current of brine supplies, in a great measure, the salt which the upper current, freighted with fresh water from the clouds and rivers, carries back.
471. The under currents going entirely to the salts of sea water.—Thus it is to the salts of the sea that we owe that feature in the system of oceanic circulation which causes an under current to flow from the Mediterranean into the Atlantic (§ 385), and another (§ 377) from the Red Sea into the Indian Ocean. And it is evident, since neither of these seas is salting up, that just as much, or nearly just as much salt as the under current brings out, just so much the upper currents carry in. We now begin to perceive what a powerful impulse is derived from the salts of the sea in giving effective and active circulation to its waters. Hence we infer (§ 461) that the currents of the sea, by reason of its saltness, attain their maxim of volume and velocity. Hence, too, we infer that the transportation of warm water from the equator towards the frozen regions of the poles, and of cold water from the frigid towards the torrid zone, is facilitated; and consequently here, in the dynamical power which the sea derives from its salts, have we not an agent by which climates are mitigated—by which they are softened and rendered much more salubrious than it would be possible for them to be were the waters of the ocean deprived of their property of saltness?
472. A property peculiar to seas of salt water.—This property of saltness imparts to the waters of the ocean another peculiarity, by which the sea is still better adapted for the regulation of climates, and it is this: by evaporating fresh water from the salt in the tropics, the surface water becomes heavier than the average of sea water (§427). This heavy water is also warm water; it sinks, and being a good retainer, but a bad conductor of heat, this warm water is employed in transporting through under currents, heat for the mitigation of climates in far-distant regions. Now this also is a property which a sea of fresh water could not have (§ 430). Let the winds take up their vapour from a sheet of fresh water, and that at the bottom if not disturbed, for there is no change in the specific gravity of that at the surface by which that at the bottom may be brought to the top; but let evaporation go on, though never so gently, from salt water, and the specific gravity of that at the top will soon be so changed as (§ 404) to bring that from the very lowest depths of the sea to the top.
