of the amplitude. Vibrate the whalebone again, and raise one end of the board so that the glass will slide out, and the vibrations will trace a beautiful wave form, as shown in Fig. 182. Since the motion of the glass strip is changing, the wave form is variable. If the strip is drawn out with uniform velocity, the wave form will closely approximate that shown in Fig. 181.

191. Water Waves of small amplitude are, as a whole, similar to those produced by transverse vibrations in a stretched cord. The particles of the water, however, move in small circles or ellipses, while the wave moves onward. This can be observed by watching the motion of a boat at a distance from the shore; the boat rises and falls with the waves, but does not advance with them. Near the shore the velocity of the wave below the surface is retarded by the sloping bottom and the outgoing water, so that the top of the wave curls over, forming a breaker, which moves in the direction of the wave.

192. Sound Defined. The physical definition of sound is any vibration that is capable of being perceived by the ear. The physiological definition includes also the effect produced upon the ear by such vibrations.

193. A Sounding Body is a Vibrating Body.-Demonstrations. - Hold a large jar, like the receiver of an air pump, horizontally by the knob, and draw a bow across the edge, or strike it lightly with a cork hammer. Is it a sounding body? Place a few carpet tacks inside the jar near the edge. Repeat the experiment. Is the jar in vibration?

FIG. 183

Bore a hole in the top of a table and firmly set into it the handle of a tuning fork. Make a cork hammer by thrusting one end of a knitting needle through a large cork. Tie a shoe button to the end of a fine silk thread. Strike one prong of the fork with the hammer, and hold the button

on the side of the fork near the top. Do its movements prove that the fork is a vibrating body? Why does not the button rebound to the same distance every time? Gradually lower it along the side. What is the effect? Hold the button between the prongs and observe.

Both these experiments prove that a sounding body is in vibration. An interesting way to show the vibration of a tuning fork is to set it in motion, and then bring it in contact with the surface of water upon which lycopodium powder has been scattered. The rapid blows of the prong will give rise to a beautiful set of waves.

- The vibrations of a

194. The Transmission of Sound. sounding body may be transmitted by any elastic substance. Gases, liquids, and most solids transmit sound, but with varying intensities and velocities.

(a) Gases. That gases transmit sound is a matter of universal experience, since the air is the common medium of sound transmission.

(b) Liquids. Sound is transmitted by liquids more readily than by gases. A person swimming under water can hear with great distinctness the sound of two stones struck together under the surface.

(c) Solids. A long wooden rod, a section of gas pipe, or the wires of a wire fence can be used as a means of proving that elastic solids are good conductors of sound. If an observer places his ear at one end of any of these, while the other end is scratched with a pin, the sound of the scratching is plainly heard through the solid body, though it may be entirely inaudible through the air.

195. Sound not Transmitted in a Vacuum. tion.

neck.

Demonstra

Fit a rubber stopper to an air pump receiver with a small
Beside this run two No. 30 insulated copper wires attached

to an electric bell. receiver air-tight.

FIG. 184

Thrust the stopper in firmly to make the Exhaust the air so far as possible, and ring

the bell by connecting the wires with a battery. Notice that the sound of the bell is very faint. Slowly admit the air, and notice how the sound increases in intensity. A perfect vacuum transmits no sound.

196. Wave Motion in Air.-Sound is transmitted by means of waves, but the particles of air do not vibrate transversely as in the coil in § 189; they vibrate longitudinallythat is, in the same direction as that in which the waves are moving. The molecules that are put in motion by

the first forward movement of a sounding body are suddenly pushed ahead of it and crowded nearer together, forming a condensation; but

their path is not

long, since they

strike other molecules, which in turn set the molecules next to them in motion. When the sounding body moves back, it leaves a partial

vacuum, or rare

faction, behind it, and into this the

molecules we have

been considering rush back. This sets up the to-and-fro motion of the air that constitutes a sound wave. The condensations and rarefactions move rapidly outward in all directions from the sounding body, and follow in regular succession as long as the sounding body continues to vibrate, at intervals that depend upon the rate of the vibration. In Fig. 185 the dark rings represent condensations and the light rings rarefactions in a train of sound waves proceeding from a sounding body at the center.

Demonstrations.

Hook one end of the wire spring as before (§ 189), and stretch the spring somewhat by pulling on the other end. Put a knife blade between two of the turns of wire and draw it toward the end held by the hand, pushing a few of the coils together. Remove the knife suddenly, and the wave will run the length of the spring and be reflected by the hook back to the hand. Tie a piece of thread to the spring at the middle, and the longitudinal vibrations will be shown by the sudden, jerking, to-and-fro motion of the thread.

That a mechanical impulse can be sent through the air as a wave form can be shown by the use of a tube 8 or 10 ft. long and 3 in.

FIG. 186

in diameter. One end of this tube is capped with a cone having an opening an inch in diameter at the small end, while the other end is covered by a sheet of thin rubber tightly stretched and tied in place. A short piece of candle is lighted and so placed that the flame comes opposite the end of the cone. When two wooden blocks are struck sharply together near the closed end, the flame suddenly flares away from the end of the tube. The same thing occurs when the rubber diaphragm is tapped lightly with the finger. There is no passage of air through the tube, for the end is closed; hence the

movement of the candle flame is the result of the blow received from the condensed wave sent out by the movement of the diaphragm.

197. Graphical Representation of a Sound Wave. — In a sound wave the motion of each particle of air is either a simple harmonic motion or a combination of two or more such motions. Though the vibration is longitudinal, the wave may be represented by a curve similar to that in Fig. 181 if we understand that all the particles really move to and fro in the direction of the horizontal line, and that the distances

A, particles at rest, when there is no sound; B, same particles at one instant in a train of sound waves; C, extent of displacement from position of rest (not direction of present motion); D, same displacements represented by vertical lines; E, the curve

A, B, etc., above this line represent displacements to the right, and C, D, etc., below the line, displacements to the left. Figure 187 illustrates how such a curve can be drawn.

198. The Velocity of Sound in Air has been found directly by taking the interval between the time when the flash of a gun is seen, and the instant when the report is heard. The distance between the two stations, divided by the time in seconds, gives the velocity per second.

If this determination is made at different seasons, it is found that the velocity is greater in summer than in winter. At the temperature of freezing water, or 0° Centigrade, the velocity of sound in air is 332.4 m., or 1090.5 ft., per