WATER AS A MECHANICAL AGENT. 217 



varices, and rises in consequence of the diminishing depth. At the break- 

 ing, or the collapse, of the wave, the waters are thrown forward, and dash, 

 for the most part, up the shore, while the trough part of the wave flows off 

 as the " undertow," followed down the beach by the returning water of the 

 collapsed wave. 



Some features of the movement are well illustrated in Hawaiian surf-riding. The 

 Hawaiian, swimming out with his plank, plunges beneath the first billow and rises beyond 

 it ; then dives beneath another, and another, until he has passed one of the great billows. 

 This he mounts, and, if rightly placed on it, rides to the beach with great speed. Should 

 his plank not keep the right angle on the crest of the billow, the surf of the following wave 

 will overtake him ; but this he would avoid by diving beneath it and swimming out farther 

 for a fresh start. 



The work done by the wave-and-current agency includes abrasion of the 

 most violent kind, as well as the gentlest, and transportation and deposition 

 as extensive as coast lines and shallow sea-borders or seas. It is the agency 

 that preserves to the continents the detritus of the discharging rivers, inas- 

 much as waves work landward ; yet it has aid in this in the fact that sedi- 

 ment drops in salt water in one fifteenth of the time required in fresh. On 

 the borders of the Gulf of Mexico, according to A. Agassiz, river sediments 

 do not extend out beyond the 100-fathom line, for at this depth there is 

 always the usual sea-bottom life. Along the Atlantic border there are sedi- 

 ments in deeper water, but this is because icebergs or icefloes have dropped 

 there loads of gravel and sand. This agency also makes impossible the 

 transportation of material from one continental land to another. If the 

 fabled Atlantis were at the surface over the Dolphin shoal (page 19), 

 the waves and currents would work about it and for it, and allow of no 

 contributions to any outside land, and least of all to America — the con- 

 tinent supposed to have needed help. 



2. Work of denudation. — The waves bring to bear the violence of a cataract 

 upon whatever is within their reach, — a cataract that girts all the continents 

 and oceanic islands. In stormy seas, they have the force of a Niagara, but 

 with far greater effects ; for Niagara falls into a watery abyss, while, in the 

 case of the waves, the rocks are made bare anew for each successive plunge. 

 They work by impact, and with enormous force. They have also great 

 abrading power added to impact, through the load of debris they take up 

 and transport. Stevenson, in his experiments at Skerry vore (west of Scot- 

 land), found the average force of the waves for the five summer months 

 to be 611 pounds per square foot, and for the six winter months, 2086 pounds. 

 He mentions that the Bell Kock Lighthouse, 112 feet high, is sometimes 

 buried in spray from ground-swells Avhen there is no wind, and that on No- 

 vember 20, 1827, the spray was thrown to a height of 117 feet, — equivalent 

 to a pressure of nearly three tons per square foot. During a westerly gale in 

 March, 1845, his dynamometer registered a pressure of 6083 pounds per square 



foot, which gives for the velocity per second, by the formula, -* /^ (-P being 



