AND OTHER WATER WAVES 229 



depth of water, for there is no water in front of 

 the advancing wedge. The rate of encroachment 

 would, in the above case, be somewhat reduced 

 by the porosity of the soil over which the flood 

 advanced. 



Colonel Gaillard, U.S.A., informs me that he once 

 saw the wave -front of a flood advancing along 

 the dry bed of a canyon in Arizona. He estimated 

 the depth of water at the crest of the wave at 

 25 feet and the horizontal distance from the ad- 

 vanced foot of the wave to this crest at about 

 300 feet. This space presented a series of frothy 

 ridges. Perhaps these were a series of roll-waves 

 over -riding one another. The mechanism can be 

 imagined from that described later for the roll- 

 waves of the Griinnbach conduit, where the larger 

 waves, i.e., those which have deepest water at their 

 crests, continually overtake the smaller ones. 



