210 PHYSICAL GEOGEAPHi OF THE SKI, AND ITS METEOEOLOGY, 



clouds in tlie Hooper 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,* and it comes in as a make-weight in the 

 system of oceanic climatology, circulation, and stabihty. Now, 

 suppose there were no trade-winds to evaporate and to counter- 

 act 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 inter- 

 vening sea with a mantle of warmth as with a garment. The 

 cool and heavy water of the polar basin, coming out as under cur- 

 rents, 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 perhaps, at some futm*e time, be considered worthy of special 

 inquiry. We have akeady 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 diminishes 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 35^ or 40"" and the 

 equator, evaporation is in excess of precipitation ; at any rate, 

 there is but little precipitation except under the equatorial cloud- 

 ring (see Storm and Kain Chart, Plate ^XIII.) ; and though, as we 

 approach the equator on either side from these parallels, the solar 

 ray warms and expands the sm-face water of the sea, the winds, by 

 the vapour they carry off and the salt they leave behind, prevent 

 it from making that water hghter. 



438. Thus two antagonistic forces are unmasked, and, being 

 Nicely adjusted, uumasked, WO discovcr in them a most exquisite 

 adjustment — a compensation — by which the dynamical forces 

 that reside in the sunbeam and the trade-wind are made to coun- 

 terbalance each other ; by which the climates of intertropical seas 

 are regulated ; and by which the set, force, and volume of oceanic 

 currents are measm'ed. This compensation is most beautiful; it 

 explains the paradox (§ 434), gives volume to the harmonies of 

 the sea, and makes them louder in their song of Almighty praise 

 than the noise of many waters. Philosophers have admired the 

 relations between the size of the earth, the force of grawity, and 

 * See chap. XXTI. on the Actinometky of the Sea. 



