silver, 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, from being displaced, will rise, and flowing in an opposite direction, spread itself at the top. The expansion and contraction of sea water by heat and cold, have in a similar manner, 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 contrary to that of 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 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° F. between the depths of 1000 and 6000 feet; and Captain Smith has shown in his survey, that the deepest part in the Straits 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 the polar ice. The rotation of the earth on its axis is another cause which can only come into play when the waters have been already 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 operates has been long recognized in the case of trade winds; thus, when a current flows from the Cape of Good Hope towards the Gulph of Guinea, it consists of a mass of water, which, on doubling the Cape, in latitude 35°, has a rotatory velocity of about 800 miles an hour; but when it reaches the line, it arrives 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 juxta position, 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. {23}