HYDROGRAPHY 157 



enclosed deep basins like the Black Sea where in the bottom 

 waters there is a marked deficiency of oxygen and a large 

 production of sulphuretted hydrogen), there must be suffic- 

 ient circulation of the bottom waters to convey the oxygen 

 into the abysses. 



In addition to what is present in combination, carbon 

 dioxide is found free in small and variable quantities in sea- 

 water. There is a free interchange of carbon dioxide between 

 the surface of the sea and the atmosphere, and this tends 

 to regulate the amount in the water, which, however, varies 

 considerably from time to time, as there are great differences 

 in the amounts used and produced by plants and animals 

 respectively in different parts of the sea and at different times 

 of year. 



Various methods have been employed to determine the 

 salinity of sea-water, such as evaporating, drying, and 

 weighing the salts ; ascertaining the specific gravity or weight 

 relatively to fresh water, at a definite temperature, such as 

 60° F. ; or estimating the amount of chlorine by titration 

 and calculating from that the total salts present, as the ratio 

 of the salts to each other is practically constant although the 

 total quantity may vary from as much as 39 parts in 1,000 

 down to 31 — or any amount less close to land or in estuaries. 

 Even in the North Atlantic (an ocean of relatively high 

 salinity) regions differ greatly. For example, in the Sargasso 

 Sea the salinity may be from 37 to 38 parts per thousand, at 

 the Azores 36, off the west of Ireland 35, and from 34 down 

 to 31 close to Newfoundland. The highest records known 

 (over 39 %o) are in the Eastern Mediterranean and the 

 Northern Red Sea, where the evaporation is great and the 

 rainfall small in amount. 



In the open sea, as a general rule, the saHnity diminishes 

 from the surface downwards to about 1,000 fathoms, but in 

 still greater depths there is generally salter water at the 

 bottom. Near land, however, there may in places be a layer 

 of fresh, or almost fresh, water on the surface. This is well 



