36 THE OCEAN WORLD. 
’ the north of the same 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 North 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 suf. The swell 
then forms a billow, which expands to half amile. 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 
