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is quite inappreciable by our senses. The weight of a stone is due, not to its attraction towards the stones in its neighbourhood, but to its attraction by every particle of the globe. A moment's reflection will convince any one that if the united attractions of every portion of this vast mass suffices only to produce the small downward energy which we call weight, then the effect of even a mountain must be quite insignificant. Small as it is, however, the delicacy of astronomical instruments has enabled us to detect it. By suspending a plumb-line, i.e., a string with a heavy ball attached to it, alternately on opposite sides of the mountain Schehallion, and by other means, it has been ascertained both that terrestrial matter does exert the attraction spoken of, and that the mass of the earth acts with a force which, on the whole, is between five and six times as great as if it were a globe of water of the same size.
2. Gravity. It has been stated that the aggregate attraction of every portion of the mass of the earth on a body placed at its surface, tends to draw it towards the centre of the earth, or gives it weight. This aggregate is called the force of gravity. It diminishes as we ascend a mountain, or rise in the air in a balloon. But the change of weight on this account is very small indeed. A pound of lead balanced by a spring at the surface of the earth, would be found to have lost about three grains at the height of a mile.
3. Capillary Attraction.—Another action of one particle on those which lie around it, is that which takes place at the junction of a solid and a fluid.
On examining the surface of a cup of tea or milk, it will be seen that it is curved upwards towards the margin. If the cup had contained quicksilver instead of tea, it would have been curved downwards. Possibly this phenomenon is a consequence of the law of universal gravitation ; but in the absence of any exact knowledge of the arrangements of the particles of solid and fluid bodies, we can explain the facts only by the assumption of another law, viz., that the particles which are very near a given particle, exert an attraction on it which is vastly greater than the aggregate of the attractions of all the other portions of the body. This is called the law of molecular attraction. To it are referred the phenomena of cohesion, by which bodies hold together-of the ascent of water in a lump of sugar, and of soap in the vessels of plants—also of melting, and the consequent tendency of water to escape through the pores of bodies, a tendency not due to its fluidity alone. Mercury is as fluid as water, but you may carry it in a pocket-handkerchief with safety.
4. Animal Strength.—This species of force needs no illustration. Suffice it to say, that when the amount of work a'man or a horse can do has to be estimated, the measure is usually stated in the number of pounds which he can lift to the height of one foot in a day. A rough estimate of a man's work is, that he can lift 10 lbs. 10 feet in a second, and continue at his work ten hours in the day. A horse-power is usually reckoned at 33,000 lbs. raised a foot per minute, and is thus equivalent to that of five or six men.
5. Friction. The forces which we have enumerated may be termed active forces ; that which we have now to consider is properly a passive force—its tendency is to prevent motion. When two hard surfaces are completely smooth, the one slides on the other with great readiness—in skating, for example. But with ordinary surfaces such is not the case. By their attrition a force is called into play in the direction of the planes of contact, which resists their relative motion. This force plays a most important part in the economy of nature. Without friction it would be impossible to walk even on a level road. But, important as this force is, its agency will be left out of account when we treat of the equilibrium of machines, seeing that it does not act uniformly, nor even at all when the other forces are in equilibrium amongst themselves. The law of friction is, that it is proportional to the pressure, other things remaining the same, and does not depend on the size of the surfaces which are in contact. In going up a hill, the heavier you are, the more friction does the ground afford, but you experience the same amount whether you press the earth with the sole of your foot or with the toe only.
HOW SOUND TRAVELS, AND HOW FAST IT TRAVELS— ECHOES
SECT. 1. How does sound travel through the air ? Let me try to answer this question. Imagine a row of boys standing close side by side, and that the last boy of the row stands close beside a wall or a glass window. Suppose somebody to give the first boy a push in the direction of the line of boys; the first boy knocks against the second and recovers himself, the second knocks against the third, the third against the fourth, and so on, each boy recovering himself after he has sent on the push to the boy next him. The last boy of the row would be pushed up against the wall, or through the window as the case might be.
Now, when a gun is fired, a percussion cap exploded, a bubble of explosive gas ignited, or when a peal of thunder occurs, the air at the place of explosion receives a sudden shock, and this shock is transmitted from particle to particle through the air, in a manner closely resembling the transmission of the push from boy to boy. There is a passage leading from the ear towards the brain; at a certain place a thin membrane called the tympanum, is drawn across this passage, the membrane and the cavity which it stops being called the drum of the ear. Well, the air is pushed against the head of this drum, just as we have supposed the last boy of our row to be pushed against the wall or the window, only with infinitely greater rapidity. The membrane is thus thrown into motion, and this motion is communicated to the nerve of hearing. It is thus transmitted along the nerve to the brain, and there produces the sensation of sound. Nobody understands how this motion is converted into a sensation ; it is one of the mysteries of life, regarding which the youngest boy who reads this page knows just as much as I do myself.
How fast does the shock travel through the air ; or, in other words, what is the velocity of sound ? The answer is, about 1100 feet a second. It travels more quickly in warm than in cold weather. Through water it travels about five times as fast as through air, and through wood it travels more than twice faster than it does through water. I once took a man and a hammer with me into Hyde Park, London, where there are very long iron rails. I placed my ear close to a rail, sent the man to a distance, and caused him to strike the rail with the hammer. For every blow he gave the rail I heard two, and the reason is, that the sound of each stroke travelled through the air and the iron at the same time; but through the iron it travelled with greater rapidity, and reached the ear sooner, the shock transmitted by the air arriving a little while afterwards. If the air were absent there could be no transmission of sound as at present; and where the air is very thin, as upon the tops of high mountains, the sound is much weakened. I fired a little cannon at the top of Mont Blanc last summer, and found the sound much weaker than when a similar cannon was fired on one of the Hampshire downs. This experiment was first made by the celebrated traveller De Saussure. I may add that sound travels just as quickly in thin air as in dense air ; it is only the intensity of the sound that is affected.
Sect. 2. Let us now seek to apply the little bit of knowledge we have gained in the foregoing section. Have you ever stood close beside a man when he has fired a gun ? If so, you will have seen the flash and heard the explosion at one and the same time. But if you stand at a distance from the man, you see the flash first, and hear the sound afterwards. The reason is, that while the light of the flash moves almost instantaneously, the sound requires some time to travel to your ear. Now let me ask you a question or two. Suppose you have a good watch, which informs you that the time which elapses between the flash and the sound is three seconds, at what distance would you be from the man who fires the gun? Of course you could tell me in a moment. These three seconds are the time required by the sound to travel from the man to you, and as the velocity of sound through air is 1100 feet a second, the man must be 3300 feet distant. An equally simple calculation enables you at once to tell whether a thunder storm is dangerous or not. Each peal of thunder appears to be preceded by a flash of lightning ; but if you were up in the clouds close to the place where the peal occurs, you would see the flash and hear the peal at the same moment, for they really occur together. If therefore a few seconds elapse between the flash and peal, it is a proof that the danger is distant ; but if the peal follow hot upon the flash, it shows that the danger is near. Never dread the sound ; if the flash pass without injury, the subsequent peal can do no harm.
SECT. 3. I want you now to turn your thoughts for a moment to the row of boys, of which I have spoken in the first section. Suppose, when the last boy is pushed up against the wall, that he, in recovering himself, pushes back against the boy next him, this second push, like the first, would propagate itself from the end to the beginning of the line of boys. In a similar way, when the pulse of air, which produces sound, strikes against a wall, it is reflected back and constitutes an echo. The reflected wave of sound moves with exactly the same velocity as the direct one. Now, suppose a gun to be fired at a distance 2200 feet from the side of
ise or of a mountain which reflects the sound, what time will elapse between the sound and the echo ? Here the sound has to travel from the gun to the wall, and back again, or a distance of 4400 feet ; and as the velocity of sound is 1100 feet a second, four seconds will elapse before the echo is heard. If you
upon the matter you will easily see that a wave of sound, after it has been once reflected, may strike upon a second object, which will reflect it a second time, and thus constitute a second echo. It is customary, when travelling up the Rhine, to fire a cannon at a certain place where the banks of the river rise in steep high rocks ; the waves of sound are reflected several times from side to side, thus producing a perfect babble of echoes, resembling the roll of thunder. The echoes which may be aroused in some of the mountain glens in Switzerland even by the human voice, are perfectly wonderful. I have known a valley to be filled with the wildest melody by a little boy singing the mountain jodel as he sat upon a rock and watched his goats.
Not only do solid bodies reflect sound in this way, but clouds do it also ; and this is undoubtedly one cause of the rumbling we hear after a peal of thunder. In firing cannon, it has been observed that when the sky was clear, the sound was sharp and echoless, but that as soon as clouds appeared above the horizon, the sonorous waves striking against the clouds were reflected back again and produced echoes. Sound is always reflected, wholly or partially, in passing from one medium to another. Even when