![]() 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." 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. If, now, the slit be placed over the dotted line" (at the bottom of Fig. 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. "The book is now slowly drawn along in the direction of the arrow, the piece of paper being held in the same position. He then holds this on the dotted line at the bottom of Fig. 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. 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. 6, if we graphically represent the backward-and-forward oscillation of a particle by such a wavy line as in Fig. After all the particles have been put into motion as shown in Fig. Longmans I am enabled to give here the illustrations which he employs. 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. Owing to the pendulum-like motion of the molecules, their relative positions vary at each instant of time. 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. 3, and obtain a wavy record on a blackened cylinder.Įxperiment shows that we have at one time a sphere of compression-that is to say, the air is packed closely together and, again, a sphere 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 or we may use the fork as shown in Fig. To one prong of this fasten a mirror, and on this mirror throw a powerful beam of light. 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. 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. 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. 4.-Shells of Compressed and Rarefied Air produced by a Source of Sound. ![]() ![]() If it came like a wind we should feel it as a wind, but as a matter of factįig. How comes it that the ear is struck? Certainly no one will imagine that the sound comes from the cannon or from the railway-whistle like a mighty rush of air. We hear the report of a gun or the screech of a railway-whistle, or any other noise which strikes the ear. Let us now pass to a disturbance of another kind, from two dimensions to three, from the surface of water to air. Hence it is that only the phase goes on- how it goes on will easily be gathered from Fig. Each particle of water moves in a circular or elliptic path in a vertical plane lying along the direction of the wave, and so comes again to its original place. Now, these appearances are as if there were an actual outpouring of water from the cavity made by the stone but if we strew small pieces of paper or other light material on the water-surface before we drop the stone, we find that it is not the water which moves outward, but only the state of things-the wave.
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