246 rnYSicAL geoghaphy of the sea, and its meteorology. 



current of brino 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 currenis oioing 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 Eed 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 per- 

 ceive 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 wann 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 projjerty 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 emploj^-ed 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 vv^inds 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. 



