ments in the ocean, a cause to which, perhaps, currents are
principally due. Whenever the temperature of the surface of the sea is
lowered, condensation takes place, and the superficial water, having its
specific gravity increased, falls to the bottom, upon which lighter
water rises immediately and occupies its place. When this circulation of
ascending and descending currents has gone on for a certain time in high
latitudes, the inferior parts of the sea are made to consist of colder
or heavier fluid than the corresponding depths of the ocean between the
tropics. If there be a free communication, if no chain of submarine
mountains divide the polar from the equatorial basins, a horizontal
movement will arise by the flowing of colder water from the poles to the
equator, and there will then be a reflux of warmer superficial water
from the equator to the poles. A well-known experiment has been adduced
to elucidate this mode of action in explanation of the "trade
winds."[389] If a long trough, divided in the middle by a sluice or
partition, have one end filled with water and the other with
quicksilver, both fluids will remain quiet so long as they are divided;
but when the sluice is drawn up, the heavier fluid will rush along the
bottom of the trough, while the lighter, being displaced, will rise,
and, flowing in an opposite direction, spread itself at the top. In like
manner the expansion and contraction of sea-water by heat and cold, have
a tendency to set under-currents in motion from the poles to the
equator, and to cause counter-currents at the surface, which are
impelled in a direction contrary to that of the prevailing trade winds.
The geographical and other circumstances being very complicated, we
cannot expect to trace separately the movements due to each cause, but
must be prepared for many anomalies, especially as the configuration of
the bed of the ocean must often modify and interfere with the course of
the inferior currents, as much as the position and form of continents
and islands alter the direction of those on the surface. Thus on
sounding at great depths in the Mediterranean, Captains Berard and
D'Urville have found that the cold does not increase in a high ratio as
in the tropical regions of the ocean, the thermometer remaining fixed at
about 55 degrees F. between the depths of 1000 and 6000 feet. This might
have been anticipated, as Captain Smyth in his survey had shown that the
deepest part of the Straits