142 WAVES OF THE SEA 



the onward flow, for it takes place at a lower 

 level . 



Below the surface trochoid are sub-surface 

 trochoids, in which the diameter of the circle which 

 the particles of water describe diminishes with 

 increasing depth. 



The diameter of the circle and the velocity of 

 the oscillating currents diminish in geometrical 

 progression as the depth increases in arithmetical 

 progression, the diameters being halved for an 

 additional depth of i-Qth wave-length below the 

 mid -level of the surface wave. Thus, at a depth 

 equal to one wave-length, the diameter of the circle, 

 and the velocity of the moving particle of water, 

 is 1-51 2th of that at the surface . Thus, in the 

 case of a wave with a length of 600 feet and a 

 height from trough to crest at the sea's surface of 

 40 feet, the particles of water at a depth of 600 feet 

 will revolve in a circle of 40 feet -r- 512 =0.94 

 inch diameter. The period of a 6oo-foot wave is 

 about 1 1 seconds, and this is the time in which the 

 particle at a depth of 600 feet would describe 

 a circle of about I inch diameter. 



We have seen that waves of the greatest height 

 are seldom more than 600 feet long in the North 

 Atlantic, and it is seldom that very long swells, 

 being much lower, would give a greater agitation 



