AND OTHER WATER WAVES 79 



known to be 280 feet. But the interval between 

 the arrival of the waves was 8| seconds, and, as 

 the vessel was travelling in a direction somewhat 

 opposed to the direction of the waves, the true 

 wave-period must have been somewhat greater. 

 An 8J-seconds wave has a length in deep water of 

 370 feet and a 9-seconds wave of 41 5 feet. 



Assuming the determination of the period to be 

 fairly accurate, as the observation is an easy one, 

 and the application of the mathematical formula 

 to be valid, as there is every reason to suppose, 

 we must seek some reason why the apparent wave- 

 length should be quite 90 feet less than the true. 



Two possibilities suggest themselves. The first 

 is that minor sea-waves are noticed in the simul- 

 taneous observations of wave-crests, but are passed 

 over by the eye when watching the progress of 

 the more rapidly moving crests of the principal 

 sea -waves. 



The second possibility is that the shortening of 

 apparent wave-length was due to the increased 

 height of the sea -waves in the vicinity of the bow 

 and the stern of the vessel. A large vessel pro- 

 ceeding at a fair speed in the above case a vessel 

 of about 6,400 tons going at 12 knots produces 

 a short, steep wave, several feet high, at the bow, 

 and a similar one at the stern. In smooth water 



