of Gibraltar is only 1320 feet, so that a
submarine barrier exists there which must prevent the influx of any
under-current of the ocean cooled by polar ice.

Each of the four causes above mentioned, the wind, the tides,
evaporation, and the expansion and contraction of water by heat and
cold, may be conceived to operate independently of the others, and
although the influence of all the rest were annihilated. But there is
another cause, the rotation of the earth on its axis, which can only
come into play when the waters have already been set in motion by some
one or all of the forces above described, and when the direction of the
current so raised happens to be from south to north, or from north to
south.

The principle on which this cause operates is probably familiar to the
reader, as it has long been recognized in the case of the trade winds.
Without enlarging, therefore, on the theory, it will be sufficient to
offer an example of the mode of action alluded to. When a current flows
from the Cape of Good Hope towards the Gulf of Guinea, it consists of a
mass of water, which, on doubling the Cape, in lat. 35 degrees, has a
rotatory velocity of about 800 miles an hour; but when it reaches the
line, where it turns westward, it has arrived at a parallel where the
surface of the earth is whirled round at the rate of 1000 miles an hour,
or about 200 miles faster. If this great mass of water was transferred
suddenly from the higher to the lower latitude, the deficiency of its
rotatory motion, relatively to the land and water with which it would
come into juxtaposition, would be such as to cause an apparent motion of
the most rapid kind (of no less than 200 miles an hour) from east to
west.

In the case of such a sudden transfer, the eastern coast of America,
being carried round in an opposite direction, might strike against a
large body of water with tremendous violence, and a considerable part of
the continent might be submerged. This disturbance does not occur,
because the water of the stream, as it advances gradually into new zones
of the sea which are moving more rapidly, acquires by friction an
accelerated velocity. Yet as this motion is not imparted
instantaneously, the fluid is unable to keep up with the full speed of
the new surface over which it is successively brought. Hence, to borrow
the language of Herschel, when he speaks of the trade winds, "it lags or
hangs back, in a direction opposite to the earth's rot