377. Upper and under currents through straits explained.—But the salt water, which has lost so much of its freshness by evaporation, becomes salter, and therefore heavier. The lighter water at the Straits cannot balance the heavier water at the Isthmus, and the colder and salter, and therefore heavier water, must either run out as an under current, or it must deposit its surplus salt in the shape of crystals, and thus gradually make the bottom of the Red Sea a salt-bed, or it must abstract all the salt from the ocean to make the Red Sea brine—and we know that neither the one process nor the other is going on. Hence we infer that there is from the Red Sea an under and outer current, as there is from the Mediterranean through the Straits of Gibraltar, and that the surface waters near Suez are salter than those near the mouth of the Red Sea. And, to show why there should be an outer and under current from each of these two seas, let us suppose the case of a vat of oil, and a vat of wine connected by means of a narrow trough—the trough being taken to represent the straits connecting seas the waters of which differ as to specific gravity. Suppose the trough to have a flood-gate, which is closed until we are ready for the experiment. Now let the two vats be filled, one with wine the other with oil, up to the same level. The oil is introduced to represent the lighter water as it enters either of these seas from the ocean, and the wine the same water after it has lost some of its freshness by evaporation, and therefore has become salter and heavier. Now suppose the flood-gate to be raised, what would take place? Why, the oil would run in as an upper current, overflowing the wine, and the wine would run out as an under current.
378. The Mediterranean current.—The rivers which discharge their waters into the Mediterranean are not sufficient to supply the waste of evaporation, and it is by a process similar to this that the salt which is carried in from the ocean is returned to the ocean again: were it not so, the bed of that sea would be a mass of solid salt. The unstable equilibrium of the seas is a physical necessity. Were it to be lost, the consequences would be as disastrous as would be any derangement in the forces of gravitation. Without doubt, the equilibrium of the sea is preserved by a system of compensation as exquisitely adjusted as are those by which the "music of the spheres" is maintained. It is difficult to form an adequate conception of the immense
