ation, that is,
from east to west,"[390] and thus a current, which would have run simply
towards the north but for the rotation, may acquire a relative direction
towards the west.

We may next consider a case where the circumstances are the converse of
the above. The Gulf Stream flowing from about lat. 20 degrees is at first
impressed with a velocity of rotation of about 940 miles an hour, and
runs to the lat. 40 degrees, where the earth revolves only at the rate
of 766 miles, or 174 miles slower. In this case a relative motion of an
opposite kind may result; and the current may retain an excess of
rotatory velocity, tending continually to deflect it eastward. Polar
currents, therefore, or those flowing from high to low latitudes, are
driven towards the eastern shores of continents, while tropical currents
flowing towards the poles are directed against their western shores.

Thus it will be seen that currents depend, like the tides, on no
temporary or accidental circumstances, but on the laws which preside
over the motions of the heavenly bodies. But although the sum of their
influence in altering the surface of the earth may be very constant
throughout successive epochs, yet the points where these operations are
displayed in fullest energy shift perpetually. The height to which the
tides rise, and the violence and velocity of currents, depend in a great
measure on the actual configuration of the land, the contour of a long
line of continental or insular coast, the depth and breadth of channels,
the peculiar form of the bottom of seas--in a word, on a combination of
circumstances which are made to vary continually by many igneous and
aqueous causes, and, amongst the rest, by the tides and currents
themselves. Although these agents, therefore, of decay and reproduction
are local in reference to periods of short duration, such as those which
history embraces, they are nevertheless universal, if we extend our
views to a sufficient lapse of ages.

_Destroying and transporting power of currents._--After these
preliminary remarks on the nature and causes of currents, their velocity
and direction, we may next consider their action on the solid materials
of the earth. We shall find that their efforts are, in many respects,
strictly analogous to those of rivers. I have already treated in the
third chapter, of the manner in which currents sometimes combine with
ice, in carrying mud, pebbles, and large fragments of rock to great