104 Mr. J. Croll on Ocean-currents in relation to 



crossing over to the northern ; and although the northern he- 

 misphere may not gain much heat by 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 undercurrents and 

 the one or two small surface-currents, that the mean temperature 

 of the water crossing the equator from the southern hemisphere 

 to the northern is very much higher than the mean temperature 

 of the water crossing from the northern hemisphere to the south- 

 ern. The mean temperature of the water crossing the equator 

 from south to north is probably not under 65°, while the 

 mean temperature of the undercurrents is probably not over 

 39°. 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°. Supposing this to be a fair estimate, here 

 we have a difference of 20°. 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 south- 

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

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

 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 Straits 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 as 1500 thermal units, it is found, as we have seen, 

 that an amount of heat is conveyed by this current equal to all 

 the heat that falls within 63 miles on each side of the equator. 

 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 

 hemisphere and added to the northern will therefore make a dif- 

 ference in the amount of heat possessed by the two hemispheres 



