59 



of this would be to concentrate salt in the Mediterranean until 

 its waters began to deposit salt in their bed. But they are of a 

 depth comparable with that of the Mediterranean; and under 

 these circumstances the under-current just described is a ne- 

 cessary result of hydrostatic principles. What has been said 

 does not explain the fact of the bottom water of the Mediter- 

 ranean being, during the summer when it was examined by 

 the Porcupine expedition, about 20 degrees colder than the 

 surface water. Mr. Murphy offered the following explana- 

 tion : — That the surface waters are not constantly sinking 

 down, but sink in the winter only, when, after being con- 

 densed by the concentration of salt in the summer, they are 

 further condensed by cooling; the permanent temperature of 

 the bottom water is consequently the winter temperature of 

 the surface. 



The case of the Baltic is converse to that of the Mediter- 

 ranean. It receives more water by rain and rivers than it loses 

 by evaporation. A surface current flows out of the Baltic 

 into the German Ocean, and an under-current in the opposite 

 direction. The currents at the entrance to Baffin's Bay ap- 

 pear to be similar to these. There is a surface-current out of 

 Baffin's Bay and the Atlantic, and the existence of an under- 

 current has been shown by a vast iceberg, the submerged part 

 of which acted as a "current drag," and caused it to drift 

 rapidly in the direction opposite to that of the surface current. 



The currents of the ocean are greatly influenced by the 

 winds. In the Equatorial seas the surface waters are driven 

 westward by the trade-winds ; they are thus forced into the 

 Gulf of Mexico, and escape through the strait between Florida 

 and Cuba into the Atlantic, forming the Gulf-stream. It is 

 often said that the current of comparatively warm water which 

 flows past our shores and into the Polar Sea is a continuation 

 of the Gulf-stream ; but in- this there is the same kind of con- 



