216 THE PHYSICAL GEOGRAPHY OF THE SEA. 



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 conse- 

 quent change which it would produce upon the specific gravity 

 of the sea, would still further augment its dynamical power, un- 

 til, 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 the tempera- 

 ture of its waters, is sent off by radiation or taken up by the va- 

 por, 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 color bears upon their heat-absorb- 

 ing properties, 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 dy- 

 namical force of the sun ; this hot and light water, by becoming 

 hotter and lighter, would flow off in currents, with almost mill-tail 

 velocity, toward the poles, covering the intervening sea with a 

 mantle of warmth as with a garment. The cool and heavy wa- 

 ter of the polar basin, coming out in under currents, would flow 

 equatorially with equal velocity. How much, if to any extent, 

 the former warm climates of the British Islands and Northern 

 Asia may be due to such a warm covering of the sea, may per- 

 haps, at some future time, be considered worthy of special in- 

 quiry. We have already seen (§ 434) that there is something 

 else besides temperature that is at work in effecting changes in 

 the specific gravity of sea water. Whatever increases or dimin- 

 ishes its saltness, increases or diminishes its specific gravity ; and 

 the agents that are at work in the sea doing this are sea shells, 

 the rivers, and the rains, as well as the winds. Between 85° or 

 40° and the equator, evaporation is in excess of precipitation ; at 

 any rate, there is but little precipitation except under the equa- 

 torial cloud-ring (see Storm and Rain Chart, Plate XIII.); and 

 though, as we approach the equator on either side from these 

 parallels, the solar ray warms and expands the surface water of 



