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



ticular place is 50 miles broad and 1000 feet deep, or that it 

 actually flows at the uniform rate of 4 miles an hour at surface 

 and bottom. All I meant was, that the Gulf-stream is equal to 

 that of a current of the above size and velocity. But in my 

 recent papers on Ocean-currents 1 have taken the volume of 

 the stream at one half this estimate, viz. equal to a current 

 50 miles broad and 1000 feet deep flowing at the rate of 2 miles 

 an hour. I have estimated the mean temperature of the stream 

 as it passes the Straits of Florida to be 65°, and have supposed 

 that the water in its course becomes ultimately cooled down on 

 an average to 40° "*. In this case each pound of water conveys 

 25 units of heat from the Gulf of Mexico, to be employed in 

 warming temperate and polar regions. Assuming these data 

 to be correct, it follows that the amount of heat transferred 

 from the Gulf of Mexico by this stream per day amounts 

 to 77,479,650,000,000,000,000 foot-pounds. This enormous 

 quantity of heat is equal to one fourth of all that is received 

 from the sun by the whole of the Atlantic Ocean from the 

 tropic of Cancer up to the Arctic Circle. 



This is the amount of heat conveyed from intertropical to 

 temperate and polar regions by the Gulf-stream. What now 

 is the amount conveyed by means of the general oceanic circu- 

 lation ? If this general interchange of equatorial and polar 

 water be, as Dr. Carpenter supposes, the great agency employed 

 in distributing heat over the globe, then surely it is not too 

 much to expect that the quantity of intertropical heat carried 

 into the North Atlantic and Arctic seas must be at least equal 

 to that carried by the Gulf-stream. 



If we assume this to be the case, then the combined amount 

 of heat conveyed by the two agencies into the Atlantic from 

 intertropical regions will of course be equal to twice that con- 

 veyed by the Gulf-stream alone. Taking the annual quantity of 

 heat received from the sun per unit surface at the equator at 

 1000, the quantities received by the three zones will be respec- 

 tively as follows : — 



Equator 1000 



Torrid zone ..... 975 



Temperate zone . . . 757 



Frigid zone 454 



* It is probable that a large proportion of the water constituting the 

 south-eastern branch of the Gulf-stream is never cooled down to 40° ; 

 but, on the other hand, the north-eastern branch, which passes into the 

 Arctic regions, will be cooled far below 40°, probably below 30°. Hence 

 I cannot be overestimating the extent to which the water of the Gulf- 

 stream is cooled down in fixing upon 40° as the average minimum 

 temperature. 



