AND OTHER WATER WAVES 287 



sharing the motion of the current, so that the only 

 movement of the water which the eye had to follow 

 was its periodic or wave motion. The changed 

 conditions of vision are well indicated by the 

 photographs then taken, in which there is no indica- 

 tion of the shutter being too slow, for the particles 

 of foam, &c., travelled along at the same speed as 

 the camera. As we passed through one after 

 another of the St. Lawrence rapids, I noticed re- 

 peatedly that the first few waves were rounded in 

 form, unbroken in surface, constant in size, and 

 practically fixed in position. Lower down, how- 

 ever, where two or more trains of waves inter- 

 sected one another, they became cusped and foam- 

 ing, breaking also and sometimes fluctuating. 



Some part of this difference may be due to in- 

 creased speed in the lower part of the rapid, but 

 it was fairly evident that the changed character 

 of the waves was mainly due to superposition. 



The most conspicuous billows, or apparent waves, 

 in torrents are usually of small lateral extension, for 

 they are produced where two standing waves cross, 

 and each standing wave is usually inclined at about 

 20 to the direction of the current. In the Niagara 

 Rapids below the Whirlpool, opposite Foster's Flats 

 (Devil's Hole Station), there is a splendid series of 

 such combination standing waves. The course of 



