while each is held upright on its resonant box (see Experiment 41). At once you hear something unusual: little bursts of sounds, followed by sudden weakenings and loss. of power, as if the forks sang forte and then piano alternately. These singular quivering changes in the tone of the two forks, when sounded together, are called "beats." The sound seems to beat with a pulse-like motion at regular intervals. Take off the wax and the beats disappear, and the two forks sound together like one instrument.

EXPERIMENT 72.-Put on a larger or smaller piece of wax and the beats change their character, coming faster or slower each time the amount of wax is changed.

These experiments succeed admirably by using the tumblers of Experiment 43, or the resonant bottles of Experiments 63, 64, in place of the resonant boxes. The tumblers or bottles should be carefully tuned, one to the loaded, the other to the unloaded fork.

To understand these singular beats, you must remember that each fork sends out sonorous waves, or alternate condensations and rarefactions, through the air. When the forks are sounded together (without the wax), each sends out the same number of waves in a second, and these travel out together, the condensations and rarefactions of each moving side by side, and reaching the ear at the same time.

When we loaded one fork with wax we caused it to move slower. The processions of waves streaming out from each may start together, but they do not keep together; as the loaded fork is going slower its waves of sound are longer and drag behind. The condensations and rarefactions no longer travel side by side. A condensation from one fork arrives at the ear at the same time that a rarefaction arrives from the other. Thus

they interfere and destroy each other, and the interference makes silence, just as we discovered in our last experiments. The condensations and rarefactions from the two forks continue to arrive at the ear, and soon two condensations or two rarefactions come side by side and arrive at the ear together, and they mutually aid or reënforce each other, and there is a sudden burst of sound as if the forks were sounding louder.

The waves of sound continue to move, and one set of waves slips past the other, till the condensations of one set arrive at the ear alongside of the rarefactions of the other, and again there is interference and silence. By such continuous actions beats of sound are produced.

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FIG. 41.

Fig. 41 represents two such series of waves traveling side by side. One series is represented by a full line, the other by a dotted one. At A the condensations of one series are shown as opposite the rarefactions of the other; but, as the waves represented by the full line are longer than those represented by the dotted line, the former pass the latter, so that at C the two series act together, and we have a beat; while at a more distant point, B, the motions in the waves are opposed, and here there is interference and silence. It is evident that the sliding of the longer waves past the shorter will cause the waves, meet

ing at B, alternately to act together and to interfere; and thus the ear, placed at B, will perceive beats of sound.

It necessarily follows that, if one fork vibrates 100 times in a second and the other 101 times, there will be one beat in every second. The number of beats made in a second is equal to the difference in the number of vibrations per second made by the two vibrating bodies.

CHAPTER IX.

ON THE REFLECTION OF SOUND.

PROFESSOR ROOD'S EXPERIMENT, SHOWING THE REFLECTION OF SOUND.

EXPERIMENT 73.-Fig. 42 represents a disk of cardboard 12 or 14 inches in diameter, and having two sectors cut out of it, on opposite sides of its centre. This is mounted on the rotator, so that it can be turned round quickly. Let some one sit beside the rotator so that he can turn the handle, and at the same time blow a toy trumpet, which I have found to be the best pipe for this experiment. Hold the trumpet so that it will be inclined. to the surface of the disk, and with its open end just in front of one of the openings, as shown in Fig. 42. While the rotating disk is being turned steadily round, and the pipe is sounding, go to a distant part of the room, and here you will perceive the sound of the pipe changing rapidly, alternately growing louder and then softer like beats.

This effect is the result of reflection. When the solid part of the disk passes before the pipe the vibrations of sound are reflected or echoed from the card. When the openings pass before the pipe, part of the vibrations pass through the open place and are lost, and the sound to the listener appears to lose power.

In performing this experiment care must be taken to place the disk in such a position that the sound will be reflected to the distant listener. As we learned in our ex

periments in "Light," there is a law governing reflections. We found by our experiments that the angle of reflection is always equal to the angle of incidence, and the same law holds good in the reflection of sound.

EXPERIMENT 74.-Another experiment in the reflection of sound may be made with a common palm-leaf fan. Let some one sound the trumpet at one end of a room, while you hold the fan upright beside one ear. While