36 THE OCEAN WORLD. 



the north of the name isthmus. It is related that a ship was cast 

 ashore upon a rock during the night, so high, that at daybreak the 

 crew found themselves and their ship suspended in mid-air far above 

 the water ! 



In the Mediterranean, which only communicates with the ocean 

 by a narrow channel, the phenomenon of tides is scarcely felt, and 

 from this cause that the moon acts at the same time upon its whole 

 surface, which is not sufficiently large to increase the swelling mass 

 of waters formed by the moon's attraction ; consequently, the swell- 

 ing remains scarcely perceptible. This is the reason why neither 

 the Black Sea or White Sea presents a tide, and the Mediterranean a 

 very inconsiderable one. Nevertheless, at Alexandria the tide rises 

 twenty inches, and at Venice this height is increased to about four 

 and a half feet. Lake Michigan is slightly affected by the lunar 

 attraction. 



Professor Whewell has prepared a map in which the course of the 

 tidal wave is traced in every country of the globe. We see there that 

 it traverses the Atlantic, from 50 of south latitude up to the fiftieth 

 parallel north, at the rate of 560 miles an hour. But the rapidity 

 with which it proceeds is least in shallow water. In the Nortrj Sea 

 it travels at the rate of 180 miles. The tidal wave which proceeds 

 round the coast of Scotland traverses the German Ocean and meets 

 in St. George's Channel, between England and Ireland, where the 

 conflict between the two opposing waves presents some very compli- 

 cated phenomena. 



The winds, again, exercise a great influence on the height of the 

 tides. When the impulse of the wind is added to that of the 

 attracting moon, the normal height of the wave is considerably 

 increased. If the wind is contrary, the flux of the tide is almost 

 annihilated. This happens in the Gulf of Vera Cruz, where the tide 

 is only perceptible once in three days, when the wind blows with 

 violence. An analogous phenomenon is observable on the coast of 

 Tasmania. 



The rising tide sometimes strikes the shore with a continuous and 

 incredible force. This violent shock is called the surf. The swell 

 then forms a billow, which expands to half a mile. The surf increases 

 as it approaches the coast, when it sometimes attains the height of 

 six or seven yards, forming an overhanging mountain of water, which 

 gradually sinks as it rolls over itself. But this motion is not in 

 reality progressive it transports no floating body. The surf is very 

 strong at the Isle of Fogo, one of the Cape de Verd Islands, in the 



