TIDES. 39 



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 Yenice this height is increased to about six feet and a 

 half. Lake Michigan is slightly affected by the lunar attraction. 



Professor Whewell has prepared maps, in which the course of the 

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

 it traverses the Atlantic, from the fiftieth degree of south latitude up 

 to the fiftieth parallel north, at the rate of five hundred and sixty 

 miles an hour. But the rapidity with which it proceeds is least in 

 shallow water. In the North Sea it travels at the rate of a hundred 

 and eighty 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 complicated 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 attract- 

 ing planet, 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 per- 

 ceptible 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 Togo, one of the Cape de Yerd Islands in the 

 Indian Ocean, and at Sumatra, where the surf renders it dangerous 

 and sometimes impossible to land on the coast. Fig. 7 represents the 

 effects of the surf at Point du Baz, on the coast of Brittany. 

 The winds adding their influence to these causes, give birth on 

 the surface of the sea to waves or billows, which increase rapidly, 

 rising in foaming mountains, rolling, bounding, and breaking one 

 against the other. "In one moment," says Malte Brun, "the 



