§ U9. THE SPECIFIC GRAVITY OF THE SEA, ETC. 225 



Amount of salt in, tbe saltness of the sea is a physical necessity. In 

 g?avitTof sea^ w^a*". somc of the aspccts 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 vapor, hold in check the 

 expansive power (§ 437) of tropical heat upon the sea ; nor coun- 

 teract, 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 lev- 

 el 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 wa- 

 ter of fresh polar seas would grow heavier and heavier by reason 

 of the cold — a condition which, by reason of evaporation and pre- 

 cipitation, is almost the very reverse of that which nature has or- 

 dained for 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 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. These specific gravity and thermal curves, as they are 

 Light cast by Plate prcscntcd ou this Plate (X.), throw liorht also on the 



X. on the open sea . p • i a • ^-v rrn 



in the Arctic Ocean. qUCStlOU 01 aU OpCU SCa lU thc ArCtlC OccaU. 1 hat 



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 the south, which, by virtue of its 

 saltness (see Fig. 2), is heavier than the cool and upper current 

 which runs out of the polar basin, and which is known as an ice- 

 bearing current. It is the same which is felt by mariners as far 

 down as the Grand Banks of Newfoundland, and recognized by 



P 



