330 DIl ALEXANDER BUCHAN ON 



At 300 fathoms and greater depths, the temperature of the western basin is 55° "5, and 

 salinity, l - 0287 ; and in the eastern basin temperature is 56 0, 5, and salinity 1'0290, accord- 

 ing to the numerous observations of the "Pola" Expeditions in 1890-92. The deep 

 water temperature of the Mediterranean is thus closely approximate to the temperature 

 of the Atlantic adjoining the Strait of Gibraltar at the depth of the ridge separating the 

 two seas, or nearly 200 fathoms, but the salinity is very greatly in excess. 



At depths of 100 and 200 fathoms specific gravities and temperatures have been 

 observed in the Mediterranean from long. 6° W. to 10° E. at seven points, giving a mean 

 specific gravity of T0290; and at eight places outside the strait in the Atlantic from 

 long. 6° to 20° W., giving a mean specific gravity of 1'0272. At 200 fathoms, a short 

 way to the east of the strait, the specific gravity is 1'0291 ; in the strait, 1"0282 ; and a 

 little way to the west, 1*0273, and a short way to west of this point, 1'0267. The 

 strong under current from the Mediterranean to the Atlantic is the inevitable consequence 

 of the extraordinary difference in the specific gravities of the two seas. The inflowing 

 surface current is, of course, due to the lowering of the level of the Mediterranean by the 

 excessive evaporation from its surface. It is therefore not winds and currents, but 

 different specific gravities and different levels, which rule the interchange of the waters 

 of this sea with the Atlantic. 



Now, just as in the case of the Red Sea, this dense warm water from the Mediterranean 

 sinks to increasingly greater depths as it advances westward through the Atlantic. This 

 result does not admit of being shown so clearly from the observations of specific gravity, 

 owing, as already stated, to their fewness, as it does by the temperature observations. In 

 the Challenger Report it is well shown by the serial maps of temperature, from which 

 it appears that, owing to the great specific gravity of the warm under-current from the 

 Mediterranean, it gradually sinks on entering the Atlantic ; but its effect in heating the 

 waters of this ocean over any considerable extent becomes strikingly apparent at about 

 500 fathoms. Beyond this depth its influence is felt over nearly the whole breadth of 

 the Atlantic to at least about 1000 fathoms. 



The accompanying woodcut, reduced from the Challenger map, show r s this in a very 

 impressive manner. The crowding of the lines near the Strait of Gibraltar, the prolonga- 

 tion of some of them across the whole ocean, and the northward movement of the higher 

 isothermals are very instructive.* 



It is evident, from what has been already advanced with respect to the influence of 



the rainfall and the winds, that the salinity of the surface of the different oceans will 



differ materially from each other. The following are the calculated averages : — 



Mean annual salinity of the surface of the N. Atlantic, ..... 1-0265 



S. Atlantic, 1-0265 



„ ,, „ Indian Ocean, 1-0262 



„ „ „ N. Pacific, P0257 



S. Pacific, 1-0261 



All Oceans, P0262 



* Sw also Atlantischer Ozean Taf. 3. Deutsche Seewarte, Hamburg, 1882. 



