1820.] Philosophical Transactio)is for 1819, Part II. 441 



This may partly account for the diflerence. Iii estimating the 

 specific Vravity of the water from the northern hemisphere, the 

 specimens marked with an asterisk (*) have been omitted, a& 

 obviously below the truth, either from ice, or from the Iresh 



water of rivers. no—T 



The mean specific gravity of water from the equator is 1-U.J/ / /. 

 This exceeds a little the 'mean gravity which prevails in the 

 northern hemisphere ; but falls short of that in the southern 

 hemisphere. The mean specific gravity in the northern, south- 

 ern hemispheres, and at the equator, is as follows : 



Northern hemisphere 1-02757 



Equator 1-02777 



Southern hemisphere 1-02919 



But it ought to be remarked, that Dr. Davy generally found 

 the specific "gravity of sea Avater both in the South Atlantic and 

 in the Indian Ocean less than what is stated in the preceding 

 tables. When sea water is taken from the surface, it may be 

 conceived to be modified a little by the quantity of ram which 

 has fallen ; for it is reasonable to believe that immediately aftei-^ 

 a heavy fall of rain, the specific gravity of water taken at the 

 surface of the sea will be a little less than it would be if takea 

 after a long tract of dry weather. 



There is no satisfactory evidence that the sea at great depths- 

 is more strongly impregnated with salt than at the surface; 

 except at the mouth of the Dardanelles where the specific gra- 

 vity of the water at the surface was 1-02028, while the specific 

 gravity of the water from the bottom was 1-02819. VVe may- 

 conceive this to be owing to a current of water from the INIedi- 

 terranean flowing into the Black Sea at the bottom, while a 

 counter current from the Black Sea flows towards the Meaiter- 

 ranean at the surface ; for the specific giavity of thewater attne 

 bottom approaches that of tlve Mediterranean, while that at the 

 surface approaches the specific gravity of the Black Sea. 



The Mediterranean Sea contains more salt, and has a higher 

 specific gravitv than that of the Atlantic Ocean ; while most 

 other inland seas, as the Baltic imd the Black Sea, contain less 

 .salt than the ocean. This is accounted for by supposino- that 

 the evaporation in that sea is greater than the supply of fresh 

 water ; the consequence of which is, that a current from the 

 Atlantic sets into the Mediterranean through the straits ot 

 Gibraltar; and in oi'der to obviate the supposed consequence, 

 that on such an hypothesis the saltness of the Mediterranean 

 would continually increase, it has been conceived that an under 

 current of salter water sets in from the Mediterranean to the 

 Atlantic at the bottom of the straits of Gibraltar. 



Dr. Marcet is of opinion, that he is entitled to conchide from 

 the specimens of water from ice taken from the sea which he has 

 examined, that the ice of sea water is always perfectly tresh ; but 



