AN EXPERIMENT IN WHICH INTERFERENCE OF SOUND IS SHOWN BY ROTATING A VIBRATING FORK OVER THE MOUTH OF A BOTTLE RESOUNDING TO THE NOTE OF THE FORK.

EXPERIMENT 63.-Get a bottle, like one of those shown in Fig. 40, holding about 5 fluid ounces when filled to its brim. Its mouth should measure 1 inch (25 millimetres) in diameter. Cut a piece of glass 1 inch long and 1 inch wide, and slide this over the mouth of the bottle while the vibrating A-fork is held over it. Fix the piece of glass with wax at the place where it makes the air in the bottle resound the loudest (see Fig. 40).

Again vibrate the fork, and holding it horizontally twirl it slowly over the partly closed bottle, just as we twirled it before the ear. You will find that whenever the corners of the fork have come opposite the mouth of the bottle the sound will have faded away to silence. In this position of the fork, one of the planes 7, 7, 7, or 7, of Fig. 39, goes directly down to the mouth of the bottle, and therefore there enter the bottle, side by side, at the same time, a condensation and a rarefaction. Hence the air in the bottle is acted on by two equal and opposed actions; it cannot vibrate to the fork, and we have rest and silence. The above experiment, and the following one, may be made as well with the tuned tumblers of Experiment 43 as with the bottles.

EXPERIMENTS IN WHICH INTERFERENCE OF SOUND IS OBTAINED WITH A FORK AND TWO BOTTLES.

EXPERIMENT 64.-Fig. 40 represents two glass bottles, of equal size, and each tuned as described in Experiment

63. Set one bottle upright, and with two bits of wax hold the other horizontally on some books, with the mouths of the bottles nearly touching, as shown in Fig. 40.

Make the fork vibrate, and, holding it horizontally, bring it down so that the space between the prongs will be opposite the mouth of the upright bottle, as shown in

Fig. 40. As it descends, you will observe that the sound first increases, and then suddenly fades away or entirely disappears. You can raise the fork and hear it still sounding, so that you may be sure it has not stopped, and yet, in a certain position between the two bottles, the sound is nearly if not wholly lost.

In this experiment, you will observe that while the face of one of the prongs is opposite the mouth of one bottle the space between the prongs is opposite the mouth of the other bottle. Therefore, while one bottle receives a condensation the other receives a rarefaction. Thus opposed vibratory motions issue from the mouths of the bottles, and they neutralize each other's action on the outside air. Hence silence is observed when the fork is in such position that the condensation or rarefaction which comes out of one bottle exactly equals in power the rarefaction or condensation which comes out of the other bottle.

You know that the air is really resounding in the bottles, even when silence is outside of them, by the following simple experiments :

EXPERIMENT 65.-Slip a piece of cardboard over the mouth of one of the bottles, and at once the other bottle resounds to the fork and sings out loudly. The balance is thus broken and sound is heard.

EXPERIMENT 66.-A piece of tissue-paper will produce another effect, because it is thin and only partly cuts off the vibrations, and the result is a feeble sound; partly an interference and partly a free action of the condensations and rarefactions, half silence, half sound.

EXPERIMENT SHOWING REFLECTION OF SOUND FROM A FLAT GAS-FLAME.

EXPERIMENT 67.—By a little care you can even slide the flat flame of a fish-tail gas-jet before the mouth of the horizontal bottle, and thus make a flame act as a guard to stop the vibrations from entering the bottle.

When two sonorous vibrations meet and make silence, they are said to "interfere." The experiments just made are experiments in the interference of sound.

EXPERIMENTS IN WHICH, BY THE AID OF A PAPER CONE AND A RUBBER TUBE, WE FIND OUT THE MANNER IN WHICH A DISK VIBRATES.

In describing Experiments 27, 28, 29, and 30, we stated that a vibrating disk always divided itself into an even number of sectors. This fact was explained by the statement that the adjoining vibrating sectors of the disk were always moving in opposite directions. The truth of this statement will be manifest on making the following experiments, which can only be explained by the fact that adjoining sectors, at the same instant, are always in opposite phases of vibration. These experiments will also afford beautiful illustrations of the interference of sonorous vibrations.

Take a piece of cardboard and roll it into a cone about 10 inches long. The small end of the cone should have in it an opening of such a size that the cone will fit into the rubber tube used in Experiment 32. If a brass disk of 6 inches in diameter is used in the experiments, the mouth of the cone should be 24 inches in diameter.

EXPERIMENT 68.—Make the plate vibrate with four sectors as in A, Fig. 23. Close one ear with soft wax; into the other put the end of the rubber tube; then place the centre of the mouth of the cone exactly over the centre of the plate with the cone quite close to its surface. In this position (which we will call No. 1, for future reference) no sound is perceived, or at least only a very faint one. This is so, because in this position of the cone it always receives, at the same instant, from the vibrating disk, four equal sound-pulses; and as two of these are condensations, and two are rarefactions, they mutually neutralize each other,

and the drum-skin of the ear remains at rest and no sound

is perceived.

EXPERIMENT 69.-Now move the mouth of the cone along the middle of a vibrating sector toward the edge of the disk. As the cone progresses the sound grows louder till it reaches its maximum when the edge of the cone reaches the edge of the disk. In this position (No. 2) the cone receives from the disk only regular sonorous vibrations, one condensation or one rarefaction alone entering the disk at a time.

EXPERIMENT 70.-Slowly move the cone along the circumference of the vibrating disk, keeping the edge of its mouth close to the border of the disk. The sound at once begins to diminish in intensity, until the circle of the mouth of the cone in its progress is divided into two semicircles by a nodal line. No sound is now perceived, because in this position (No. 3) a condensation and a rarefaction enter the ear together, for on the opposite sides of a nodal line the plate has always opposite directions of motion.

EXPERIMENTS WITH BEATING SOUNDS.

EXPERIMENT 71.-In purchasing the two A-forks, you took special pains to get two which were tuned accurately to unison; otherwise they are of no value for our experiments. Take one of these in each hand and make them sound together. Hold them near each other close to the ear, and you will observe that while both sound there appears to be but one note. The two sounds blend together perfectly, so that we cannot distinguish one from the other. Having tried this thoroughly, place a bit of wax on the end of one of the forks, and then make them sound