The preceding figure (Fig. 6) will show bow 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.
Prof. 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 figures in the following way: He cuts a narrow slit, S S, in a piece of stiff paper, either black or of a dark color, 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, the rate of motion being, however, much slower. If, now, the slit be placed over the dotted line" (at the bottom of Fig. 8), "and the book drawn along underneath it in the direction of the arrow, a representation is obtained of the motion of a series of particles of air which are acted on by a number of successive equal undulations or waves. Each particle merely moves a little right and left, and always comes back again to its starting-point; but the condensations and rarefactions, represented by the lines being respectively closer together or farther apart, are gradually transmitted through the whole series of air-particles from one end to the other."
In dwelling upon sound-phenomena, we have the advantage of deal-
- "Experimental Physics," p. 332.