218 PHYSICAL GEOGRAPKY OF THE SEA, AND ITS METEOllOLOGY. 



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 inter- 

 tropical 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 adjust- 

 ments 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 

 da}^. 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 Avaters, 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. Kor 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,* 

 and it comes in as a make-weight in the system of oceanic clima- 

 tology, circulation, and stability. Xow, 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. 



