]go SEA AND LAND 



a depth of five hundred feet — they beg-in to have a sensible 

 effect upon the bottom, oj)erating- to brush the finer materials 

 in the direction in which the surges are moving; attaining 

 yet shallower water, this rubbing action is proportionately in- 

 creased. At the depth of a hundred feet the effect of these 

 waves in sweeping sands in toward the shore may be con- 

 siderable. The action is probably of sufficient energy to 

 drive even small pebbles up the slope toward the land. 

 Owing to the friction which the front part of the wave 

 encounters beyond that which the following part meets as 

 it passes over the upward slope of the bottom, the surge 

 becomes ever narrower in cross section as it approaches 

 the shore — that is to say, it is higher in proportion to its 

 width. As this friction of the bottom of the wave on the 

 lloor of the sea increases, the upper part of the surge, for 

 the reason that the lluid there is less hindered in its for- 

 ward movement by the resistance of the bottom, shoots 

 forward, cjuickly acquires a wall-like front, and finally its 

 upper part flies clear beyond the base and combs over in 

 the form of a "roller." Owing to the long-continued friction 

 on the bottom which the wave has encountered in its move- 

 ment over the shallows toward the shore, its volume and 

 energy are commonly very much reduced from the conditions 

 presented in the open sea. Usually the surge when it breaks 

 upon the shore has not more than the fourth to the tenth 

 of the power which it had in the water a thousand feet 

 deep. Were it not for this loss of energy, the effect of the 

 ocean surges on the land would be vastly greater than it 

 usually is. 



Watching the action of ocean waves along a gently slop- 

 ino- shore, we observe that the lesser undulations, such as 



