228 On the Loss of Heat by the Souther?! Hemisphere. 



means of them, it ; on the other hand, does not lose much ; for 

 the heat which they give out in their progress along the southern 

 hemisphere does not belong to the northern hemisphere. 



But after making the fullest allowance for the amount of heat 

 carried across the equator from the northern hemisphere to the 

 southern, we shall find, if we compare the mean temperature of 

 the currents from the southern hemisphere to the northern with 

 the mean temperature of the great compensating undercurrent 

 and the one or two small surface-currents, that the mean tempe- 

 rature of the water crossing from the southern hemisphere to the 

 northern is very much higher than the mean temperature of the 

 water crossing from the northern to the southern. The mean 

 temperature of the water crossing the equator from south to north 

 is probably not under 65° F., while the mean temperature of 

 the undercurrent is probably not over 39° F. But we must add 

 to them the surface-currents from north to south. And let us 

 assume that this will raise the mean temperature of the entire 

 mass of water flowing from north to south to, say, 45° F. Here 

 we have a difference of 20° F. Each cubic foot of water which 

 crosses the equator will in this case transfer about 1250 units of 

 heat from the southern hemisphere to the northern. If we had 

 any means of ascertaining the volume of those great currents 

 crossing the equator, we should then be able to make a rough 

 estimate of the total amount of heat transferred from the southern 

 hemisphere to the northern ; but as yet no accurate estimate has 

 been made on this point. Let us assume, what is probably much 

 below the truth, that the total amount of water crossing the 

 equator is at least double that of the Gulf-stream as it passes 

 through the Strait of Florida, which amount we have already 

 found to be equal to 133,816,320,000,000 cubic feet daily*. 

 Taking the quantity of heat conveyed by each cubic foot of water 

 of the Gulf-stream at 1500 thermal units, it is found that an 

 amount of heat is conveyed by the current equal to all the heat 

 that falls within 63 miles on each side of the equator f. Then, 

 if each cubic foot of water crossing the equator transfers 1250 

 thermal units, and the quantity of water is double that of the 

 Gulf-stream, it follows that the amount of heat transferred from 

 the southern hemisphere to the northern is equal to all the heat 

 falling within 105 miles on each side of the equator, or equal to 

 all the heat falling on the southern hemisphere within 210 miles 

 of the equator. This quantity taken from the southern hemi- 

 sphere and added to the northern will therefore make a differ- 

 ence in the amount of heat possessed by the two hemispheres 

 equal to all the heat which falls on the southern hemisphere 



* Phil. Mag. for June 1867, p. 433. Geol. Mag. for April 1869. 

 t Ibid. p. 434. 



