CHAPTER XIII.

ON THE INTENSITIES OF SOUNDS.

EXPERIMENT SHOWING THAT, AS THE SWINGS OF A VIBRATING BODY BECOME LESS, THE SOUND BECOMES FEEBLER.

YOUR experiments have shown you that the pitch of a sound rises with the frequency of the vibrations. You no doubt have observed that sounds may be loud or soft without regard to their pitch. Thus, just after we have vibrated a tuning-fork, its sound is the loudest, then it gradually grows feebler and feebler, and slowly dies out.

EXPERIMENT 93.-Let us make an experiment which will tell us the cause of this gradual change in the intensity of its sound.

Vibrate the fork, as shown in Experiment 25, and very slowly draw the smoked glass under the pointed piece of foil which is fastened to one of the prongs. As the glass slowly moves under the vibrating fork you will observe that the sound grows feebler and feebler, and at last it dies out.

Take the glass and examine the trace made by the vibrating fork. You see that the lamp-black has been

scraped from the

shown in Fig. 48.

glass in a triangular-shaped space, as This shows that, as the sound dimin

grew less and less.

CHAPTER XIV.

ON CO-VIBRATION.

EXPERIMENTS WITH TWO TUNING-FORKS.

EXPERIMENT 94.-Take the two tuning-forks that we used in the experiments in interference, and holding one upright before you make the other vibrate, and then bring the two close together, with the surfaces of their prongs opposite each other. One is silent and motionless, the other is vibrating. Hold them there for a few seconds, and then bring the fork that was silent quickly to the ear, and you will discover something quite surprising. It is not silent, it is sounding faintly. It has not been touched, and yet it is vibrating. Why should a fork begin to vibrate merely because a sounding fork is near it?

EXPERIMENT 95.-Get the two wooden boxes or resonators we made for these forks (A-forks) and place them on a table with the open ends facing each other, and a few inches apart. Hold one of the forks upright on one of the boxes, and then, making the other fork sound, place it on the other box. It now sounds clear and loud. Stop this vibrating fork by touching it with the finger, and the other fork will be heard sounding alone. This is certainly a most curious matter. That a vibrating fork can cause another near it to sound seems im

possible, and yet our experiment shows that it is possible.

EXPERIMENTS ON THE CO-VIBRATION OF TWO WIRES ON THE SONOMETER.

EXPERIMENT 96.-Stretch the two wires on the sonometer (Fig. 46) so that they come in tune with each other. If you cannot do this, get some one familiar with music to help you, and let him bring the two strings into unison. When this is done pull one of the strings at the centre and let it go and then watch the other string. At first it is at rest and silent, but in an instant it too begins to quiver and sound. You may repeat this several times, and each time you will observe the same thing. One string sounding near another causes it to sound also.

EXPERIMENT 97.-Loosen the second wire slightly and put it out of tune with the first, and the experiment fails completely. Take another fork, not in tune with the one that sounds, and Experiment 95 will also fail. Here we are coming on a fact in this matter that may help us out. When the two forks are alike, when the two strings of the sonometer are in tune, the sounding fork or string makes its neighbor sound with it.

This remarkable fact, that a vibrating body may cause another elastic body in tune with it also to vibrate, is called co-vibration; which means, vibrating with (another body).

The fork (or string, or any body), in vibrating, gives to the molecules of the surrounding air the same number of pushes and pulls in one second as the silent fork does when it vibrates.

Suppose that the silent fork receives a feeble push from the vibrating air which touches it. The prong of the fork

is pressed forward, but through a very minute distance; then it swings back by its own elasticity, but it swings back with the air, which now pulls it. Then, on reaching the end of this backward swing, it at once gets another push from the air, and this push aids it on its forward swing, and makes it swing a very little more than it would have done if it had not received this push. Thus the little pushes and pulls of the air keep exact time with the tiny forward and backward swings of the fork, and, as several hundred of these pushes and pulls act on the fork in a second, they soon get it swinging sufficiently to make it act with power enough on the air to give us a sound when the other fork is stopped.

An exact understanding of how these feeble pushes and pulls of the air can set into vibration such a stiff and heavy body as a steel fork may be rendered clear by the following:

EXPERIMENT OF SWINGING A HEAVY COAL-SCUTTLE BY THE FEEBLE PULLS OF A FINE CAMBRIC THREAD.

EXPERIMENT 98.-Take a very heavy body, like a scuttle full of coal, and suspend it by a stout piece of twine. Then tie a piece of the finest cambric thread to the handle on the back of the scuttle. When the scuttle is hanging motionless give the cambric thread a feeble pull, being careful not to pull too hard or you will break it. Now you will see, by looking sharply, that you have set the scuttle swinging, but through a very small distance. Again gently pull the thread when the scuttle is swinging toward you, and repeat this pull several times, always keeping time with the swing of the scuttle. Now the scuttle is swinging through an inch or two, and by