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THE POPULAR SCIENCE MONTHLY. 



The preceding figure (Fig. 6) will show bow this backward-and-for- 

 ward 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 graphi- 

 cally 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, introdu- 

 cing the change of phase, have such an arrangement 

 of wavy lines as is represented in Fig. 8. 



Now, the beauty of Weinhold's illustration con- 

 sists 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, JS 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 gradu- 

 ally transmitted through the whole series of air-particles from one end 

 to the other." 1 



In dwelling upon sound-phenomena, we have the advantage of deal- 



1 "Experimental Physics," p. 332. 



Fig. 7. 



