1/2 PHYSIOGRAPHY. [chap. 



wind, however, urges the surface-water forwards to a small 

 extent. A fresh breeze tears off the water from the crest 

 of a wave, and scatters it as spray, and a heavy gale con- 

 verts this into blinding showers of salt rain. The wind too 

 catches the top of the wave, and causing it to move faster 

 than the water below, urges it to leeward in the form of a 

 graceful curl, the edge of which breaks into foam. On 

 reaching a shore, the retardation of the deeper part of the 

 wave by friction against the sea bottom, increases the 

 relative velocity of the superficial part, and the latter 

 rolls over; the water bursts with great force upon the 

 land, and then sweeps back, as a powerful " undertow," 

 to the sea. 



However agitated the surface of the sea may be, there is 

 reason to believe that the disturbance never extends far 

 downwards. The more violent the wind, the greater of 

 course will be the agitation which it is capable of pro- 

 ducing ; but, even during a storm, the waves never attain 

 to anything like the height which is often popularly ascribed 

 to them. It is not uncommon to hear of the sea runninsr 

 "mountains high ;" yet, in a strong gale in the open ocean 

 the height of a wave, from crest to trough, rarely exceeds 

 forty feet. In the shallow seas around our own islands, 

 they are far from attaining to such a magnitude ; the largest 

 waves, even in a storm, not exceeding 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 imperceptible at a 

 depth of about 300 fathoms, or 1,800 feet; whil'e the agita- 

 tion 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 100 fathoms. Indeed 



