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 saltness 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 harmonize 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 ray from unduly disturbing the aqueous equilibrium of our planet. As evaporation goes on from a sea of fresh water, the level only, and not the specific gravity, of the remaining water is changed. The waters of fresh intertropical seas would, instead of growing heavy by 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 very 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 the 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
