Sometimes, when the genie rushes out of the bottle which has imprisoned him, he is caused to work or drive various tools known as pneumatic tools. These tools are operated on a principle not unlike that of the steam engine in which a piston is caused by the pressure of the air to move rapidly to-and-fro and thus operate various devices. An example of pneumatic tools is to be found in the pneumatic hammer, such as is employed for heavy chipping, for calking or for riveting. Pneumatic hammers can be made to give from 1,500 to 2,000 blows per minute, or even a greater number. Pneumatic hammers have also been designed for the cutting of stone or for sculptor work generally. Pneumatic tools have also been devised for drilling blast holes in rocks, as well as for the drilling of holes in metals or wood or other work.

I have thus given you a brief account of but a small part of the work which the bottled genie has been compelled to do for man. There are many other kinds of work he can perform, but it will not be practicable to give you any further account of such work by reason of our limited space.

When the atmosphere is subjected to a very considerable pressure and, at the same time, means are taken not only for removing the high temperature that is produced by the compression, but at the same time to reduce and keep the compressed air at a very low temperature, the air loses its gaseous condition and becomes changed into a liquid closely resembling water, except that its color is light blue. Liquid air when relieved of the pressure so rapidly evaporates that it begins to boil, and thus produces a very intense cold. In this manner temperatures as low as 346° below the zero of the Fahrenheit thermometer have been obtained.

Some very curious effects are produced on certain food. products when cooled to exceedingly low temperatures by the evaporation of liquid air. Meats, for example, become so hard that when struck by a hammer they ring

like steel and are readily broken into a fine powder, as would a piece of glassware or porcelain. The chopped pieces of cabbage in sauer-kraut become as hard as flakes of mica, and the mass holds together so firmly that it can only be removed from the barrel or tub in which it has been stored, by a chisel and hammer.

Liquid air possesses many curious properties and has been applied to a number of practical purposes. Its further discussion, however, properly belongs to the "Wonder Book of Heat," in which it will be described.

CHAPTER VI

THE SOUND WAVES OF THE ATMOSPHERE

There are various phenomena, constantly going on around us, that are dependent for their existence on differences in the presence of the atmosphere. Some of the most important of these are the phenomena of sound. Were our earth's atmosphere removed and other means provided for our continued living, practically all sounds would at once cease and there would be an eternal silence over the earth.

This is so interesting a matter that we will take some little time to explain it.

Three things are necessary for the production of sound: a vibrating or sonorous body; a medium capable of being set into to-and-fro motions or vibrations, and an ear connected with the brain of an animal. The medium usually consists of the air or atmosphere. In it the sonorous body produces sound waves that move outwards in all directions, and, provided they affect the ear and through it the brain, produce the sensation of sound.

The atmosphere, however, is not the only medium in which sound waves can be set up. Such waves may also be produced either in liquids or in solids.

While a full description of the phenomena of sound belongs properly to the "Wonder Book of Sound," yet so many of the wonders of the atmosphere are due, either directly or indirectly, to the action of sound waves, that it will be necessary before describing these wonders to give a brief explanation of some of the elementary principles of acoustics.

Let us suppose that a sound is produced, such as by striking a bell, hung in the middle of a large empty room. When

the clapper or hammer strikes the bell, what happens is very simple. The walls or sides of the bell begin to shake to-and-fro, and, striking the air particles, move them away from the sides. When this motion reaches the ear of an observer, a sound is produced.

You must not suppose, however, that the air particles thrown off from the bell continue to move straight forward until they reach the observer's ear. On the contrary, they only move away from the sides of the bell for a comparatively short distance, when, striking against other particles of air, they give up their motion to them, and then come to a state of rest. In the meanwhile, the particles of air last set in motion move the particles around them, and these the particles around them; and this is continued until a series of pulses or waves is set up in all the air in the room.

I will not take space to explain more fully to you the true nature of the motion of the air particles in sound waves. In reality, this motion is more complex than you might suppose from the preceding explanation. There is, however, a simple experiment which will enable me to briefly show you the character of the motions of the sound waves.

FIG. 23. EXPERIMENT WITH TWELVE IVORY BALLS

If you will examine Fig. 23, you will see a row of twelve ivory balls resting in a smooth groove in a block of wood or stone, so that all the balls touch one another. If now, one of these balls be taken in the hand, as shown in the figure, and thrown against the ball nearest it, the blow or impact will be transmitted through the remaining balls. If the experiment is well performed, you cannot see any of the other balls move, except the last ball, which suddenly flies away from the rest. It is evident, however, that in some

way or other the force of the moving ball has been transmitted through the row of balls to the last ball, which flies off as a result of the blow it has thus received.

What has taken place is as follows: when the first ball strikes the ball nearest it, it gives up its motion to it and comes to rest. The second ball gives up the motion it thus receives to the next ball and comes to rest, and this is repeated successively through the balls, each ball giving up its motion to the one next to it and then coming to rest. When, however, the twelfth ball has received the blow from the eleventh, there being no ball to which it can give up its motion, it moves away from the others.

Now the sounding beli sends a pulse or wave through the particles of air around it in a similar manner. As the sides move outwards, they throw the particles of air around them outwards. These particles, striking the particles of air in front of them, give them their motion and then come to rest, and this motion goes on through successive air particles until the particles of air that fill the cavity of the ear of an observer are caused to move onward until they strike against the tympanum, a tightly stretched drum-head in the ear, and shake it. This motion is transmitted to the brain, thus producing the sensation of sound.

If you have any difficulty in picturing to yourself what takes place among the successive particles of air when a sound wave, or pulse, is passing through it, you will, I think, be aided if I describe a simple but beautiful experiment suggested by Professor Tyndall. Air particles, of course, are so small that you cannot see them; nor, indeed, could you at a distance see very distinctly the separate ivory balls. Suppose we represent each air particle, or ivory ball, by a boy. In the air these particles extend in all directions around the bell; but what is true of any single set of particles that reach in any direction outward from the bell, will be true of all similar sets of particles reaching in any other direction. Therefore, let us take only five of such particles and represent