Here each

them by the five boys placed as shown in Fig. 24. boy's hands rest on the shoulders of the boy in front of him. Suppose, now, the boy A, at one end of the line, receives a sudden push against his shoulders in such a direction as to move him against B. A will give up this motion to B, and will come to rest standing in an upright position. B, in a similar manner, now imparts his motion to C, and then comes to rest; and this goes on until D, the fourth boy, gives his motion to E, who, having no boy in front of him, is thrown forward.

Now suppose these boys represent the particles of air through which a sound pulse is passing, E being the air

FIG. 24. EXPERIMENT WITH FIVE BOYS

particles near the drum of the ear. As the particles at E are thrown forward, they strike the drum, making it shake; and this shaking, being transmitted to the brain, produces a sound.

If, instead of having twelve ivory balls as shown in Fig. 23, we have 100, or even 1,000, the motion would be transmitted through them in the same manner, and as soon as the last ball was reached, it would be moved forward. The time required to do this, however, would be greater as the number of balls increased, for time is required for the transmission of a sound pulse.

Coming back to the illustration of the row of boys, I do not think you will have any difficulty in understanding that the time required to transmit the pulse or push of the last boy will vary considerably with the manner in which each boy transmits his push to the boy on whose back his hands are resting. If each boy gives up his motion sluggishly, it will require a much longer time for the motion to be transmitted through all the chain, than if each boy gives up his motion quickly. In other words, the time required for the transmission of a pulse through a row of particles will vary with the nature of the particles. Very elastic balls, such as a series of ivory, or glass balls, will transmit a pulse much more rapidly than a series of wood, or cork balls.

There is another interesting experiment, which I believe was also first proposed by Professor Tyndall. This experiment

FIG. 25. LIGHT EXTINGUISHED BY SOUND PULSE

shows the manner in which a sound pulse is sent through the tin tube, shaped as shown in Fig. 25, and supported on a stand. Now, this particular tube is fifteen feet long. As you will see, it is open at both ends; the opening at one end, however, is the entire width of the tube, while the other end has open only the small part of a cone that is attached to it. Suppose, now, a lighted candle be placed with its flame near the opening at the small end, and a loud sound is made at the larger opening in anyway, as by clapping two pieces of board together. The sound pulse transmitted through the tube will now instantly extinguish or blow out the candle-flame.

This extinguishment of the flame is not, as you might

suppose, due to a current of air blown though the tube by the striking together of the pieces of wood; for, if the inside of the tube be filled with smoke, and the candle-flame be relighted the clapping together of the boards will, as before, instantly extinguish the flame, but no smoke will be seen escaping from the small end as would have been the case had the air been actually forced through the tube.

In the case of the ivory balls, the distance the last ball is thrown away from the others will depend on the amount of force that is used. If the first ball is thrown against the other with more or less force, the last ball will be thrown off to a greater or less distance. Then, too, if the same force be employed, the last ball will be thrown off with less force if the pulse has passed through 100, or 1,000 balls, than if it had only passed through twelve balls.

Of course you know the sound of a bell is greater when you are near it than when you are a great distance from it; for, since the bell sets up sound waves in all the air of a room, if the room is large the particles of air furthest from the bell will have received their motion by transmission through a very great number of air particles. Therefore, the force with which the particles strike against the drum of the ear is comparatively feeble, and hence the sound produced is not very strong.

Suppose, instead of permitting the sound waves to spread in all directions through the air of a room, they are passed through a closed tube, smooth on the inside, so as to decrease the friction. The tube shown in the preceding experiment will answer very well for this purpose. It is, as you will remember, fifteen feet in length. Now, if I stand at a distance of fifteen feet from a person and say something to him in a faint whisper, he will be unable to understand me, since the sound pulse has not only passed through the air of the room in the straight line between us, but also in all other directions. If, however, this person places his ear at the small end of the tube, and I whisper at the larger end

with the same strength as I did before, he will be able to hear all I say, but no one else in the room can hear; for all the energy with which I speak is now limited to the air inside the tube, so that the last particles that are shot off from the end of the tube strike the drum of his ear with considerable force. Try to imagine how many people might be placed at a distance of fifteen feet with my lips as centre, and that I speak just loud enough for all these people to hear. I do not know how many people can be placed in such a position, but let us suppose, simply for the sake of convenience, that there are a 100 or more, and I speak loud enough to make all the people hear. Then, if I speak equally loud in the tube, all this sound would now be concentrated on a single ear, and, as you can easily understand, the sound produced would be much louder; therefore the distance through which conversation could be held by means of a tube would be very much greater.

This is the principle of what is called a speaking-tube. A speaking-tube consists merely of a smooth tube, generally of tin, connecting two different parts of the house. The person holding his ear near one end of the tube can distinctly hear whatever a person whispers in the other end.

Provided the tubes are air-tight, are smooth on the inside, and do not change their direction too frequently or abruptly, it is possible to readily hold conversation in a faint voice over considerable distances. The French philosopher, Biot, made some interesting experiments with the water-pipes of Paris before the water was turned on, and found that there was no difficulty in holding conversation in a low tone over a distance of about three-fourths of a mile. But if tubes are to be used for great distances, their diameter must not be very great, and the insides must be kept very smooth.

You can try an amusing experiment with a fairly wide India rubber hose, provided the inside is smooth. Pass such a tube out of the window of a third story room into the

yard. If the hose is not long enough, the second story will do, although the third story is preferable. A person holding his ear at either end of this tube can distinctly hear what is whispered at the other end; and a conversation can easily be carried on through it.

By placing a small, conical tube inside a human figure, the sounds from the yard will appear to come from the mouth of the figure, so that it will seem as if it was speaking; and, since it would be quite possible in this way to transmit sounds through distances of many hundred feet, you can easily understand what very odd effects could be obtained. But air is not the only medium through which sound waves can be sent. Any other elastic medium can be employed for this purpose. For example, water answers admirably. Some time when you are in swimming, hold your head near the water, so that one ear is immersed; or, still better, dive under the water and have a companion strike two stones together and you will be surprised how distinctly the sound waves, or pulses are transmitted through the water to your

ear.

You must be careful, however, in trying this experiment, not to be too near the source of the sound, and to caution the person making the sound not to make it too loud, since water is so good a conductor of sound that the drum of the ear might be seriously injured by the intensity of the sound waves. It would be very dangerous to have your head partially or completely under the water when submarine blasting is going on. Under such circumstances, the chances for losing your hearing would be very good. Indeed, as you are probably aware, the pulse produced by the explosion of dynamite under water is so severe, that it generally partially stuns, or completely kills all the fish in the neighborhood.

Sounds can also be transmitted through solids. If you place an ear against one end of a long, wooden bench, you can distinctly hear the scratching of a pin against the other