AND OTHER WATER WAVES 105 



of fetch the greater is the distance from which 

 the surface-water draws the reinforcement of its 

 long-period heaving, and the greater, therefore, 

 is that part of the wave disturbance which is of 

 greater wave-length than the dominant wave. 



What, now, precisely is this dominant or storm- 

 wave, and how is it evolved? The answer is not 

 difficult if we think of the mode of motion of the 

 wind as it blows over the wave -water. There 

 must be a continual give and take between the 

 wind and the water, such that the air above tends 

 to go into a regular series of travelling vortices or 

 eddies, with long-extended horizontal axes, rolling 

 along in the hollows between the crests of a regular 

 series of travelling water-ridges. Above this series 

 of travelling eddies the air must flow in undulating 

 lines, the amplitude of the undulations diminishing 

 with the height above the water -surf ace, so that 

 at a considerable altitude the air flows in straight 

 lines. When the sea has attained to an approxi- 

 mately steady wave condition under the action of 

 the wind, there is superposed upon it a train of 

 wind-eddies (and above them, aerial undulations), 

 which are of regularly increasing size for a long 

 distance from the windward shore. At each suc- 

 cessive position as we recede from the windward 

 shore there is a characteristic, and successively 



