712 



Professor Andrew Gray 



[April 29, 



same for each moving particle. The lines of motion and the relative 

 positions of the particles in a wave are shown in the first diagram above 

 (Fig. 1). As the motion is kept up at the place of excitation it is 

 propagated out by the elastic resistance of the medium to displace- 

 ment, and the configuration of particles travels outwards with the speed 

 of light, traversing a wave-length (represented 

 in the diagram by the distance between two 

 particles of the row in the same phase of motion) 

 in the period of complete to-and-fro motion of a 

 particle in its rectilineal path. 

 Fig. 9. Now, a to-and-fro motion such as this can be 



conceived as made up of two opposite uniform and 

 equal circular motions. Think of two distinct particles moving in the 

 two equal circles A B in this diagram (Fig. 9), with equal uniform 

 speeds in opposite directions. Let each particle be at the top of its 



A B a' b' 



cb cb .cp 



O0 



d^e i^P ^r> 



/ \ 



?q:> cq:^ cr;^ <cp 

 <i^ ct> c^ c^ 



2 2, 



<^ C46 Jct^ ct> 



Fig. 10. 



circle at the same instant ; then at any other instant they will be in 

 similar positions, but one on the right, the other on the left of the 

 vertical diameter of the circle. Thus at that instant each particle is 

 moving downward or upward at the same speed, while with whatever 

 speed one is moving to the left, the other is moving with precisely 



