30 THE WATER 



1,000 pounds of the water solution. " Specific gravity " 

 is the weight of a unit volume of the water solution ; 

 it varies with temperature as well as with salinity, 

 since water expands when heated and contracts when 

 cooled. 



That the determinations of salinity must be made 

 accurately appears from the fact that the total range 

 of variation in the open sea is at most between 33 and 

 37 per 1,000, the average salinity running between 

 35 and 36. 



The chart which follows shows the general distri- 

 bution of salinity in the oceans so far as it is known, 

 but it must be understood that in many regions, where 

 actual observations are few and far between, the 

 " isohalines " (lines of equal saltness) are drawn with 

 a free hand (compare Chart IV. with this). 



A study of these charts shows at once the general 

 nature of some of the problems which a knowledge of 

 salinity helps to elucidate, and which depend for their 

 complete solution, in part at least, on a more intimate 

 acquaintance with the local distribution in certain 

 seas, and the changes which occur. 



It appears (a), that the saltest surface water is 

 found in the trade wind belts, where strong sunshine 

 and steady winds favour great evaporation. From 

 maps of specific gravity it appears that the high 

 temperature of the water keeps it relatively light, 

 notwithstanding its great saltness, and hence we have 

 to inquire as to the sources whence the deficiency of 

 water due to removal by evaporation is made good. 

 Again (b), the freshest surface water occurs in either 

 regions of widespread excessive rainfall — such as 

 the Western Pacific — or regions in high latitudes, 

 where cold fresh water is set free by the melting of 

 floe-ice and icebergs. The investigations of the last 



