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stone. Rest the ball on this, and observe the size of the circular spot made on it. Now let the ball fall on the stone, and observe the larger circular spot made by the fall. This shows that when the ball struck it flattened and touched a larger surface on the stone.

The first ball rolls down and strikes a hard blow on the side of ball No. 2. This ball is flattened between balls Nos. 1 and 3, as shown in Fig. 30.

Ball No. 2 at once springs back again into its former spherical figure, and in doing so it brings No. 1 to rest and flattens No. 3, as shown in Fig. 31.

Ball No. 3 now springs back into its spherical form, and in doing so acts on No. 2 and brings it to rest, and acts on No. 4 and flattens it. Thus each ball passes the blow on to the next by its elasticity, and each in turn flattens and then springs into its natural form, and thus we have a series of contractions and expansions running through the whole series of balls. The last ball is finally flattened, and, when it expands immediately afterward, it presses against the ball that gave it the blow and brings it to rest; at the same time, finding no resistance in front of it, its back-action on the ball behind it causes it to start up the railway. Thus the last ball, No 7, is shot up the railway by a force derived from ball No. 1, and which was sent through all the balls by their successive contractions and expansions.

EXPERIMENTS WITH A LONG SPRING, SHOWING HOW VIBRATIONS ARE TRANSMITTED AND REFLECTED.

EXPERIMENT 57.-Obtain a brass wire, wound in the form of a spiral spring, about 12 feet long. Get an empty starch-box or cigar-box, and take off the cover, and then stand it on one end at the edge of a wooden

table, with the bottom of the box facing outward. Screw this box firmly to the table, and then screw a small iron or brass hook to the bottom of the box, as shown in Fig. 33. Slip over this hook the loop at the end of the long spiral spring. Hold the other end of the spring in the hand, letting it hang loosely between the hand and the box. Insert a finger-nail or the blade of a knife between the turns of the wire, near the hand, and pull the turns

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asunder. Free the nail suddenly, and a vibration or shock will start and run from coil to coil along the whole spring, and a loud rap or blow will be heard on the box, thence to be reflected to the hand, and then again to the box, and so on. Here we have a beautiful illustration of the manner in which a vibration may travel along an elastic substance, and make itself heard as a sound at the

other end, there to be reflected back to the place whence it came, to begin over again its forward journey.

EXPLANATION OF THE MANNER IN WHICH SONOROUS VIBRATIONS ARE PROPAGATED.

If the student clearly understands the actions in the experiments with the glass balls and spring-coil, he can have no difficulty in perceiving how a shock or vibration may in like manner pass through the elastic air.

For simplicity of illustration, imagine a very long tube, in which, at one end, fits a piston or plug. Suppose this piston moves quickly forward in the tube through a short distance say, one inch-and then stops. If the air were inelastic, then one inch of air would move out of the other end of the tube while the piston moved forward one inch. But air is elastic; it gives before the motion of the piston; and it takes some time, after the piston has moved forward, before the air moves at the other end of the tube. If the tube is 1,100 feet long, and the temperature of the air 42°, it will be one whole second before the end of the air-column moves; for it takes that time for a sound-vibration to traverse 1,100 feet, and a mechanical action on air of the above temperature cannot be sent through it with a greater speed than that.

Now, suppose that the piston takes second to make its forward motion in the tube, how far will the air be compressed in front of it at the instant the piston stops? Evidently the answer is found by taking of 1,100 feet, which is 110 feet. If the piston takes of a second in moving forward, then at the end of that time the air is compressed before the piston to a depth of of 1,100 feet, or 11 feet. The length of the column of air, compressed by the forward motion of the piston, in every case

is found by dividing the velocity of sound by the fraction of a second during which the piston was moving.

This compressed air cannot remain at rest in the tube, for it is now exactly like the compressed ball No. 2 of Fig. 30. It expands, and in expanding it acts backward against the immovable piston, but in front it compresses another column of air equal to it in length; this, in turn, acts like ball No. 3 of Fig. 31, bringing to rest the column of air behind it and compressing another column in front of it; and in this manner the compression will traverse a tube 1,100 feet long in one second.

If the piston moves backward in the tube, then a column of rarefied or expanded air will be formed in front of the piston, caused by the air expanding into the space left vacant by its backward motion; and this rarefaction will go forward through the air exactly as did the compression.

Now imagine the piston to move to and fro in the tube; it will send through the column of air condensations and rarefactions, following each other in regular order. If we have a body vibrating freely in the open air, then it will form spherical shells of compressed and rarefied air all around it, these shells constantly expanding outward into larger and larger shells, and following each other in regular order and motion, like the regular movement of the circular water-waves which spread outward around a point of agitation on the surface of a pond. Thus the sound-vibrations are sent out in all directions from a vibrating body just as light is diffused in all directions around a luminous body. In our experiments in "Light," page 34, we found that the illumination of a given surface varies in brightness inversely as the square of its distance from the source of light. In like manner the loudness of a sound decreases inversely as the square

of our distance from the vibrating body. Thus, at 100 feet, the loudness of the sound is of what it was at 50 feet, and at 200 feet its loudness is of what it was when we were 50 feet distant.

Now what will be the effect on any portion of air— like that, for example, which touches the drum-skin of the ear-if these condensations and rarefactions reach it? Evidently, while the condensations are passing, the molecules (the smallest parts) of the air will move nearer each other, then regain their natural positions, to be separated yet farther by the rarefaction which at once follows. Therefore, the effect on any molecule will be to swing it to and fro. Hence the air, touching the drum-skin of the ear, moves forward and then backward, and forces the drum-skin in and then out. This swinging motion is conveyed to the fibres of the auditory nerve, and causes that sensation called sound.

But we have seen that vibrating bodies swing to and fro like the pendulum, hence those vibrating bodies which are causing sound make all the molecules of air around them swing to and fro like the bobs of very small pendulums, each pendulum beginning its swing just a little sooner than the one in front of it.

All this, however, and much more than we have time to write about, will be taught you very clearly by an instrument which I shall now show you how to make.

EXPERIMENTS WITH CROVA'S DISK, SHOWING HOW SONOROUS VIBRATIONS TRAVEL THROUGH AIR AND OTHER ELASTIC MATTER.

EXPERIMENT 58.-In Fig. 34 A is a cardboard disk mounted on a whirling machine or rotator B, and C is a

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