F. GEOGRAPHY. 115 



through the seas of the temperate zone, may be predicted as 

 a physical necessity. The reduction in temperature of the 

 polar column, the whole of which may be brought down by 

 the continued exposure of the surface to atmospheric cold al- 

 most to its freezing point, must diminish its height while aug- 

 menting its density, and thus the water of the surrounding 

 area must flow in to maintain the level thus lowered. But 

 when the column has been restored to an equality of height, 

 it will possess such an excess of weight that its downward 

 pressure must force out a portion of its deeper water, and 

 thus an outflow of ice-cold water will be occasioned from the 

 polar toward the equatorial area, over the sea-bed of the 

 deepest oceanic basins, while at the same time there will be 

 a continual indraught of warmer surface water into the polar 

 basin, which can only be supplied by a general poleward 

 movement of the upper stratum of the equatorial water. 

 These movements will not have the character of currents ; 

 for it is .only where the communication between the two 

 bodies of water takes place through a narrow strait that dif- 

 ferences so inconsiderable can give rise to a perceptible move- 

 ment between them. But the movement is not the less real 

 when diffused than it is when concentrated ; and the same 

 vertical circulation would take place between the two ex- 

 tremities, or between the centre and circumference of the 

 same continuous basin, under opposite conditions as to heat 

 and cold, as would exist if they were connected by a com- 

 paratively narrow channel or communication. Contempora- 

 ry Review, 1871. 



THE SARGASSO SEA. 



Dr. Collingwood has recently published an interesting ac- 

 count of the Sargasso Sea of the North Atlantic, one of sev- 

 eral immense areas of floating meadows of sea-weed found in 

 mid-ocean in different parts of the globe. The one to which 

 our author refers is that which occupies the greater portion 

 of that breadth of the Atlantic Ocean between the coast of 

 Africa and the region of the West Indies, from 20 to about 

 65 of west longitude, and from the parallel of 20 to that of 

 45. This area is compared to that of the Mississippi Val- 

 ley ; and this immense bed of floating sea-weed was at one 

 time supposed to be derived from plants originally attached 



