r in the latitudes in question; and even
when it does, has time to affect only a comparatively thin stratum of the
surface water before the return of warm weather.

How does this apparently anomalous state of things come about? If we
suppose the globe to be covered with a universal ocean, it can hardly be
doubted that the cold of the regions towards the poles must tend to cause
the superficial water of those regions to contract and become
specifically heavier. Under these circumstances, it would have no
alternative but to descend and spread over the sea bottom, while its
place would be taken by warmer water drawn from the adjacent regions.
Thus, deep, cold, polar-equatorial currents, and superficial, warmer,
equatorial-polar currents, would be set up; and as the former would have
a less velocity of rotation from west to east than the regions towards
which they travel, they would not be due southerly or northerly currents,
but south-westerly in the northern hemisphere, and north-westerly in the
southern; while, by a parity of reasoning, the equatorial-polar warm
currents would be north-easterly in the northern hemisphere, and south-
easterly in the southern. Hence, as a north-easterly current has the same
direction as a south-westerly wind, the direction of the northern
equatorial-polar current in the extra-tropical part of its course would
pretty nearly coincide with that of the anti-trade winds. The freezing of
the surface of the polar sea would not interfere with the movement thus
set up. For, however bad a conductor of heat ice may be, the unfrozen
sea-water immediately in contact with the undersurface of the ice must
needs be colder than that further off; and hence will constantly tend to
descend through the subjacent warmer water.

In this way, it would seem inevitable that the surface waters of the
northern and southern frigid zones must, sooner or later, find their way
to the bottom of the rest of the ocean; and there accumulate to a
thickness dependent on the rate at which they absorb heat from the crust
of the earth below, and from the surface water above.

If this hypothesis be correct, it follows that, if any part of the ocean
in warm latitudes is shut off from the influence of the cold polar
underflow, the temperature of its deeps should be less cold than the
temperature of corresponding depths in the open sea. Now, in the
Mediterranean, Nature offers a remarkable experimental proof of just the
kind needed