464 THE POPULAR SCIENCE MONTHLY. 



tively thin stratum of the surface-water before the return of warm 

 weather. 



How does this apparently anomalous state of things come about ? 

 If we suppose the globe to be covered with a universal ocean, it can 

 hardly be doubted that the cold of the regions toward the poles must 

 tend to cause the superficial water of those regions to contract and 

 become specifically heavier. Under these circumstances, it would 

 have no alternative but to descend and spread over the sea-bottom, 

 while its place would be taken by warmer water drawn from the adja- 

 cent regions. Thus, deep, cold, polar-equatorial currents, and super- 

 ficial, warmer, equatorial-polar currents, would be set up ; and, as the 

 former would have a less velocity of rotation from west to east than 

 the regions toward which they travel, they would not be due southerly 

 or northerly currents, but southwesterly in the Northern Hemisphere, 

 and northwesterly in the Southern ; while, by a parity of reasoning, 

 the equatorial-polar warm currents would be northeasterly in the 

 Northern Hemisphere, and southeasterly in the Southern. Hence, as 

 a northeasterly current has the same direction as a southwesterly 

 wind, the direction of the northern equatorial-polar current in the 

 extratropical part of its course would pretty nearly coincide with that 

 of the anti-trade winds. The freezing of the surface of the polar sea 

 would not interfere with the movement thus set up. For, however 

 bad a conductor of heat ice may be, the unfrozen sea-water imme- 

 diately in contact with the under surface of the ice must needs be 

 colder than that farther off; and hence will constantly tend to descend 

 through the subjacent warmer water. 



In this way it would seem inevitable that the surface-waters of the 

 northern and southern frigid zones must, sooner or later, find their 

 way to the bottom of the rest of the ocean ; and there accumulate to 

 a thickness dependent on the rate at which they absorb heat from the 

 crust of the earth below, and from the surface-water above. 



If this hypothesis be correct, it follows that, if any part of the 

 ocean in warm latitudes is shut off from the influence of the cold polar 

 underflow, the temperature of its deeps should be less cold than the 

 temperature of corresponding depths in the open sea. Now, in the 

 Mediterranean, Nature offers a remarkable experimental proof of just 

 the kind needed. It is a land-locked sea which runs nearly east and 

 west, between the twenty-ninth and forty-fifth parallels of north lati- 

 tude. Roughly speaking, the average temperature of the air over it 

 is 75 Fahr. in July, and 48 in January. 



This great expanse of water is divided by the peninsula of Italy 

 (including Sicily), continuous with which is a submarine elevation car- 

 rying less than 1,200 feet of water, which extends from Sicily to Cape 

 Bon in Africa, into two great pools an eastern and a western. The 

 eastern pool rapidly deepens to more than 12,000 feet, and sends off 

 to the north its comparatively shallow branches, the Adriatic and the 



