impressed on an expansive ocean, by winds blowing for many
months in one direction, may easily be conceived, when we observe the
effects produced in our own seas by the temporary action of the same
cause. It is well known that a strong southwest or northwest wind
invariably raises the tides to an unusual height along the west coast of
England and in the Channel; and that a northwest wind of any
continuance causes the Baltic to rise two feet and upwards above its
ordinary level. Smeaton ascertained by experiment, that in a canal four
miles in length, the water was kept up four inches higher at one end
than at the other, merely by the action of the wind along the canal; and
Rennell informs us that a large piece of water, ten miles broad, and
generally only three feet deep, has, by a strong wind, had its waters
driven to one side, and sustained so as to become six feet deep, while
the windward side was laid dry.[385]

As water, therefore, he observes, when pent up so that it cannot escape,
acquires a higher level, so, in a place _where it can escape_, the same
operation produces a current; and this current will extend to a greater
or less distance, according to the force by which it is produced. By the
side of the principal oceanic currents, such as the Lagullas and the
Gulf Stream, are parallel "counter-currents" running steadily in an
opposite direction.

Currents flowing alternately in opposite directions are occasioned by
the rise and fall of the tides. The effect of this cause is, as before
observed, most striking in estuaries and channels between islands.

A third cause of oceanic currents is evaporation by solar heat, of which
the great current setting through the Straits of Gibraltar into the
Mediterranean is a remarkable example, and will be fully considered in
the next chapter. A stream of colder water also flows from the Black Sea
into the Mediterranean. It must happen in many other parts of the world
that large quantities of water raised from one tract of the ocean by
solar heat, are carried to some other where the vapor is condensed and
falls in the shape of rain, and this, in flowing back again to restore
equilibrium, will cause sensible currents.

These considerations naturally lead to the inquiry whether the level of
those seas out of which currents flow, is higher than that of seas into
which they flow. If not, the effect must be immediately equalized by
under-currents or counter-currents. Arago is of