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Let him round off the edges, and cut a hole just inch (5 millimetres) in diameter in its centre. If the brass disk has been hammered, put it in a stove till it is red hot, and then take it out and lay it away where it will cool slowly. Cut about 6 inches (15.2 centimetres) from a rake or broom handle, and set it upright and firm in a heavy block of wood. With a knife pare off the sharp edges at the top of this upright, and then fit a screw tightly to the hole in the brass disk, and screw the disk to the top of the upright. We have now a flat disk of brass resting firmly on a stout upright support.

EXPERIMENT 27.-Draw a violin-bow carefully over the edge of the disk, and after a little practice you will be able to make the disk give a clear, strong sound. The bow, alternately catching and slipping on the edge of the disk, causes it to vibrate, and the vibrations result in sound. To make these vibrations visible, get some of the sand we used in the sand-pendulum, and scatter it thinly over the disk. Now, when the violin-bow causes the disk to vibrate, the sand will be agitated. Each grain will spring up and down with a curious, dancing motion. This movement of the sand shows that the disk is violently agitated; is beating up and down, and tossing the sand about at every vibration.

EXPERIMENT 28.-Touch one finger on the edge of the disk, and draw the bow at a point one-eighth of the way round the disk, measuring from the finger. We now get a most singular result. The sand dances furiously about, as before, and immediately gathers in lines, which are two diameters at right angles, and one of the diameters starts from the point where the finger touches the disk, as is shown in A of Fig. 23.

EXPERIMENT 29.-Draw the bow at a point 30° distant from where the finger touches the disk, and 6 lines of sand will be formed, as at B in Fig. 23.

EXPERIMENT 30.-By a little practice, a place still nearer the finger may be found, where the bow will make 8 lines of sand, as at C in Fig. 23.

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To understand these experiments, you will observe that, when the bow throws the disk into vibration, the sand on its surface is driven about in a kind of tiny waltz. Where the finger touches the disk the trembling of the disk is nearly stopped, and where the disk is bowed the swings of the plate are the greatest; hence the disk is forced to break up into vibrating sectors, each sector being twice the width of the distance of the finger from the bow. While any one sector is swinging up the adjoining sectors are swinging down, and vice versa. If this be so,

then the disk must always break up into an even number of sectors. It always does so. That the disk really vibrates as we have stated, you will conclusively prove for yourself by Experiments 68, 69, 70, and 122.

The lines separating the different sectors remain nearly at rest, exactly like the places just above the crotch of the tuning-fork, found out in Experiment 26. The sand, dashed about on the vibrating parts, is driven to these

places of rest, and gathers in narrow windrows running from the centre of the disk. These lines of rest are called nodal lines.

EXPERIMENT 31.-To increase the interest in these experiments, mix some dry lycopodium powder (from the druggist's) with the sand, and strew the mixture on the disk. These powders will behave in the most extraordinary manner when the disk vibrates, forming little heaps and whirlwinds that seem to smoke and boil furiously. This singular phenomenon is caused by the rotating currents of air set in motion by the vibrating disk. The heavy sand dashes through these little whirlwinds, but the light powder is caught up and whirled about in strange fantastic dances.

EXPERIMENT IN WHICH A SUBMERGED FLAGEOLET IS SOUNDED BY FORCING WATER THROUGH IT.

EXPERIMENT 32.-Fig. 24 represents a glass jar (a bucket will do as well) standing in a sink near a waterfaucet. A tin flageolet, such as may be bought in the toy-shops, has the highest finger-hole in it closed with. wax, as shown in Fig. 24. All the other holes are left open. A rubber tube leading from the faucet is slipped over the mouth of the flute. The water is turned on, and flows through the tube into the flute, and thence out into the jar. The jar overflows, and the water runs away down the sides. These things, placed in this manner, will give an experiment showing that a liquid, like water, may vibrate and give a sound. If the flow of the water is carefully regulated, and the flute is of the right pattern, you will hear a low but distinct musical note from the water. Touch the glass jar and a piece of paper laid on the surface of the water, and you will feel them quivering with the vibrations. Here we have a flute

blown by water under water, and giving a sound which is caused solely by the vibrations of the water. A queer

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flute, certainly, and an experiment as surprising in its effects as it is instructive.

PROFESSOR KUNDT'S EXPERIMENT, MADE WITH A WHISTLE AND A LAMP CHIMNEY, SHOWING THAT, AS IN WIND INSTRUMENTS, A VIBRATING COLUMN OF AIR MAY ORIGINATE SONOROUS VIBRATIONS.

EXPERIMENT 33.-The chimneys of student-lamps have a fashion of breaking just at the thin, narrow part

near the bottom. Such a broken chimney is very useful in our experiments. At A, in Fig. 25, is such a broken chimney, closed at the broken end with wax. A cork is fitted to the other end of the chimney, and has a hole bored through its centre. In this hole is inserted part of a common wooden whistle. At B is an exact representa

A

C

B

FIG. 25.

tion of such a whistle, and the cross-line at C shows where it is to be cut in two. Only the upper part is used, and this is tightly fitted into the cork.

Inside the tube is a small quantity of very fine precipitated silica, probably the lightest powder known. Some of this powder you may purchase of Mr. Hawkridge, of Hoboken, New Jersey. Hold the tube in a horizontal position and blow the whistle. The silica powder springs up into groups of thin vertical plates, separated by spots of powder at rest, as in the figure. This is a very beautiful and striking experiment.

EXPERIMENT 33 a.-The following experiment shows that the sound is caused by the vibrations of the column of air in the tube and whistle, and not by the vibrations of these solid bodies. Grasp the tube and whistle tightly in the hands. These bodies are thus prevented from vibrating, yet the sound remains the same.

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