The breath driven through the mouth of the whistle strikes on the sharp edge of the opening at the side of the whistle, and sets up a flutter or vibration of air. The air within the glass tube now takes part in the vibrations, the light silica powder vibrates with it, and makes the vibrations visible.

To exhibit this experiment before a number of people, lay the tube carefully on the water-lantern before the heliostat, and throw a projection of the tube and the powder on the screen. When the whistle is sounded, all in the room can see the fine powder leaping up in the tube into thin, upright plates.

EXPERIMENT 34.-Mr. Geyer has made the following pleasing modification of this experiment: Take a glass tube about 2 feet (61 centimetres) long and inch (19) millimetres) diameter. One end of this tube is stopped with a cork; then some silica is poured into it. The other end is placed in the mouth. Singing into the tube, a note is soon struck which causes the silica to raise itself in groups of vertical plates, separated by places where the powder is at rest, the number of these groups and their positions in the tube changing with the note sung.

We have now seen how solids, like steel or brass, may vibrate and give a sound. We have heard a musical sound from vibrating water, and these last experiments prove that a gas, like air, may also vibrate and give a sound. In the next chapter you will find experiments which show how these vibrations move through solids, through liquids, and through the air.

CHAPTER VI.

ON THE TRANSMISSION OF SONOROUS VIBRATIONS THROUGH SOLIDS, LIQUIDS, AND GASES, LIKE AIR.

EXPERIMENT WITH A TUNING-FORK AND WOODEN ROD.

In this chapter it is shown that a solid, a liquid, or a gas, like air, may conduct to a distant point the vibrations made at the place of origin of the sound.

EXPERIMENT 35.-Get the tuning-fork and one of the pine rods we used in Experiments 9 and 17. Let one hold the rod horizontally and lightly pressed against the panel of a door. Let another make the fork vibrate, and then press the end of its handle against the free end of the rod. At once the door-panel gives the note of the fork. Take the fork away from the end of the rod and the sound is no longer heard. Why the panel gives so loud a sound will be explained by other experiments. Just now we are merely observing the fact that the vibrations of the fork move through the rod to the door.

EXPERIMENT 36.-Hold the rod to the ear, and touch the fork to the other end, and the sound will be heard distinctly.

EXPERIMENT 37.-If you hold the rod in the teeth and close the ears the sound will be heard, showing that the vibrations of the fork travel through the rod, through the teeth, and through the bones of the head to the ear.

EXPERIMENT 38.-Touch the handle of the vibrating fork to the head and you will perceive the sound.

EXPERIMENT 39.-Open the mouth and place in it your watch, taking care that your teeth do not touch it, and take note of the force of the sound you hear. Now gently bite the watch, and note how distinctly the ticks are heard.

EXPERIMENT 40.-At the toy-shops you can buy a little instrument sometimes called the "lovers' telegraph," or "telephone." It consists of two short pieces of tin tube, each having a membrane fastened over one end, and a long piece of twine joining the two membranes. Let one boy hold the open end of one of the tins to his ear, and let another take the other tin to the distance at which the twine is pulled out tight. Then let him sound the fork and touch its foot to the tin; immediately the boy at the other end hears the note. The vibrations of the fork travel through the first tin and membrane, then along the twine to the other membrane and tin to the ear of the listener.

AN EXPERIMENT IN WHICH SONOROUS VIBRATIONS ARE SENT THROUGH WATER.

EXPERIMENT 41.-At the carpenter's procure two wooden boxes, measuring on the outside 73 inches (19.7 centimetres) long, 33 inches (9.84 centimetres) wide, and 24 inches (6.4 centimetres) deep, using pine inch (6 millimetres) thick. In making these boxes, neither dovetailing nor nails need be used; they may be glued together. One end of the box is left open. Make the tuning-fork sound, and then hold it upright, resting the handle on the centre of the top of the box. You observe the sound is now very much louder. Why this is so will be shown by other experiments.

EXPERIMENT 42.-In Fig. 26 is the box. A tumbler filled with water is standing on the box. The tuning-fork stuck in a block of wood rests on the water. Take the fork and block in the hand, and make the fork vibrate; then immediately plunge the block in the water as represented in the figure. At once you will hear the sound of

the fork apparently coming out of the box. The vibrations of the fork pass through its handle to the block, and from this through the water and through the bottom of the glass to the box. Our experiment thus shows that vibrations may easily pass through a liquid. A sheep's bladder filled with water may replace the tumbler and water in this experiment.

EXPERIMENTS SHOWING THAT THE AIR IS CONSTANTLY VIBRATING WHILE SONOROUS VIBRATIONS ARE PASSING THROUGH IT.

We must now add to our apparatus an open metaì Apipe, about 7 inches (19 centimetres) long, shown at

C in Fig. 27. This pipe the organ-builder will accurately tune to your "A-philharmonic" fork.

EXPERIMENT 43.-Get a glass tumbler about 2 inches in diameter and about 3 inches deep, though any tumbler will do. Take a piece of window-glass about 3 inches square and place it over the tumbler. The glass must touch the edge of the mouth of the tumbler all around.

Now slowly slide the glass so that the opening into the tumbler gets larger and larger, while the vibrating fork is held all the time over this opening, as shown at A in Fig. 27. Presently you will get an opening of a size that causes an intense sound, much louder than any you have ever before heard from the fork alone. This is because the air in the tumbler is set in vibration, and adds the vibrations of its mass to those of the fork. That