106 WAVES OF THE SEA 



larger, size of dominant or storm -wave, and this 

 is the wave of length identical with that of the 

 air-eddy. These air-eddies, as has been already 

 said, hamper the development of shorter waves 

 whose full growth would require the existence of 

 shorter air -eddies ; but they have little effect upon 

 the longer and flatter swell, which possesses great 

 energy, stirs the water to considerable depths, and 

 is continually reinforced by gravitational transmis- 

 sion from great distances. 1 Thus it seems that in 

 the attempt to extract precise and even numerical 

 results for the visual observation of waves at sea, 

 we may neglect that part of the wave disturbance 

 which is of shorter wave-length than the domi- 

 nant or storm-waves (" the waves " of common 

 parlance), but that we must not ignore the swell, 

 'which is of greater wave-length. 2 



1 I suspect the existence of another mechanism contributing 

 to the same result. Referring once more to the swell running 

 by gravity suppose this to have attained a regular gradation 

 of wave-length. Then each section will presumably be opaque 

 to and absorb vibrations of its own period and fransmit or 

 be transparent to vibrations of greater period. 



2 The theory of the action of wind to increase the height of 

 waves already running before it is that the horizontal velocity 

 of the air being greatest at the crest, the downward pressure of 

 the atmosphere is least there. Conversely at the trough, where 

 horizontal velocity is least, downward pressure is greatest. 

 Hence the trough is pushed farther down and the crest is 

 sucked up. 



