84 WAVES OF THE SEA 



be given to unravelling this part of the subject. 

 My explanation of the systematic effect of the 

 ship's waves to shorten the apparent wave-length 

 I believe to be an important part of the whole 

 explanation, but it may not be the sole cause. 



It is possible that the observed elevation of the 

 wave-crest above the ship's flotation line is some- 

 times increased on account of the presence of a 

 ship-wave tending to make the recorded heights 

 of waves too great, especially when observed from 

 large, fast ships. 



On the other hand, a large vessel, among waves 

 shorter than herself, neither rises to the crests nor 

 sinks to the troughs, so that in observations such 

 as my own on the Minnehaha and the Ivernia the 

 recorded height of the wave -crest above the still - 

 water-line of the ship is probably less than the 

 height above the trough. 



Thus, these two possible sources of error tend 

 to neutralise each other, and as the heights 

 recorded by eye on large ships agree with those 

 recorded on smaller ones, and both are in accord- 

 ance with aneroid determinations, as far as these 

 have been carried, we may regard them as probably 

 free from any large systematic error such as that 

 which the ship-wave, and perhaps other superposi- 

 tion, introduces in one of the methods of measuring 

 wave-lengths. 



