THE SPECIFIC GRAVITY OF THE SEA, ETC. 227 



order to weigh the seas in this manner, it is necessary that the little 

 hydrometric balance by which it is to be done should be well and 

 truly adjusted. 



448. Amount of salt in, and mean specific gravity of sea water. — 

 From these premises it would not be difficult to show that the 

 sal tn ess of the sea is a physical necessity. In some of the aspects 

 presented, the salts of the sea hold the relation in the terrestrial 

 mechanism that the balance-wheel does to the machinery of a 

 watch. Without them the climates of the earth could not harmo- 

 nize as they do ; neither could the winds, by sucking up vapour, 

 hold in check the expansive power of tropical heat upon the sea ; 

 nor counteract, by leaving the salts behind, the thermal influence 

 of the sun in imparting dynamical force to marine currents ; nor 

 prevent the solar ra}" from unduly disturbing the aqueous equi- 

 librium of our planet. As evaporation goes on from a sea of fresh 

 water, the level only, and not the specific gravity, of the remain- 

 ing water is changed. The waters of fresh intertropical seas 

 would, instead of growing heavy b}^ reason of evaporation between 

 the tropics, become lighter and lighter by reason of the heat ; 

 while the water of fresh polar seas would grow heavier and 

 heavier by reason of the cold — a condition which, by reason of 

 evaporation and precipitation, is almost the very reverse of that 

 which nature has ordained fur the salt sea, and which, therefore, 

 is the wisest and the best. The average amount of salts in sea 

 water is not accurately known. From such data as I have, I 

 estimate it to be about 4 per cent. (.039), and the mean specific 

 gravity of sea water at 60" to be about 1.0272. Supposing these 

 conditions to be accurate — and they are based on data which 

 entitle them to be considered as not ver^^ wide of the mark — 

 the hydrometer and thermometer, with the aid of the table (§ 441), 

 will give us a direct measure for the amount of salt in any specimen 

 of sea water into which the navigator will take the trouble to dip 

 these two instruments. 



449. Light cast by Plate X. on tJie open sea in the Arctic Ocean. — 

 These specific gravity and thermal curves, as they are presented 

 on this Plate (X.), throw light also on the question of an open sea 

 in the Arctic Ocean. That open sea is like a boiling spring 

 (§ 427) in the midst of winter, which the severest cold can never 

 seal up ; only it is on a larger scale than any spring, or pool, or 

 lake, and it is fed by the under currents with warm water from 



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