The Sea and Its Work 



eight or ten feet in height. The disturbance 

 produced by such waves extends downwards to 

 only a comparatively small depth. In fact, the 

 motion of the largest waves is almost imper- 

 ceptible at a depth of about 300 fathoms, or 1,800 

 feet; while the agitation produced by ordinary 

 waves must be quite insignificant at one-third of 

 this depth. So far, then, as the destruction of 

 the land by the sea depends on the mechanical 

 action of such waves, it must cease at about one 

 hundred fathoms. Indeed it is probably very 

 feeble at depths much less than this; and, in most 

 cases, on our own shores, it is not very marked 

 below the limit of the lowest tide. 



Winds not only agitate the sea and produce 

 irregular waves, but where they are constantly 

 blowing over the ocean in a definite direction 

 they cause the surface-water to take a similar 

 course, and thus produce steady drifts or currents. 

 Dr. Croll has shown that the direction of the great 

 ocean currents agrees very closely with that of 

 the prevailing winds. Bottles thrown overboard 

 from ships in the open ocean may be carried by 

 these currents for hundreds of miles, and ulti- 

 mately cast upon distant shores. Pieces of wood, 

 and nuts and seeds, known to be native to the 

 West Indies and tropical America, are occasionally 

 drifted across the Atlantic, and are washed on 

 to the western shores of England, Scotland and 

 Ireland, and even across to Norway. In like 

 manner, the Portuguese men-of-war (Physalia, 

 Velella) and those oceanic snails with violet shells 

 161 



