SALINITY AND SPECIFIC GEAVITY. 645 



considers the limits are not so exact, but it appears certain that tiie 

 maximum lies not far from lat. 25° N., 'ong. 35° W., and that it never 

 exceeds 37-5 parts per 1,000. Northward of lat. 45°, the Salinity does 

 not exceed 36 parts, and in Davis Strait it is 34^ parts per 1,000. 



Several forms of apparatus have been devised for securing samples of the 

 water from great and intermediate depths. On the voyage of H.M.S. 

 Challenger, samples were collected at various depths in various parts of 

 the world, and Professor Dittmar, from an analysis of 77 specimens, con- 

 cluded that the average composition of the salts of sea- water is as follows : — 

 Chloride of Sodium, 77-758 ; Chloride of Magnesium, 10-878 ; Sulphate of 

 Magnesium, 4-737 ; Sulphate of Lime, 3-600 ; Sulphate of Potash, 2-465 ; 

 Bromide of Magnesium, 0-216 ; and Carbonate of Lime, 0-345. 



The Specific Gravity of ocean water, free from land and other influences, 

 varies within very narrow limits, 1-024 and 1-028 at 60° F., or 15*56° Cent. 

 In the regions of great rainfall near the Equator, the surface is frequently 

 quite fresh ; and in the Temperate zone there is nearly an equilibrium 

 between the rain and evaporation. To these sub-divisions of concentration 

 and precipitation is assigned an important part in the formation of Ocean 

 Currents. 



The following is also by Mr. J. Y. Buchanan, M.A. (the chemist of the 

 Challenger expedition), from a paper read before the Eoyal Geographical 

 Society in 1877 :— 



The Specific Gravity* of the surface water was determined (at a tem- 

 perature of 39° F.), every day when at sea, and from the bottom and inter- 

 mediate depths as often as opportunity offered. 



As far as the surface is concerned, the general results were as follows. 

 The concentration of the waters of the Atlantic Ocean is greater than that 

 of either the Pacific or the Southern Ocean, and it is greater in the North 

 Atlantic than in the South Atlantic, although the actual maximum may 

 be slightly higher in the South Atlantic. In the North Atlantic, the 

 maximum (1.0275) was observed in lat. 22° N., long. 40° W., from which 

 point it diminishes in all directions. (On the South Coast of Iceland it 

 was 1.0260). 



If we consider the water below the surface, as shown in the vertical 

 sections, we find, in the Atlantic, that in the concentration-areas the 

 Specific Gravity diminishes until a minimum (1.0260) is reached at a depth 

 of about 800 or 1,000 fathoms, after which it increases slightly down to the 

 bottom. In investigating the causes of the variations in the Specific Gravity 

 of the Ocean, we find that they depend on the means available for remov- 

 ing or supplying water. Thus, the areas of greatest concentration coincide 

 with those where the dry Trade Winds are constantly blowing, taking their 

 rise in the lower Temperate latitudes, and proceeding in their course always 

 from colder to warmer regions, so that, for the first part of their journey, 

 at least, although they are continually taking up moisture, their capacity 



* According to Erman's elaborate investigations, the weight of salt in 1,000 parta of 

 sea- water, of different specific gravities, is — 



Specific Gravity 1.025 1.026 1.027 1.028 



Salts per 1,000 33.765 35.049 36..343 37.637 



