80 WAVES OF THE SEA 



these are stationary relatively to the ship, and 

 have a constant height. In a slight head sea, 

 however, the stationary bow-wave is replaced 

 by an intermittent wave, rising up, when the 

 sea-wave meets the bow, and subsiding at the 

 trough of the sea-waves. In a heavy head sea 

 the ship's own bow-wave becomes inconspicuous, 

 but, nevertheless, contributes as much as before 

 to the total height of the sea-waves as they reach 

 the bow and the stern. Now, looking aft, the 

 steeper ship-wave will maintain the apparent crest 

 of the flatter sea -wave for some distance after the 

 crest of the true sea-wave has passed. Conversely, 

 looking forward, the steep bow-wave will cause 

 an anticipation of the true crest of the advancing, 

 flatter sea -wave. Consequently the above measure- 

 ment of wave-length by simultaneous position of 

 crests along the ship's side was too small by twice 

 the distance through which the steeper ship-wave 

 shifted the position of the combined wave-crest. 1 



Where the sea waves are large and regular and 

 long, and the ship small and slow, this error is 

 least. With a moderate sea and a large and swift 

 ship the error is greatest. 



Officers of the Atlantic liners generally estimate 



1 The presence of ship's waves need not affect the deter- 

 mination of wave-length from observed velocity, for the interval 

 between the arrival of a sea-wave at st^rn and bow is the same 

 as if there were no ship-wave. 



