I 9 2.j 



PLANE MOTION. 



103 



191. To show the connection of the present subject with the 

 theory of wave motion, imagine a flexible cord AB of which one 

 end B is fixed while the other A is given a sudden jerk or 

 transverse motion from A to C and back through A to D, etc. 

 (Fig. 49). The displacement given to A will, so to speak, run 

 along the cord, travelling from A to B and producing a wave. 

 The figure exhibits the successive stages of the motion up to 

 the time when a complete wave has been produced. 



A' 



Fig. 49. 





192. The distance A'K (Fig. 49) is called the length of the 

 wave. Denoting this length by X, and the time in which the 

 motion spreads from A ' to K by T we have for the velocity of 

 propagation of the wave 



V=j: (18) 



It is to be noticed that the motion of any particular point of 

 the cord is supposed to be rectilinear and at right angles to 

 AB ; this is the case with the simple transverse vibrations 

 in an elastic medium such as the luminiferous ether regarded 

 as the vehicle of light waves. 



