ELECTRIC OSCILLATIONS AND ELECTRIC WAVES. 261 



in Fig. 192. This body of elevated water is an impure wave in- 

 asmuch as its velocity of flow v is zero, and therefore its potential 

 energy of elevation cannot be equal to its kinetic energy of flow. 

 Such an elevated portion of still water breaks up into two oppo- 

 sitely moving pure waves, and the initial stage of this process of 

 breaking up is indicated in Fig. 193. 



When a wave like A, Fig. 190, travels along a canal, the 

 velocity of flow v is continually decreased by friction, whereas 

 there is no action tending to reduce the elevation h. Therefore 

 that portion of the elevation which is in excess of what is required 

 to give a pure wave with what remains of the velocity of flow, 

 behaves exactly like the elevation A in Fig. 192, that is, this 

 excess of elevation breaks up into two pure waves a and b, Fig. 

 193, the portion a merges with the original wave A and the 

 portion b shoots backwards. 



The upper part of Fig. 194 represents, on an exaggerated 

 scale, the elevated portion of water in a pure wave. The velocity 



head 



tail of wave 

 Fig. 194. 



of flow v in this wave is continually reduced by friction as the 

 wave travels along the canal, the excess of elevation which is 

 being thus continually left in the wave causes a long drawn-out 

 wave to shoot backwards, and after a time the wave has the 

 form shown in the lower portion of Fig. 194, The head of the 

 wave is greatly reduced in intensity (energy value) partly be- 

 cause of the loss of energy by friction and partly because of the 



