170 WAVES OF THE SEA 



during the previous on-shore wind. I found that 

 when the water was but little above my knees 

 the crest of the wave about to break close by 

 was considerably above my head, showing that the 

 wave remained unbroken when the depth of water 

 immediately in front was considerably less than 

 the height of the wave from trough to crest. In 

 a rough sea with on-shore wind the cusped break- 

 ing waves were formed much beyond the limit 

 of a man's depth. These waves not only broke 

 farther out, but also when the cusp was not per- 

 fectly formed. The upper part of the crest folded 

 itself down upon the front face, becoming a mass 

 of seething water, and the wave then travelled on 

 as a bore with its head of froth. The motion in 

 the frothing head was violently shorewards, and 

 it was impossible to swim in it. 



Many measurements have been made by en- 

 gineers to determine the depth of water in which 

 waves break. Very roughly the general result may 

 be expressed by saying that the depth of water 

 (reckoned from the undisturbed sea -level) is equal 

 to the height of the crest above the trough. I 

 shall not pause to discuss the somewhat unsatis- 

 factory mode of expression, the point I wish to 

 bring out being that the more those measurements 

 have multiplied the clearer it has become that the 



