cannon or from the railway whistle like a mighty rush If it came like a wind we should feel it as

of air.

a wind, but as a matter of fact no rush of this kind is felt. It is clear, therefore, that we do not get a bodily

transmission, so to speak, as we get it in the case of the ball thrown from one boy to the other. We have a state of things passing from the sender of the sound

to the receiver; the medium through which the sound passes being the air. A sounding body in the middle of a room for instance, must send out shells of sound

as it were, in all directions, because people above, below, and all round it would hear the sound. Replace the stone by a tuning fork. To one prong of this fasten a mirror, and on this mirror throw a powerful beam of light. When this tuning fork is bowed, and a sound is heard, the light thrown by the attached mirror shows the fork to be vibrating, and when the tuning fork is moved we get an appearance on the screen which reminds us of the rope, of we may use the fork as shown in Fig. 3, and obtain a wavy record on a blackened cylinder.

Experiment shows that we have at one time a sphere of compression—that is to say the air is packed

au o'

FIG. 5.-Propagation of sound waves along a cylinder.

torn further apart than The state of things then is a state of compresHence the particle of

closely together; and, again, a sphere of rarefaction, when the particles of air are they are in the other position. that travels in the case of sound, sion and rarefaction of the air. air travels differently from the particle of water; it moves backwards and forwards in a straight line in the direction in which the sound is propagated.

The annexed figure will show how this backward

and-forward movement results in the compressions and rarefactions to which reference has been made, in consequence of the impulse having been imparted to one molecule after the other. Owing to the pendulum-like motion of the molecules, their relative positions vary at each instant of time.

FIG. 6.-Sound Waves. Particles of air, a, b, c, d, e, are in position I at rest. The remaining positions show how they are situated at successive instants, when a continuous series of impulses reaches them from the left. In position 2, e.g., only one particle has begun its oscillation. In position 3, only two, while in position 6, all are in motion.

Professor Weinhold has given in his "Experimental Physics" a good method of obtaining on a plane a mental image of what goes on in a so-called sound wave, and by the courtesy of Messrs. Longmans I am enabled to give here the illustrations which he employs.

After all the particles have been put into motion as shown in Fig. 6, if we graphically represent the backward and forward oscillation of a. particle by such a wavy line as in Fig. 7, we shall, when we put a large

number of such waves side by side introducing the change of phase, have such an arrangement of wavy lines as is represented in Fig. 8.

Now the beauty of Weinhold's illustration consists in this; he almost makes each element of each line-each element representing of course a particle of air-appear to be actually in motion by treating the above figs. in the following way. He cuts a narrow slit SS in a piece of stiff paper, either black or of a dark colour as shown in Fig. 9. He then holds this on the dotted line at the bottom of Fig. 7. "The book is now slowly drawn along in the direction of the arrow, the piece of paper being held in the same position. At first the lower extremity of the curved line in A is seen through the slit; but as the book is drawn along, the portions to the right and those to the left come successively in view; the small white dot which is the only visible portion of the curved line, appears as a point which moves first to the right and then to the left, and imitates closely the motion of a vibrating particle of air,

FIG. 7.