388 T. C. CHAMBERLIN 



whence the warmer upper and middle layers originate. This is 

 especially true of the outpour from the Mediterranean later to be 

 discussed. Near the great intercontinental dam over which the 

 waters pour in both directions, much mixing and irregularity of 

 motion and constitution is to be expected, and present data are 

 quite inadequate for a discussion of details. Here, however, the 

 general fact of a difference in thickness of the polar and the Atlantic 

 water-columns will perhaps suffice. At any rate, this difference 

 between the polar and Atlantic columns is only a striking intensi- 

 fication of what should arise normally in less degree in all similar 

 cases, for the polar water-column should be relatively thickest in its 

 cold, dense, basal layer and the Atlantic in its warm, saline layer, 

 and these dominant layers should actuate the circulation so far as 

 dependent simply on temperature and salinity. A comparison of 

 the two columns is thus a necessary step toward interpreting the 

 great features of the polar- Atlantic interchange so far as dependent 

 on density, and density is the radical feature of oceanic circulation. 

 Let us then consider these two columns, bearing in mind, however, 

 that the layers of water are not stationary, but moving; that they 

 are not fixed in temperature and salinity, but always changing 

 toward equilibrium.^ 



THE THREE MAIN LAYERS OE THE COLUMN OE THE POLAR SEA 



From the determinations of Nansen"" and others it appears that 

 a typical column of the waters of the Polar Basin from surface to 

 bottom embraces three main layers:^ 



' The data from which these generalized comparisons are made have been gathered 

 chiefly from Murray's weU-known publications, particularly the Challenger Reports, 

 The Ocean Depths, and The Ocean; the papers of Nansen and associates, particularly 

 The Scientific Results of the Norwegian North Polar Expedition (1893-96); Jenkins' 

 Textbook of Oceanography (igii); and James Johnstone's "Oceanography," Enc. Brit., 

 newVol. XXXI (1922). 



^ "Norwegian North Polar Expedition," Scientific Results, Vol. III. 



3 Nansen gives four layers, but for the present general discussion the distinction 

 between the two upper, rather thin layers seems immaterial as both are cold layers of 

 less than mean salinity, and have a common origin in surface conditions. These two 

 layers are thus defined by Nansen: 



" (i) A surface-current of water with low salinity (from about 29 0/00 to about 

 3:? 0/00, perhaps 20 or 30 m. deep, running towards the northwest and west; 



" (2) An underlying, slow current of water with a higher salinity and a very low 

 temperature, running in a different direction, and consisting of surface water from other 

 parts of the Polar Sea. The absolute minimum of temperature is situated in this cur- 

 rent, at about 50 or 60 m." — Ibid., Vol. Ill, p. 346. 



