We have already spoken of a submarine current which appears to carry the waters of the Mediterranean into the Atlantic Ocean. Its existence is in some respects established by calculations which prove that the quantity of salt water supplied by the upper current through the Straits of Gibraltar is equal to seventy-two cubic miles per annum, while the quantity of fresh water brought down by the rivers is equal to six, and the quantity lost by evaporation to twelve cubic miles per annum. This would leave an annual excess of sixty-six cubic miles, if the equilibrium was not re-established by an under current flowing into the Atlantic. This hypothesis would appear to have been confirmed by a very curious fact.
Towards the end of the seventeenth century, a Dutch brig, pursued by the French corsair Phœnix, was overhauled between Tangier and Tarifa, and seemed to be sunk by a single broadside; but, in place of foundering and going down, the brig, being freighted with a cargo of oil and alcohol, floated between the two currents, and, drifting towards the west, finally ran aground, after two or three days, in the neighbourhood of Tangier, more than twelve miles from the spot where she had disappeared under the waves. She had therefore traversed that distance, drawn by the action of the under current in a direction opposite to that of the surface current. This ascertained fact, added to some recent experiments, lend their support to the opinion which admits of the existence of an outward current through the Straits of Gibraltar. Dr. Maury quotes an extract from the "log" of Lieutenant Temple, of the United States Navy, bearing the same inference. At noon on the 8th of March, 1855, the ship Levant stood into Almeria Bay, where many ships were waiting for a chance to get westwards. Here he was told that at least a thousand sail were waiting between the bay and Gibraltar, "some of them having got as far as Malaga only to be swept back again. Indeed," he adds, "no vessel had been able to get out into the Atlantic for three months past." Supposing this current to run no faster than two knots an hour, and assuming its depth to be four hundred feet only, and its width seven miles, and that it contained the average proportion of solid matter, estimated at one-thirtieth, it appears that salt enough to make eighty-eight cubic miles of solid matter were carried into the Mediterranean in those ninety days. "Now," continues Dr. Maury, "unless there were some escape for all this solid matter which has been running into the sea, not for ninety days, but for ages, it is very clear that the Mediterranean would long ere this have been a vat of strong brine, or a bed of cubic crystals."
For the same reason, Dr. Maury considers it certain that there is an under current to the south of Cape Horn, which carries into the Pacific Ocean the overflowings of the Atlantic. In fact, the Atlantic is fed unceasingly by the Great American rivers, while the Pacific receives no important affluent, but ought to be, and is, subjected to enormous losses, in consequence of the evaporation continually taking place at the surface.
Tides.
Tides are periodical movements produced by the attraction of the sun and moon. This action, which influences the whole mass of the earth, is made manifest by the swelling movement of the waters. The attractive force exercised by the moon is three times that of the sun, in consequence of its approximation to the earth, as compared to the greater luminary.
In order to comprehend the theory of tides, we shall first consider the lunar influences, putting aside for a moment the solar action.
Fig. 5. Lunar Tides.
The attraction which the moon exercises upon any point on the earth's surface is in the inverse ratio of the square of its distance. If we draw a straight line from the moon passing through the centre of the earth, this line will meet the surface of the waters at two points diametrically opposite to each other—namely, z and n (Fig. 5); one of these points would be to the moon its zenith, the other its nadir. The point of the sea which has the moon in the zenith—namely, that above which the moon is perfectly perpendicular—will be nearest to the planet, and will consequently be more strongly attractive to the centre of the earth, while the points diametrically opposite to which the moon is the nadir will be more distant, and consequently less strongly attracted by that luminary. It follows that the waters situated directly under the moon will be attracted towards it, and form an accumulation or swelling at that point; the waters at the antipodes being less strongly attracted to the moon than to the centre of the earth, will form also a secondary swelling on the surface of the sea, thus forming a double tide, accumulating at the point nearest the moon and at its antipodes. At the intermediate points of the circumference of the globe, where the waters are not subjected to the direct attraction of the moon, the sea is at low water, as represented in Fig. 5.
The earth, in its movement of rotation, presents, in the course of twenty-four hours, every meridian on its surface to the lunar attraction; consequently, each point in its turn, and at intervals of six hours, is either under the moon, or ninety degrees removed from it: it follows, that in the space of a lunar day—that is to say, in the time which passes between two successive passages of the moon on the same meridian—the oceanic waters will be at high and low tide twice in the month on every point of the surface of the globe. But this result of attraction is not exercised instantaneously. The moon has passed from the meridian of the spot before the waters have attained their greatest height; the flux reaches its maximum about three hours after the moon has culminated; and the watery mountain follows the moon all round the globe, from east to west, about three hours in its rear.