AND OTHER WATER WAVES 309 



inches. The distance between the wave-crests in 

 the lower part of the channel was on this day 

 66 feet. 



The process of growth is easily observed, the 

 larger wave -crests catching up the smaller and 

 incorporating them, the wave -velocity being greater 

 where the depth is greatest. As soon as the differ- 

 ence of depth between troughs and crests of the 

 larger waves becomes considerable, the process of 

 incorporation of the smaller wave -crests is much 

 hastened ; for when they are situated in the troughs 

 of the larger waves their total speed is much 

 diminished. 



On several days the speed of the current and 

 the rate at which the waves travelled past the 

 observer were determined. The latter, which may 

 be called the apparent speed of the waves, is the 

 velocity of the current plus the rate at which 

 the wave travels through the water in a down- 

 stream direction. Deducting the velocity of the 

 current from the apparent velocity of the wave, 

 we obtain the true velocity of the wave. If this 

 should be found to be nearly the same as the calcu- 

 lated velocity of a long wave (viz., V = >/ g h, 

 where V = velocity, g = acceleration of gravity, 

 and h = depth, all expressed in feet and seconds), 

 it may be safely concluded that these roll -waves 



