In The Seas Of The Tropics We Find, That At Great Depths The Thermometer Marks 7 Or 8 Centesimal Degrees.

Such is the result of the numerous experiments of commodore Ellis and of M. Peron.

The temperature of the air in those latitudes being never below 19 or 20 degrees, it is not at the surface that the waters can have acquired a degree of cold so near the point of congelation, and of the maximum of the density of water. The existence of this cold stratum in the low latitudes is an evident proof of the existence of an under-current, which runs from the poles towards the equator: it also proves that the saline substances which alter the specific gravity of the water, are distributed in the ocean, so as not to annihilate the effect produced by the differences of temperature.

Considering the velocity of the molecules, which, on account of the rotatory motion of the globe, vary with the parallels, we may be tempted to admit that every current, in the direction from south to north, tends at the same time eastward, while the waters which run from the pole towards the equator, have a tendency to deviate westward. We may also be led to think that these tendencies diminish to a certain point the speed of the tropical current, in the same manner as they change the direction of the polar current, which in July and August, is regularly perceived during the melting of the ice, on the parallel of the bank of Newfoundland, and farther north. Very old nautical observations, which I have had occasion to confirm by comparing the longitude given by the chronometer with that which the pilots obtained by their reckoning, are, however, contrary to these theoretical ideas. In both hemispheres, the polar currents, when they are perceived, decline a little to the east; and it would seem that the cause of this phenomenon should be sought in the constancy of the westerly winds which prevail in the high latitudes. Besides, the particles of water do not move with the same rapidity as the particles of air; and the currents of the ocean, which we consider as most rapid, have only a swiftness of eight or nine feet a second; it is consequently very probable, that the water, in passing through different parallels, gradually acquires a velocity correspondent to those parallels, and that the rotation of the earth does not change the direction of the currents.

The variable pressure on the surface of the sea, caused by the changes in the weight of the air, is another cause of motion which deserves particular attention. It is well known, that the barometric variations do not in general take place at the same moment in two distant points, which are on the same level. If in one of these points the barometer stands a few lines lower than in the other, the water will rise where it finds the least pressure of air, and this local intumescence will continue, till, from the effect of the wind, the equilibrium of the air is restored.

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