AND OTHER WATER WAVES 171 



ratio is not even approximately constant. It is 

 quite different, not only for shores of different 

 slopes, but also for a given shore it is quite dif- 

 ferent during on-shore and off-shore wind respec- 

 tively. Thus Colonel D. D. Gaillard, U.S.A., 

 found ' that with a given locality and a given slope 

 a wave broke in a depth of water equal to ij times 

 its height when there was a strong wind blowing 

 shorewards, while with an equally strong wind 

 blowing off-shore the wave remained unbroken 

 until the depth of the water was only f of its height. 



When there was no wind and the breakers were 

 due solely to the ocean swell the wave broke in 

 a depth of water equal to the height of the crest 

 above the trough when the slope was i in 100, 

 but when the slope was i in 1 2 the wave broke 

 when the depth was more than twice the wave 

 height. 



The breaking of a wave is, of course, due to the 

 water near the surface in the crest of the wave 

 moving much more quickly forward than the water 

 immediately below it. This condition must be 

 reached sooner or later when ordinary waves from 

 the deep sea advance in the continually shallow- 

 ing water caused by a sloping bottom. On the 



1 "Wave Action in Relation to Engineering Structures," 

 Washington, 1904, Chapter VII. 



10 



