We speak here only of currents, whose extent and rapidity are very considerable, for in every sea there are an infinity of currents, though of no great importance. The flux and reflux, the winds, and all other causes which agitate the waters, produce currents, more or less perceptible, in different parts. We have observed that the bottom of the sea, like the surface of the earth, is overspread with mountains intersected with inequalities and divided by banks of sand. In all mountainous places currents will be violent; in all places where the bottom of the sea is level they will be almost imperceptible; the rapidity of the current will increase in proportion to the obstacles the water meets with, or rather to the contraction of the spaces through which they incline to pass. Between two chains of mountains the current will be so much the stronger as the mountains are near. It will be the same between two banks of sand, or two neighbouring islands. It is also remarked in the Indian ocean, which is divided with an infinity of islands and banks, there are rapid currents throughout, which render the navigation of that sea dangerous.
It is not inequalities at the bottom of the sea alone which form currents, but the coasts themselves have a similar effect, as the water is repelled at greater or lesser distances: this repulsion of the waters is a kind of current which circumstances can render continual and violent; the oblique position of a coast, the vicinity of a bay, or of some great river, a promontory; in one word, every particular obstacle which opposes the general motion, will always produce a current. Now, as nothing is more irregular than the bottom and borders of the sea, we must cease from being surprised at the great number of currents which every where appear.
All currents have a determinate breadth, which depends on that of the interval between the two eminences which serves it for a bed. The currents flow into the sea as rivers flow on land, and they produce similar effects. They form their bed, and give to eminences corresponding angles. In one word, it is these currents which hollowed our vallies, formed our mountains, and gave to the surface of the earth, when it was under water, the form it now retains.
If any doubt of the correspondence of the angles of mountains remains, I appeal to the sight of every man who makes the observation. Every traveller, with the smallest attention, will perceive that the opposite sides of a hill exactly correspond. Whenever the hills to the right of the valley form a projection, those opposite recede to the left. These hills have also nearly the same elevation, and it is very rare to see any great inequality of height in the two hills separated by a valley. I can assert, that the more I have looked on the circumference and heights of hills, the more I have been convinced of the correspondence of the angles, and of the resemblance they have with the beds and borders of rivers; and it is by reiterated observations on this surprising regularity and resemblance that my first ideas of this Theory of the Earth arose. Let us add to these observations that of the parallel and horizontal situation of the strata, that of the shells being dispersed throughout the earth, and incorporated in every matter; and it must be admitted, that on a subject like this we cannot have a greater degree of probability.
OF REGULAR WINDS.
Nothing can appear more irregular and variable than the force and direction of winds in our climates; but there are countries where this irregularity is not so great, and others where the winds constantly blow in one direction, and with almost the same degree of strength.