256 Mr. J. Croll on the Physical Cause of Ocean- currents. 



of Florida is equal to all the heat received from the sun by 

 1,560,935 square miles at the equator. The annual quantity of 

 heat received from the sun by the torrid zone per unit surface, 

 taking the mean of the whole zone, is to that received by the 

 equator as 39 to 40, consequently the quantity of heat conveyed 

 by the Gulf-stream is equal to all the heat received by 1,600,960 

 square miles of the Atlantic in the torrid zone. 



Dr. Carpenter is mistaken in supposing that " all the calcu- 

 lations which have been made as to the quantity of water which 

 issues from the Narrows, and the amount of heat which it con- 

 veys, are based upon the assumption that both its temperature and 

 its rate of movement are the same throughout its depths as they 

 are at its surface"*. The surface-temperature of the stream at 

 the Narrows is somewhat about 85°; but I have taken the mean 

 temperature of the water at this place as only 65°. The cold 

 return current, according to Dr. Carpenter, has a temperature 

 as low as 30° or 32° ; but, not to overestimate the quantity of 

 heat derived from the Gulf-stream, I have taken the return 

 current at 40°. In this case the quantity of heat conveyed 

 through the Narrows I estimate to be $5 thermal units per 

 pound of water. But had I taken the surface-temperature of 

 the stream and Dr. Carpenter's estimate as to the temperature 

 of the cold return current, I should have had 53 or 55 thermal 

 units per pound as the amount conveyed. My data were de- 

 rived, not from popular treatises on physical geography, but 

 from a careful analysis of the sections and charts of the United- 

 States Coast Survey ; and any one who will be at the trouble to 

 examine these will easily satisfy himself that I have underesti- 

 mated both the temperature and volume of the stream. 



But if, according to Dr. Carpenter's views, the quantity of 

 heat conveyed from the tropical regions is double that conveyed 

 by the Gulf-stream, the amount of heat in this case conveyed 

 into the Atlantic in temperate regions will be equal to all the 

 heat received from the sun by 3,201,920 square miles of the 

 Atlantic between the equator and the tropic of Cancer. This 

 is |~| of all the heat received from the sun by that area. 



Taking the annual quantity received per unit surface at the 

 equator at 1000, the quantities received by the three zones would 

 be respectively as follows : — 



Equator 1000 



Torrid zone 975 



Temperate zone 757 



Frigid zone 454 



Now, if we remove from the Atlantic in tropical regions ff 



* ' Nature/ vol. ii. p. 334. 



