§ 480, 481. 



THE SALTS OF THE SEA. 



251 



that average sea water invariably assumes during the process of 

 congelation (§ 442). Moreover, the specific gravity of the surface 

 water which Eodgers measured in the Arctic Ocean was (§ 427) less 

 than that of average sea water — a fact in confirmation of this con- 

 jecture as to the office of rain and river water in the polar seas. 

 The freezing-point of the strongest brine is 4° ; consequently, the 

 freezing-point of water in the sea varies according to the propor- 

 tion of salts from 4° all the way up to just below 32°. Thus the 

 salts of the sea impart to its waters an elasticity, as it were, giving 

 a law, a sort of sliding-scale both for the thermal dilatation and 

 of congelation, which varies between that of fresh water and the 

 saltest sea waters according to the degree of their saltness. 



480. Eodgers tried with his hydrometer and thermometer the 

 Layers of water of watcrs of the Arctic Occau at the surface, below, 

 tiifrin' thl^Trctic ^^^ ^t the bottom, and as often as he tried he found 

 ^^^^^' this arrangement : warm and light water on the top, 



cool in the middle, " hot and heavy" at the bottom. His experi- 

 ments were made near Behring's Straits in August, 1855, between 

 the parallels of 71°-2°, and are as per example following : 



1 Near bottom. 



Assuming the surface water which Eodgers used for these ex- 

 periments to be a fair average of arctic surface waters generally, 

 this table affords data that show the proportion of rain and river 

 water that the Arctic Ocean receives annually. The quantity 

 may be inferred from the fact that average sea water has ten per 

 cent, more salt than Eodgers's arctic. 



481. Eeturning now to the drift of the ice, and the drift of the 

 The ice-bearing drift Advaucc and her followers, we see that, so far as 

 thT ordi^a^i^"'' drift currcuts arc concerned, we have in the Arctic 

 from the Baltic. Qccau a repetition merely of the more familiar phe- 

 nomenon that is seen in the Baltic, where (§ 383, note) an under 

 current of salt water runs in, and an upper current of brackish 

 water runs out. Then, since there is salt always flowing out of 

 the north polar basin, we infjr that there must be salt always 



