the winds to evaporate, and the quantity of salts in the sea—these are so proportioned and adjusted that when both the wind and the sun have each played with its forces upon the intertropical waters of the ocean, the residuum of heat and of salt should be just such as to balance each other in their effects, and so the aqueous equilibrium of the torrid zone is preserved.
437. Compensating influences.—Nor are these the only adjustments effected by this exquisite combination of compensations. If all the intertropical heat of the sun were to pass into the seas upon which it falls, simply raising the temperature of their waters, it would create a thermo-dynamical force in the ocean capable of transporting water scalding hot from the torrid zone, and spreading it, while still in the tepid state, around the poles. The annual evaporation from the trade-wind region of the ocean has been computed, according to the most reliable observation, to be as much as 15 feet, which is at the rate of half an inch per day. The heat required for this evaporation would raise from the normal temperature of intertropical seas to the boiling-point a layer of water covering the entire ocean to the depth of more than 100 feet. Such increase of temperature, by the consequent change which it would produce upon the specific gravity of the sea, would still further augment its dynamical power, until, even in the Atlantic, there would be force enough to put in motion and feed with boiling-hot water many Gulf Streams. But the trade-winds and the seas are so adjusted that this heat, instead of penetrating into the depths of the ocean to raise inordinately the temperature of its waters, is sent off by radiation or taken up by the vapour, or borne away by under currents, or carried off by the winds, and dispensed by the clouds in the upper air of distant lands. Nor does this exquisite system of checks and balances, compensations and adjustments, end here. In equatorial seas the waters are dark blue, in extra- tropical they are green. This difference of colour bears upon their heat-absorbing properties,[1] and it comes in as a make-weight in the system of oceanic climatology, circulation, and stability. Now, suppose there were no trade-winds to evaporate and to counteract the dynamical force of the sun; this hot and light water, by becoming hotter and lighter, would flow off in currents with almost mill-tail velocity, towards the poles, covering the intervening sea with a mantle of warmth as with a garment. The cool and heavy water of the polar basin, coming out as under currents, would flow equatorially with equal
- ↑ See chap. XXII. on the Actinometry of the Sea.
