith hung by a silk cord 
 from the end of a standard. 
 
 The glass rod, after being rubbed with the flannel cloth, 
 will show its electricity by attracting the pith-ball. Chi 
 coming in contact with the glass the pith itself becomes 
 electrified, and then jumps away from the glass. 
 7*
 
 154 
 
 NATURAL PHILOSOPHY. 
 
 What is an electric machine ? The glass rod and 
 sealing-wax will give electricity enough only to show 
 itself distinctly. When it is to be obtained in greater 
 
 Fig. 80. 
 
 force other apparatus must be used. Any apparatus l>y 
 which electricity of considerable force is obtained may be 
 called an electrical machine. 
 
 The most common form of the electrical machine con- 
 sists of a large circular glass plate, with its axle resting 
 upon pillars. This plate is turned with a crank, and in 
 turning it rubs between two rubbers. This friction gives 
 the electricity. Then there is a brass ball or cylinder 
 resting upon a glass pillar which takes the electricity from 
 the glass plate. This ball or cylinder is called the prime 
 conductor, and the electricity for experiments is taken 
 from it.
 
 ELECTRICITY BY FRICTION. 155 
 
 In what two ways does electricity act? The 
 
 experiment with the electroscope described a little while 
 since, shows that electricity acts both by attraction and 
 repulsion. Look back, and read that experiment again. 
 
 Will glass and sealing-wax act alike ? The elec- 
 troscope will help us to show that the electricity from 
 glass and that from sealing-wax do not act alike. 
 
 Let the glass rod be rubbed and once brought in con- 
 tact with the pith-bail (Fig. 80). The ball will after this 
 be repelled by the glass. Next rub a stick of sealing-wax, 
 and then hold it near to the pith : the little ball will 
 quickly fly toward it, being attracted by the sealing-wax. 
 Do not let it touch the sealing-wax, and you will find that 
 every time the glass comes near it the ball will be r<-pell<?d, 
 and every time the wax approaches it the ball will be 
 attracted. The glass and sealing-wax act in exactly oppo- 
 site ways. 
 
 How are these two actions named? The elec- 
 tricity of the glass has been called positive electricity, and 
 that from the sealing-wax has been called neyative elec- 
 tricity. 
 
 Now, whenever other non-conductors are rubbed, some 
 of them will give positive electricity and others negative 
 electricity. But when we speak of positive electricity we 
 mean simply that it is like the electricity from glass, and 
 the term negative electricity means only that the elec- 
 tricity is like that obtained from sealing-wax. 
 
 What is the law of attraction and repulsion ? 
 The action of these two forces is always in obedience to 
 the following law : 
 
 Bodies having the same kind of electricity repel each 
 other, but having opposite kinds they attract each other.
 
 156 NATURAL PHILOSOPHY. 
 
 
 
 Show that electricity is only on the outside 
 surface of a body. Fig. 81 shows a curious experiment 
 which teaches us where the electricity of a body is to be 
 
 Fig. 81. 
 
 found. A sack made of muslin and in the shape of a 
 cone is fastened to a metallic ring upon a standard. The 
 sack is to be held out by a long silk cord. When the 
 ring is brought in contact with any body already charged 
 with electricity, the force will spread into the ring and 
 from it into the sack. On examining the sack its outside 
 surface will show the presence of electricity, but its inside 
 surface will not. Now by taking hold of the other end 
 of the cord the sack may be turned inside out ; and, on 
 examining again, the electricity is found to be on what is 
 now the outside, while not a trace of it can be found upon 
 the inside. No matter how often nor how quickly the 
 sack is turned inside-out, the electricity will always be 
 found on the outside. 
 
 Can a conductor be charged with electricity ? 
 
 Since electricity will pass freely over the surface of any
 
 ELECTRICITY BY FRICTIOX. 157 
 
 conductor, it would at first seem to be impossible to make 
 it remain, or, in other words, to charge the conductor. 
 But k-t the conductor be placed upon a glass support, and 
 the electricity will have no means of escape, so that it will 
 be compelled to stay. 
 
 A body which does not touch any other conducting sur- 
 face is said to be insulated. The electricity can not stay 
 on the surface of any conductor unless it be insulated. 
 
 Can a pointed conductor be charged? A con- 
 ductor with a pointed wire reaching out from its surface 
 will not retain electricity even when insulated. The force 
 passes off from the point into the air, and seems to be lost. 
 
 A candle flame held in front of such a point will be 
 blown as if struck by a breeze of air, and indeed it is, for 
 the air electrified by the point is repelled and moves 
 away : it is this current of air that wafts the flame. 
 
 Suppose a pointed conductor brought near to an 
 electrified body. If we put the pointed end of a wire 
 near to any body which is electrified it will draw the 
 electricity away without touching it. To point the finger 
 at the prime conductor of an electrical machine will be 
 almost enough to keep the conductor from being charged : 
 and an open penknife held in the hand and pointed at 
 the conductor will be found quite enough to draw away 
 the electricity as fast as it is evolved. 
 
 For -what purpose are pointed conductors used ? 
 Lightning-rods are pointed conductors, which are used 
 to protect buildings from being struck by lightning. 
 
 Lightning is nothing more than the light caused by 
 electricity in the air or clouds, and thunder is only the 
 noi.se that is made by the .electricity when it passes from 
 one cloud to another or to the earth.
 
 158 NATURAL PHILOSOPHY. 
 
 "When a cloud full of electricity floats along over a 
 house the electricity sometimes leaps into the building 
 and tears it to pieces, or perhaps sets it on lire ; but ii' the 
 house has a good lightning-rod reaching above the roof, 
 the point of the rod will take the electricity out of the 
 cloud silently and gradually, and in this way the disaster 
 may be prevented. 
 
 Who first took lightning from the clouds ? Dr. 
 Franklin first drew electricity from the clouds in such a 
 way as to be able to examine it, and prove that lightning 
 is nothing but electricity. 
 
 How did he doit? This discovery of the nature of 
 lightning was one of the most important ever made in 
 the science, and yet, Dr. Franklin made it simply byj^y- 
 ing a kite in a thunder-shower (Fig. 82). 
 
 He made his kite of silk instead of paper, and sent it up 
 with a hempen cord ending in a piece of silk cord, by 
 which the kite was held. It is said that he fastened a door- 
 key to the lower end of the hempen cord, and that after 
 his kite had been for some time sailing among the clouds 
 he touched this key with his knuckle and drew a spark 
 of electricity from it. The electricity in the cloud entered 
 the kite, and came down the hempen string to the key, 
 but could not go any farther because t" e silk cord was a 
 non-conductor. When the doctor presented his hand the 
 electricity in the key leaped into his knuckle. 
 
 Can electricity act through no --conductors ? 
 
 The following experiment will show that electricity can 
 and will act through non-conductors. 
 
 "We will suppose the ball C (Fig. 83) to be insulated 
 and charged with electricity. Another insulated conduc- 
 tor, AB. is slowly moved toward the ball, and when the
 
 ELECTRICITY BY FRICTION. 
 
 Fig. 82.
 
 160 NATURAL PHILOSOPHY. 
 
 end of it is still at some distance, the pith-balls, which have 
 all the time hung vertically, suddenly jump away and 
 remain as shown in the picture. 
 
 Fig. S3. 
 
 The action of the pith-balls shows that the insulated 
 conductor AB is electrified, and we see that the electricity 
 of the ball C must have acted through the air. It would 
 have the same effect through glass or other non-con- 
 ductors. 
 
 What is this action called ? This action of a 
 charged body through non-conductors is called induction. 
 
 What is its effect ? It will be found that the insu- 
 lated body AB is not electrified all over its surface alike. 
 Both kinds of electricity are found upon it. The ends 
 are most powerfully electrified, and the two ends are in 
 opposite conditions. The end B, most distant, has the 
 same kind of electricity as the charged ball C. 
 
 Such will always prove to be the case. Induction al- 
 ways causes ~both kinds of electricity to appear on the 
 surface of an insulated body. 
 
 How do we describe this condition of an insu-
 
 ELECTRICITY BY FRICTION. IQ[ 
 
 lated body? When botli kinds of electricity are devel- 
 oped on the surface of a body, it is said to be polarized. 
 
 Describe the electrical bells. Fig. 84 shows a 
 chime of bells, which are to be rung by electricity. Notice 
 how they are arranged. The two Fig. 84 
 
 outside bells are fastened by 
 metal chains to a rod of metal 
 which hangs from the end of the 
 prime conductor of an electrical 
 machine. The middle bell is 
 hung by a silk thread and has a 
 chain passing from it to the floor. 
 Finally, notice two little balls of 
 metal between the bells : these 
 balls are hung by silk threads 
 also. When the machine is in 
 
 operation, these little balls will fly back and forth and 
 ring the bells loudly. 
 
 Explain this experiment. The electricity from the 
 machine passes down the chains into the outside bells, but 
 it can not get into the middle bell nor into the little balls, 
 because their silk cords are not conductors. Now when 
 one outside bell is charged, its electricity will act through 
 the air upon the little ball and polarize it, and the elec- 
 tricity on that side of the ball nearest the bell is the other 
 kind from that in the bell itself. Then the two unlike 
 kinds attract each other, and this is what brings the ball 
 to strike the bell. 
 
 But when the ball touches the bell it takes electricity 
 from it, and then the two, having the same kind of elec- 
 tricity, repel each other. This is what throws the little 
 ball against the middle bell. But when it touches the 
 middle bell it gives away its electricity and is ready to be
 
 102 NATURAL PHILOSOPHY.- 
 
 polarized over again. It is first polarized, then attract- d, 
 then i'1'pcllt-d over and over again, perhaps fitly times 
 while we are giving the explanation once. 
 
 The chain from the middle bell is to conduct away the 
 electricity brought by the little balls. This is needed to 
 keep the middle bell from becoming charged, and stopping 
 the operation. 
 
 Describe the dancing pith-balls. If a plate of 
 metal is hung from the machine just above another simi- 
 lar plate which lies upon the table, and then if a handful 
 of pith-balls is just between the two, these balls will per- 
 form a lively dance whenever the machine is put in opera- 
 tion. Sometimes a glass shade is put over the disks to 
 keep the balls from Hying away from between them, as 
 they are otherwise very sure to do. 
 
 Each little ball is first polarized by the electricity in 
 the upper plate, and then attracted and afterward re- 
 pelled. 
 
 What do these experiments illustrate? These 
 experiments illustrate the fact that no light body is ever 
 attracted until after it is polarized by the charged body 
 toward which it flies. 
 
 Describe the charged goblet of water. A long 
 time ago a gentleman in France was experimenting with 
 electricity to see how it would affect water. A chain 
 from the conductor of his machine hung down into a gob- 
 let of the liquid, but it seemed to produce no effect,- and 
 he was about to take it away. He seized the goblet in 
 one hand and took hold of the chain with the other. The 
 moment that his fingers touched the chain, he received a 
 shock which convulsed his hands and gave him such a 
 fright that he did not quite get over it in two days.
 
 ELECTRICITY BY FRICTION. 163 
 
 Explain this experiment. The water in the goblet 
 was charged with electricity 1'rom the machine, and when 
 the hand was placed around the outside, it and the glass 
 were polarized, so that the outside and inside of the goblet 
 were in opposite conditions. When the other hand 
 touched the chain the arms and body made a conducting 
 road through which the two electricities could get together, 
 and their action through the person caused the curious 
 and unexpected feeling called the shock. 
 
 What should we notice in the arrangement of 
 the goblet ? Now we notice that there were three 
 things the water inside, the hand outside, and the glass 
 letw<. en, arid that the water and the hand are good con- 
 ductors, while the glass is not To put it in few words, 
 we see that there were two conductors kept apart by a non- 
 conductor. 
 
 Describe the Leyden jar. Any two conducting sub- 
 stances kept apart by glass will answer just as well, or 
 indeed very much better than the water and the hand. 
 Tinfoil is pasted over the surfaces of a glass jar, both 
 inside and outside, to within a few inches of the top, and 
 then the jar is covered with a cover made of hard wood, 
 through which passes a brass rod. There is a ball on the 
 top of this rod, and a chain at its lower end that reaches 
 down to the bottom of the jar. 
 
 How may the jar be charged? Let the jar be held 
 in the hand with its knob very near to the conductor of 
 the machine. Sparks of electricity will fly into the knob; 
 in a dark room they look like little flashes of lightning. 
 After a while, when the sparks are .feeble, the jar is said 
 to be charged. 
 
 How may it be discharged ? Should a person acci- 
 dentally touch the knob of the charged jar with one hand
 
 1G4 NATURAL PHILOSOPHY. 
 
 and the outside tinfoil with the other, he would feel a 
 shock which would startle and perhaps injure him. His 
 body is a good conductor, and the shock is due to the 
 discharge of the jar through it. A discharge will always 
 occur when any conductor reaches from the knob to the 
 outside coating of the jar. A bent wire with a glass 
 handle is generally used for the purpose.
 
 ELECTRICITY BT CHEMICAL ACTION. 
 
 What is meant by chemical action ? "When a 
 piece of paper burns it ceases to be paper, as every one 
 knows, and changes into smoke and ashes. The nature 
 of the substance is changed during the action. Now this 
 shows what we mean by chemical action. It is an action 
 by which the nature of a substance in changed. 
 
 We will mention another case. Put some bits of zinc 
 into a goblet and pour upon them some weak sulphuric 
 acid. The fluid will soon begin to boil violently, and 
 bubbles of gas will be given off, so that often, if a lighted 
 match is held near, the gas will take fire. This will give 
 the curious appearance of water on fire. After a while 
 the action will stop, but not until much and perhaps all 
 the zinc has been used up. 
 
 Why is this action a chemical action? In this 
 case both the zinc and the acid are changed into other 
 substances, and on this account the action is called a 
 chemical action. 
 
 Will it produce electricity ? Now let a strip of 
 zinc and another of copper be placed side by side in a glass 
 vessel nearly full of weak sulphuric acid, but do not let 
 them touch each other. Have a wire fastened to the 
 upper end of each metal. It will be found that whenever 
 th se wires come together electricity will act through
 
 166 NATURAL PHILOSOPHY. 
 
 them. This electricity is due to the chemical action 
 going on in the vessel. 
 
 What is this apparatus called ? This simple appa- 
 ratus is called a Voltaic circuit. The electricity it gives 
 is ol'ten called Galvanism and often Voltaic electricity. 
 These names were given in honor of Galvani, who first 
 studied this force, and of Yolta, who also made ii a study, 
 and found out many new things about it. 
 
 What are the poles of the circuit? The ends of 
 the wires are commonly called the poles, of the circuit. 
 One is called the positive pole and the other is called the 
 negative pole. 
 
 When the poles are in contact, or when there is any 
 conductor which joins them toge.ther, the circuit is said 
 to be closed, but when they are separated the circuit is 
 said to be open. 
 
 What effects can this circuit produce ? The elec- 
 tric force in this simple circuit is very weak. It can make 
 its presence known by a feeble spark seen in the dark at 
 the moment when the wires are separated, but which can 
 be seldom seen at all in daylight. It also causes bubbles 
 of gas to rise alongside of the copper plate when the circuit 
 is closed. And, what is more curious still is that it will 
 turn a magnetic needle, when the wires are laid length- 
 wise of the needle, without touching it. You will learn 
 more of these effects at some future time. 
 
 Can greater effects be obtained? Very much 
 greater effects can be obtained by using a different appa- 
 ratus. One of the best kinds is the Bunseris lattery. 
 
 The picture Fig. 85 will help us to understand how 
 the Bunsen's battery is arranged. It consists of 
 
 1. A glass or earthenware vessel, containing 
 
 2. Dilute sulphuric acid. In this stands
 
 ELECTRICITY BY CHKMICAL ACTION. 
 
 3. A hollow cylinder of zinc. Inside of this is 
 
 4. A porous earthen cup, filled with 
 
 5. Strong nitric acid. And finally in this 
 
 6. A rod or block of carbon is immersed. 
 
 Fig. So. 
 
 167 
 
 One wire or metallic bar goes from the carbon ; this is 
 the positive pole: another goes from the zinc, and this is 
 the negative pole. 
 
 How may the power of the battery be increas- 
 ed? By joining several of these single batteries together 
 electricity of almost any power may be obtained. 
 
 The zinc of each cell may be joined to the carbon of 
 the next. The first carbon is the positive pole, and the 
 last zinc is the negative pole. When these poles are 
 joined together a powerful electric force is obtained, by 
 which most wonderful effects are caused.
 
 168 
 
 NATURAL PHILOSOPHY. 
 
 Describe the electric heat. If a fine iron wire is 
 stretched between the poles of a strong battery it will be 
 quickly heated white hot, and actually burned up, even 
 when several inches long. Copper, zinc, and other metals 
 melt readily and are burned by electricity. 
 
 How is the electric light obtained ? In the pic- 
 
 tare, Fig. 80, the electric light is represented as in use. 
 The battery, a very strong one of 50 or perhaps 100 cells,
 
 ELECTRICITY BY CHEMICAL ACTION. 169 
 
 stands upon the floor. The lamp stands upon the table. 
 The light is obtained by having the poles tipped with 
 charcoal and then drawing them a little way apart. It 
 is of the most dazzling brightness. The light is not 
 due to the burning of the charcoal, for it is as bright 
 when made in a vacuum where the carbon cannot burn 
 at all. 
 
 In the picture this light is being used in a microscope. 
 It is made to pass through an insect and afterwards 
 through convex lenses, which form a magnified image of 
 the creature upon the screen. 
 
 "With this instrument it is possible to show the smallest 
 objects magnified almost indefinitely. A human hair 
 will appear as large as a broomstick ; an ordinary flea 
 will look the size of a sheep, and the smallest animalcules 
 will be visible in all their beauty of form and color as 
 clearly as if they were seen with the naked eye." 
 
 The electric light has been used also, sometimes, to en- 
 able workmen to labor at night. When it is necessary to 
 accomplish a great work speedily, it may go on without 
 stopping, the sun giving the workmen light by day and 
 the electric light by night. See Fig. 87. 
 
 How may water be decomposed ? We need only 
 put the poles of a strong battery into water and bubbles 
 of gas wiil rapidly rise from both. The water is changed 
 into two gases. 
 
 The picture, Fig. 88, will show us how the experiment 
 is usually made. 
 
 We see that the wires from a battery go up into a 
 vessel of water and that two tall tubes are placed over 
 them. These tubes are there to catch the little bubbles 
 of gas into which the water is changed. They were filled 
 with water at the beginning of the experiment, but as the
 
 170 
 
 NATURAL PHILOSOPHY. 
 
 Fi?. 87.
 
 ELECTRICITY BY CHEMICAL ACTION. 
 
 171 
 
 bubbles rise they drive the water out. One tube is tilling 
 twice as fast as the other, you may notice. 
 
 Fig. SS. 
 
 What gases are these ? One of these gases is hydro- 
 gen, the other is oxygen. The hydrogen is given off most 
 rapidly. 
 
 The electricity in this case helps- us to learn that water 
 is made up of two very light and colorless gases, hydrogen 
 and oxygen, and that it contains twice as much of the 
 first a^ of the second. 
 
 A great many other substances may be decomposed by 
 electricity.
 
 ELECTRO-MAGNETISM. 
 
 What apparatus needed to show the effect of 
 electricity upon iron ? Take a long piece of covered 
 copper wire and wind it many times around a rod of soft 
 iron. Fix one end of this wire to one pole of the battery 
 and the other end to the other pole. The electricity will 
 then act around the iron rod and we can study its effects. 
 
 What effect is produced ? Open the circuit by tak- 
 ing the wire away from one pole of the battery, and we 
 may by trial find that the iron has no especial attraction 
 for small bits of iron which are brought in contact with 
 it ; but close the circuit, and instantly the bits of iron will 
 cling to the rod and be held by it as long as the elec- 
 tricity acts. 
 
 This experiment shows that electricity makes a bar of 
 iron magnetic by acting around it. 
 
 What are such magnets called ? Such magnets are 
 called electro-magnets because their magnetism is caused 
 by electricity. 
 
 In what form are they usually made? These 
 electro-magnets are generally made in the shape of the 
 horse-shoe magnet, and the wire is wound a great many 
 times around each branch. 
 
 In the picture (Fig. 89) we see one of the electro-mag- 
 nets fastened in a frame.
 
 ELECTRO-MAGNETISM. 
 
 173 
 
 This picture also shows the result of a curious experi- 
 ment. A box of nails is shown 'below. This box was 
 lifted until the magnet touched the nails and then was 
 
 Fi R . 89. 
 
 slowly let down again. The magnet lifted all the nails it 
 touched : these nails lifted others, and others then clung 
 to them until, as you see in the picture, a chain of nails 
 hung from the ends of the magnet and rested upon the 
 box below. 
 
 How long will the iron stay magnetic ? Just at 
 the moment the electric circuit is opened the nails will 
 drop, every one into the box. The iron will be a magnet 
 only while the electricity is acting around it. 
 
 What instrument acts on this principle ? The
 
 174 NATURAL PHILOSOPHY. 
 
 electric telegraph acts upon tins principle. In one city 
 there is a " key " by which a person may open and close 
 an electric circuit as often as he pleases. The wires of 
 this circuit reach over the country to a distant city an I 
 are there joined to the coils of an electro-magnet. Just 
 above the poles of this magnet is an armature kept a little 
 way from them by a spring. 
 
 Now let a person press the key with his finger and close 
 the circuit : the electricity will dart through the wires to 
 the distant city and around the electro-magnet, and the 
 magnet will pull the armature down. When he lifts his 
 finger the electricity will not act; the magnet ceases to 
 be magnetic and the armature is lifted by the spring. 
 Just as often as he presses the key the armature will be 
 drawn down. 
 
 The next thing to know in order to understand this 
 wonderful instrument, is that there is a steel point fastened 
 to the armature, so that every time the armature is drawn 
 to the magnet, the point is pulled against a strip of 
 paper and makes a mark upon it. A person in one city 
 can thus be making marks upon paper in another city 
 many miles away. 
 
 The marks consist of dots and straight lines, and each 
 letter of the alphabet is represented by some arrangement 
 of these marks. For instance, a dot followed by a dash, 
 
 thus : , means A ; and a dash followed by a dot, thus, 
 
 - - stands for N ; while a dash and two dots stands 
 
 for D. You see, then, how the man at the key may write 
 the word AND by making these dots and dashes. It 
 would look like this, - - - - -, and one who knows the 
 
 alphabet by dots and dashes can read these characters 
 even when a thousand miles away from the place from 
 which the message is sent.
 
 ELECTRO-MAGNETISM. 175 
 
 This description is only an outline of the plan of the 
 electric telegraph. Every boy and girl should seek a 
 chance to see the instrument itself, for from it can be 
 learned, better than from any book, just how the messages 
 are sent and taken.
 
 MACHINERY. 
 
 What is shown in Fig. 90 ? In Fig. 90 we see the 
 picture of a man trying to move a block of stone. It is. 
 much too heavy for him to lift, and you notice that he has 
 taken a long bar to assist him. Putting one end of it 
 under the stone, and resting the bar upon a block C, he 
 pushes down upon the other end, and in this way lifts the 
 stone. 
 
 Fig, 90. 
 
 Explain the action more fully. We see that the 
 bar rests upon the prop C, and that the end B cannot be 
 pushed down without moving the other end A, up. Now 
 when the part C B is so much longer than the part C A, 
 the man by his own weight can lift a stone very much 
 heavier than he himself is. 
 
 What is this bar called ? This bar is called a lever. 
 This name is given to any bar that can be used in this 
 manner. A lever is an inflexible bar which can turn 
 freely upon a pivot or prop. 
 
 Define Power, Weight, and Fulcrum. The strength 
 which the man exerts at B is called the power/ we speak
 
 MACHINERY. 177 
 
 of the stone to be moved as the weight, and call the prop 
 C the fulcrum.. 
 
 Levers are not always moved by hand, nor are they 
 always used to move stones, and yet these same terms are 
 used. The power is any force by which the lever is to be 
 moved. The weight is any resistance which is to be over- 
 come. The fulcrum is the support on which the lever 
 moves. 
 
 What does Fig. 91 A show ? In Fig. 91 A, the lever 
 is shown without the laborer and the stone. P represents 
 the place of the power ; "VV represents the place of the 
 weight, and F the fulcrum. The fulcrum is in this case 
 between the power and the weight. 
 
 Fig. 91. 
 
 II 
 
 
 Is this always the case ? But this is not always the 
 order of arrangement. In Fig. 91 B, the weight W is be- 
 tween the fulcrum F and the power P. The weight being 
 near the fulcrum, is lifted by raising the more distant end 
 of the lever. 
 
 In Fig. 91 C. the power is between the fulcrum and the 
 weight. Here also the power must pull upward to lift the 
 weight. Now these three figures represent levers of the 
 three classes. 
 
 Describe the three classes of lever. In a lever of
 
 178 NATURAL PHILOSOPHY. 
 
 the first class the fulcrum is between the power and the 
 weight (Fig. 91 A). In a lever of the second class the 
 weight is between the power and the fulcrum (Fig. 91 B). 
 In a lever of the third class the power is between the ful- 
 crum and the weight (Fig. 91 C). 
 
 All levers, of whatever form or use, belong to these 
 three classes. 
 
 Mention some levers of the first class. The handle 
 of a common pump is a lever of the first class : the piston 
 and the water are the weight, the hand of whomsoever does 
 the work is the power, while the pivot on which the handle 
 turns is the fulcrum. 
 
 The balance of a .tradesman is another example : the 
 body to be weighed, put into one scale-pan, is the weight ; 
 the weights put into the other pan are the power ; while 
 the pivot on which the beam turns is the fulcrum. 
 
 Examples of the second class. The handles of a 
 whealbarrow are levers of the second class: the axle of 
 the wheel is the fulcrum on which they turn when lifted ; 
 whatever is placed in the barrow is the weight, while the 
 hand of the laborer is the power to lift it. 
 
 An oar is another example. The boat is the weight to 
 be moved ; the hand of the boatman is the power to move 
 it ; while the water against which the other end of the oar 
 presses is the fulcrum. 
 
 Example of the third class. When a ladder is raised 
 by resting one end on the ground and lifting upon a round 
 somewhat farther up, it is a lever of the third class. The 
 end on the ground is the fulcrum; the hand of the man is 
 the power ; while the weight of the ladder, most of which 
 is beyond the hand, is the weight. 
 
 What relation exists between power and weight ? 
 In Fig. 90, if the distance from A, the place where the 
 weight rests on the lever, to C, the fulcrum, is one-fourth 
 the distance from C to B, then the weight may be balanced
 
 MACHINERY. 179 
 
 by a power only one-fourth as great. The power will be 
 j.ust as many times less than the weight as the distance 
 from it to the fulcrum is times greater than the distance 
 from the weight to the fulcrum. If the distance of the 
 power from the fulcrum is ten times as far as the distance 
 from the weight to the fulcrum, then the weight will be 
 balanced by a power only one-tenth as great as itself. 
 f> State the law. This principle, briefly stated, is as fol- 
 lows : " The power and weight will balance each, other 
 when they are to each other inversely as their distances 
 from the fulcrum." 
 
 This principle is called the law of equilibrium for the 
 lever. It holds good in all the three classes. 
 
 To move the weight, the power must be a little greater 
 than this principle would make it. 
 
 What is a compound lever ? When two or more 
 levers are made to act one upon another in succession, so 
 that a power applied to the first lifts a weight applied to 
 the last, the instrument is called a compound lever. 
 
 What does Fig-. 92 show? Fig. 92 shows how a 
 weight may be lifted by fastening one end of a rope to it 
 
 Fig. 92. 
 
 c 
 
 and winding the rope up on a cylinder. In this way water 
 is often raised from deep wells. By turning the crank
 
 180 NATUKAL PHILOSOPHY. 
 
 the upper part of the rope is wound upon the cylinder, and 
 the bucket, hooked upon the lower end, is raised. 
 
 What often takes the place of the crank? Instead 
 of a crank 13, to turn the cylinder, a wheel C is very often 
 used. The power is applied to the circumference of the 
 /heel, sometimes by means of a rope, sometimes by means 
 i a band, sometimes by means of cogs, and in various 
 other ways. 
 
 What are such machines called? A machine such 
 as represented in Fig. 92 is called a " Wheel and axle.' 1 '' 
 The cylinder upon which the rope winds is the "<:?&>," 
 while the "wheel" may be an actual wheel or a crank, 
 both of which are shown in the picture, or it may have 
 other forms still. Whatever shape this part may have 
 the machine is called the " wheel and axle." 
 
 What is the relation between the power and the 
 weight? When the weight is just as many times greater 
 than the power as the radius of the wheel is greater than 
 the radius of the axle, the two forces will just balance. 
 In other words : " The power and weight will balance, 
 when the power is to the weight as the radius of the axle 
 is to the radius of the wheel." 
 
 This principle is called the law of equilibrium for the 
 wheel and axle. To move the weight, the power must be 
 made greater than this law requires. 
 
 How are wheels and axles often combined ? Many 
 wheels and axles are sometimes turned by a single power. 
 In this case motion is communicated from wheel to axle 
 or from Tixle to wheel by means of bands or cogs. If by 
 u power on one wheel its axle is turned, and a band passes 
 around this axle and a second wheel, the second axle will 
 be turned also. Great power may in this way be ob- 
 tained. 
 
 How may rapid motion be secured? If the power 
 be applied to the circumference of the axle instead of
 
 MACHINERY. 
 
 181 
 
 Fig. 93. 
 
 the wheel, and if a band pass around the wheel and a second 
 axle, the second wheel will be put into very rapid motion. 
 
 What is shown in Fig. 93 ? In this figure we can 
 see how a heavy weight may be 
 lifted by fastening it to the end 
 of a rope which passes up over a 
 grooved wheel, and then pulling 
 downward upon the other end. 
 
 What is such a grooved 
 wheel called? A grooved wheel 
 used for such a purpose is called 
 a pulley ; and in this case, since it 
 is firmly fastened in a fixed sup- 
 port, it is called a fixed pulley. 
 
 What advantage in its use? The only advantage 
 gained by means of the fixed pulley consists in being able 
 to change the direction in which the power acts. A man, 
 for example, can exert his strength to better advantage 
 putting downward than lifting upward, and if a load is 
 to be lifted the fixed pulley allows him to use his power in 
 this better way. He gains in no other 
 way ; if the load weighs 100 Ibs., he must 
 pull Avith a force equal to 100 Ibs., and in- 
 deed a little more, since the rope and pulley 
 take up some of his strength to move them. 
 What is a movable pulley? The case 
 is very different when the pulley is ar- 
 ranged as in Fig. 94. In this arrangement 
 the weight is hung from the axis of the pul- 
 ley, B, and is to be lifted by means of the 
 rope which is fastened to the beam at A, 
 and then after passing under the pulley 13, 
 goes over the fixed pulley C. By pulling upon the rope 
 at P, the pulley B will be lifted and will carry the weight 
 
 Fig. 94.
 
 182 
 
 NATURAL PHILOSOPHY. 
 
 up with it. A pulley which moves with the weight is 
 called a movable pulley. 
 
 "What advantage is gained? Now it is easy to see 
 that the weight is held up by the two branches of rope, m 
 and n, and that each branch holds one-half of it. But the 
 half which rests on m is sustained by the beam, leaving 
 only the other half, which rests on n, to be lifted by the 
 power at P. 
 
 Fig. 95. When, with a movable pulley, there are 
 
 two branches of rope to sustain the weight, 
 the power ma}' be only one-half the weight. 
 In Fig. 95 there are three branches of 
 the rope which hold the weight and share 
 it equally between them. In this case the 
 power need be only one-third as great as 
 the weight to balance it. 
 
 Wnat general principle does this il- 
 lustrate ? In all cases the power needed 
 to balance any weight w r ill be found by 
 dividing that weight by the number of 
 branches of the rope which supports it. 
 
 Will this law apply in all cases ? In the cases con- 
 sidered you will notice that there is a single rope winding 
 around all the pulleys. Now the law holds good when- 
 ever the weight is supported in this way, provided the 
 branches of the rope are parallel. .There are a great many 
 other ways of arranging the pulleys, not as common as 
 this, however, and in such cases the law stated above does 
 not hold good. 
 
 Mention some purposes for -which pulleys are 
 used. Pulleys are often used for lifting heavy articles of 
 merchandise to the upper stories of warehouses. They 
 may be seen also where buildings of stone are being erect- 
 ed, and heavy blocks are to be raised to considerable 
 height. But more numerous than anywhere else, you will
 
 MACHINERY. 183 
 
 find pulleys on shipboard, where they are used by the 
 seamen in managing the rigging of the ship. 
 
 What is an inclined plane ? "When a drayman wishes 
 to lift a cask of sugar from the sidewalk to his dray he 
 does not lay hold of it and raise it vertically, as he migh* 
 do with a lesser weight, but he accomplishes the work 
 far more easily by rolling it up along a plank reaching 
 obliquely from the ground to the dray. The inclined 
 surface of the plank is called an inclined plane. Any 
 inclined surface over which weights are to be moved is 
 an inclined plane. 
 
 What is meant "by the terms length and height of 
 the plane ? In Fig. 96 a weight W is shown resting on 
 an inclined surface A B, balanced by a smaller weight P. 
 Kow the distance A B is called the length of the inclined 
 plane, and the vertical distance C B is called the height of 
 the plane. 
 
 Fig. 96. 
 
 In the plane used by the drayman the length of the 
 plank from the sidewalk to the dray is the length of the 
 inclined plane, while the height of the dray above the 
 walk is the height of the plane. 
 
 What relation exists between power and weight ? 
 If the weight W is 100 Ibs. and the height of the plane 
 is one half the length, then the power to balance the 
 weight need be only one-half the weight, or 50 Ibs. 
 
 If the height is ^ O f the length of the plane, the power 
 need be only y 1 ^ the weight. 
 
 The general principle or law is this ; the power and
 
 184 
 
 NATURAL PHILOSOPHY. 
 
 weight will balance when the power is to the weight as 
 the height of the plane is to its length. 
 
 Under wfrat conditions will this law hold good ? 
 This law holds good only when the power is exerted in a 
 direction parallel to the length of the plane; in any other 
 direction the relation is different. Moreover, friction of 
 the weight upon the plane has much to do with the re- 
 lation of power to weight. The law would be quite true 
 only when friction did not exist, a case which never occurs 
 in practice. If the weight is to be balanced on the plane, 
 then friction helps the power to do it ; but if the weight is 
 to be moved up the plane, friction is a hindrance instead 
 of a help. 
 
 Explain Fig. 97. This picture, Fig. 97, shows how 
 barrels are drawn up from or let down into a cellar. It is 
 a case of the inclined plane which you can easily under- 
 stand and explain without further help. 
 
 Fig. 97. 
 
 How does a woodman sometimes split his blocks ? 
 When blocks of wood are to be split, a smaller block of 
 wood or metal, made thick at one end and tapering to an
 
 MACHINERY. 185 
 
 edge at the other, is driven into the end of it, as shown in 
 Fig. 98. The tapering block is called a wedge. 
 
 Fig. 98. 
 
 What are the power and weight in this case ? 
 
 The energy of the woodman's blows upon the back of the 
 wedge is the power / the cohesion of the wood is the 
 weight, and it is not easy to find the relation between these 
 two forces. Hence we cannot here state any law accord- 
 ing to which the ratio of power and weight, when they 
 balance each other, can be found. 
 
 What familiar instruments act on the principle of 
 the wedge ? The chisel of the carpenter is a wedge ; so 
 is the blade of a pocket-knife ; each having a sharp edge, 
 and being thicker at a distance from it. The chisel is 
 usually driven by blows; the knife urged by pressure; but 
 the cohesion of the wood is the resistance, or the weight to 
 be overcome by both. All cutting and piercing instru- 
 ments are different forms of the wedge. 
 
 Describe the screw. The screw consists of a cylinder 
 having a spiral groove cut around its circumference. The 
 small screw used by carpenters for joining the parts of 
 their work, on a small scale illustrates this arrangement. 
 When the screw is to be used as a machine, the cylinder is 
 made larger, sometimes a few inches, sometimes several 
 inches in diameter. The projecting edges left between the 
 parts of the spiral groove are called threads, and the screw* 
 works through an opening in a firm block having a spiral 
 groove cut upon its interior surface into which these
 
 186 NATURAL PHILOSOPHY. 
 
 threads just fit. This block is called the nut, or sometimes 
 the concave screw. 
 
 The top ol the screw is called its head : it is the part to 
 which the power is applied. The power is generally 
 applied by means of a lever reaching outward from the 
 head. 
 
 Explain Fig. 99. This cut represents a screw set in a 
 firm framework, as it is often used when a great pressure is 
 rig. 99. to be exerted. C represents tiie head 
 
 of the screw, and B the lever by which 
 it can be turned. The block N through 
 which the screw works is the nut. 
 
 AVhen the screw is turned by the 
 lever it advances through the nut and 
 pushes the movable block E F down 
 upon the body to be pressed. An 
 enormous pressure may, in this way, 
 be exerted upon this body. 
 
 The power is applied at B, and the pressure exerted is 
 the weifjht. 
 
 What is the relation between the power and 
 weight ? As the screw is turned the power at B must 
 travel around a large circle, and when it has gone once 
 around, the screw will have advanced through the nut a 
 distance a c, the distance betweeix two contiguous parts of 
 the thread. Now it is found that the weight will be as 
 many times greater than the power, as the circumference 
 through which the power travels is times greater than this 
 little distance a c. In other words, the law may be stated : 
 " The power and weight will balance, when the power is to 
 the weight as the distance between two contiguous threads 
 is to the circumference in which the power moves.'' 
 
 How many machines have now been described ? 
 We have now described six simple machines by which
 
 MACHINERY. l,s; 
 
 weights may be lifted or resistance overcome. Let us bring 
 their names together. 
 
 The Lever. The Inclined plane. 
 
 " Wheel and axle. " Wedge. 
 
 " Pulley. " Screw. 
 
 There are no others. All forms of machinery are made 
 by combining these six. Just as when you learned the 
 twenty-six letters of the alphabet you possessed all the 
 characters used in the many thousands of words of oar 
 language, so, having learned of these six simple machines, 
 you have learned of all the elements out of which in- 
 genious men have contrived the wonderful variety of 
 mac hinery to be found among civilized nations. 
 
 What one principle holds good in the action of all 
 these machines ? A small power acting swiftly may 
 put a large weight in slow motion ; or a great power act- 
 ing slowly may put a small weight in rapid motion. 
 
 This law holds good in all forms of machinery, what is 
 lacking in force must be made up in velocity. And in 
 just this lies the advantage of using machinery. It helps 
 man to transform velocity into power to overcome resist- 
 ance. No power can be created by it, and wherever 
 power is gained it is bought and. paid for in velocity. 
 
 By what agents is power to move machinery ex- 
 erted ? Animals, water, wind, and steam are the agents 
 or powers most commonly applied to move machinery. 
 Men or horses, by means of pulleys or wheels, may bo 
 seen lifting stones where large buildings are being erected. 
 Saw-mills and flouring-mills are often worked by u water 
 power" acting upon the circumference of large wheels. 
 Wind-mills for pumping water and for other purposes are 
 turned by the wind, while in the steam-engine and all the 
 vast machinery in the arts moved by it, the motive power 
 is steam.
 
 188 NATURAL PHILOSOPHY. 
 
 What is meant by the term " Horse-Power " ? Tl o 
 term horse-power is used in estimating the effect which a 
 steam-engine or other machine can produce. The term 
 has no reference to the animal whose, name is used : it 
 simply means a certain amount of work. A power to lift 
 33,000 Ibs. through 1 foot in 1 minute is a ' horse-power.'' 
 So when an engine is described as an engine of "'10 horse- 
 power," we understand that it is able to do a work equal 
 to that of lifting 10 times 33,000 Ibs. 1 foot in 1 minute. 
 
 THE STEAM-ENGINE. 
 
 How can steam be applied to machinery ? The ex- 
 pansive power of steam (p. 142) was known lonj before 
 any means were devised to use it in machinery. Animals 
 could be harnessed to a machine easily : water and wind 
 would act upon wheels and turn them, but how could 
 steam be harnessed or applied ? In the open air steam is 
 as feeble as an insect, but when confined in a close vessel 
 its efforts to expand produce enormous pressures. If it 
 can be confined and yet be properly brought against a 
 machine, the machine will be put in motion. This is 
 accomplished in the steam-engine. 
 
 How is this accomplished ? For this purpose a cylin- 
 der is provided, having a piston fitting its interior nicely, 
 and able to move smoothly from one end to the other back 
 and forth. Steam is made to enter the cylinder first at 
 one end of the cylinder, then at the other, and the pressure 
 of the steam behind the piston knocks it back and forth 
 from end to end with enormous force. 
 
 .Explain Fig. 100. Fig. 100 will illustrate this brief 
 description. C represents the cylinder, and P the pis'on 
 which can move freely from end to end. S represents the 
 pipe through which the steam comes from the boiler. It 
 enters the box shown by the open space, ?/, and the steam
 
 MACHINERY. 
 
 189 
 
 passes from there in a direction shown by the upper arrow, 
 
 and enters the cylinder at the top. Once in the cylinder 
 
 it expands against the piston and pushes it down, the 
 
 old and useless steam below, Flg J00 
 
 passing at fie same time through 
 
 an opening at the bottom, and 
 
 thence up to the space O, and 
 
 iway through a pipe not shown 
 
 in the cut. 
 
 When the piston has almost 
 
 reached the bottom of the cylin- 
 der, the block y slides up, and 
 
 covers the passage leading to the 
 
 top, but uncovers the one leading 
 
 to the bottom. The steam is 
 
 then forced through the lower 
 
 passage to the bottom of the 
 
 cylinder, and, expanding there, 
 
 drives the piston to the top again- 
 The block y then slides down 
 and opens the upper passage : the 
 steam goes to the top behind the piston and drives it 
 down. The block y then slides up again and opens the 
 lower passage : the steam goes to the bottom behind the 
 piston, and drives it to the top. 
 
 You see that by the slide valve, y, the steam goes 
 first to one end of the cylinder, then to the other, and thus 
 drives the piston P back and forth. 
 
 Now this piston P has a rod firmly fastened to it which 
 reaches up through the top of the cylinder, and which is 
 pushed out and drawn in by the moving piston. It is 
 called the "piston-rod." The outer end of this piston-rod 
 acts as a power to move levers or turn cranks, and thus 
 give motion to machinery. 
 
 The steam moves the piston, the piston moves tlio pis-
 
 190 
 
 NATURAL PHILOSOPHY. 
 
 ton-rod, and the piston-rod gives motion to the machinery 
 outside. 
 
 Explain Fig. 101. The picture, Fig. 101, shows how 
 the piston-rod gives motion to inadmiery in one form of 
 
 the steam-engine. 
 
 Fig. 101. 
 
 '''' ' _ ' ' '' ' !" 
 
 In the first place, at the left, we see the cylinder with 
 0110 side cut away, sj that we may see the piston (P) inside. 
 The steam is supposed to be entering the valve-box at S 
 and going to the upper part of the cylinder, pushing the 
 piston down, just as we described when we explained Fig. 
 100. 
 
 The piston-rod A D is fastened to one end of the large 
 and strong lever II K. As the piston goes down it pulls 
 this end of the lever down and throws tlie other end K, up. 
 When the piston rises in the cylinder the piston-rod pushes 
 the lever end II, up, and throws the other end K, down.
 
 MACHINERY. 191 
 
 Now as the lever at K goes up and down, it pulls and 
 pushes upon the strong arm J, and in this way turns the 
 crank C. The large wheel W W, fixed upon the axle, will 
 thus be put in motion. 
 
 Where may -we find engines of this form ? This 
 form of engine is often used on steamboats. The great 
 lever II K may be seen above decks moving alternately up 
 and down when the steamer is in motion. It is sometimes 
 called the "walking-beam." The strong arm J reaches 
 clown into the boat and tarns an enormous iron axle, which 
 reaches quite through the boat from side to side, and has a 
 paddle-w/ieel at each end. 
 
 Is motion always communicated in this -way? 
 The way in which the piston-rod gives motion to other 
 parts is very different in different engines. There is no 
 " walking-beam " in a locomotive, you know. You can see 
 in Fig. 101 that the piston-rod is jointed at B. Now in 
 the locomotive engine the outer end of the jointed part D 
 is fastened directly to the "drive-wheels'" of the locomotive 
 at a point between its centre and its circumference. The 
 cylinder lies horizontal, and as the rod A moves back and 
 forth the " drive- wheel " is turned and the locomotive 
 rolled forward. 
 
 Engines used for other purposes than those just mention- 
 ed are made Jn different forms. A visit to some manu- 
 factory, where you may see for yourself the construction 
 and action of one of these most wonderful machines, will do 
 more to make you acquainted with its parts and their uses 
 than can be gained from books, even though you study 
 long and faithfully. 
 
 What is a high-pressure engine ? Sometimes the 
 steam, after pushing the piston, is made to escape into the 
 air in puffs. This is done in the locomotive engine, and it 
 gives rise to the irregular puffs heard especially when the 
 engine starts. Now this steam must \>& puihed out against
 
 192 NATURAL PHILOSOPHY. 
 
 the air, which presses with a force of 15 Ibs. to the square 
 inch to keep it in the cylinder. The steam which moves 
 the piston must exert force enough to overcome this pres- 
 sure before it can exert any to move the machinery outside. 
 Its pressure must be at least 15 Ibs. to the square inch 
 higher than the machinery would otherwise require. On 
 this account the engine is called a high-pressure engine. 
 
 What is a low-pressure engine ? In other engines 
 the steam, after having moved the piston, is allowed to 
 pass through a pipe into a closed chamber, where, by cold 
 water, it is condensed. The withdrawal and condensation 
 of the steam removes the pressure from in front of the 
 piston, and the force of the steam behind it may be all 
 expended in moving the machinery. Since the steam 
 has less work to do, it can do it with less pressure than in 
 the high-pressure engine, and this form is on this account 
 called the low-pressure engine.
 
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