1 86 POPULAR LECTURES AND ADDRESSES. 



But instead, you have these two wave-models (see 

 Fig. 34, page 157), each of which shows you the 

 displacement and motion of a line of particles in 

 the propagation of a wave through our imaginary 

 three-dimensional solid ; the line of molecules 

 chosen being those which in equilibrium are in 

 one direct straight line of the cubic arrangement, 

 and the supposed wave having its wave front 

 perpendicular to this line, and the direction of 

 its vibration the direction of one of the other two 

 direct lines of the cubic arrangement. 



You have also before you this series of diagrams 

 (Figs. 36 to 41) of waves in a molecularly-consti- 

 tuted elastic solid. These two diagrams (Figs. 

 36 and 37) illustrate a wave in which there are 

 twelve molecules in the wave-length ; this 'one 

 (Fig. 36) showing (by the length and position of 

 the arrows) the magnitude and direction of 

 velocity of each molecule at the instant when 

 one of the molecules is on the crest of the wave, 

 or has reached its maximum displacement ; that 

 one (Fig. 37) showing the magnitude and direction 

 of the velocities after the wave has advanced such 



