340 GEOLOGY, 



ing depth (Fig. 299), but the rate of diminution itself diminishes, and 



Fig. 299. — Figure illustrating the decrease in the amount of wave-movement with 

 increase of depth, (Fenneman.) 



fchere seems no theoretic reason for assigning any definite Hmit to the 

 downward propagation of the oscillation. 



At the surface, the radius of the circular orbit which a particle of 

 water in a wave tends to describe is half the height of the wave. At a 

 depth equal to one wave-length, the radius of the circle described by a 

 particle is 3-J-5- as great as at the surface, and at a depth equal to two 

 wave-lengths, 3o-oV"oo- If "the height of a wave be 43 feet, the radius 

 of the circle described by a surface particle is 21 J feet. If the length of 

 the wave be 300 feet, the radius of a particle at a depth of 300 feet is 

 only about -/q- of an inch, and at 600 feet y^Vo of ^^ inch.^ These 

 figures make it clear that effective agitation of the water does not ex- 

 tend to great depths. 



So long as the velocity of the wind remains constant, the velocity of 

 the current which the wind-waves generate is less than that of the wind, 

 and there is always a differential movement of the wa'er, each layer 

 moving faster than the one beneath. The friction is thus distributed 

 through the whole vertical column of the water in movement, and is 

 even borne in part by the sea-bottom if the movement extends so far 

 down. The greater the depth, the smaller the share of the friction 

 each layer of water is called upon to bear, and the greater the velocity 

 of the current generated by a given wind. But while the wave-motion 

 extends indefinitely downward, the lower limit of agitation effective 

 in erosion is soon reached. Engineering operations have shown that 

 * Dana. Manual of Geology, 4th ed., p. 213. 



