270 Dr. J. Croll on the Cause of 



ocean-currents. It was shown at considerable length (' Cli- 

 mate and Time/ pp. 27-30, and other places) that the amount 

 of heat that can be conveyed from the equator to the poles 

 by means of aerial currents is trifling, and that, consequently, 

 the transference must be referred to the currents of the ocean. 

 It became obvious then that the influence of ocean-currents in 

 the distribution of heat over the globe had been enormously 

 underestimated. In order to ascertain with greater certainty 

 that such had been the case, I resolved on determining, if pos- 

 sible, in absolute measure the amount of heat actually being 

 conveyed from the equator to temperate and polar regions 

 by means of ocean-currents. 



The onry great current whose volume and temperature had 

 been ascertained with any degree of certainty was the Gulf- 

 stream. On computing the absolute amount of heat con- 

 veyed by that stream, it was found to be more than equal to 

 all the heat received from the sun within 32 miles on each 

 side of the equator. The amount of equatorial heat carried 

 into temperate and polar regions by this stream alone is 

 therefore equal to one fourth of all the heat received from the 

 sun by the North Atlantic from the Tropic of Cancer up to 

 the Arctic circle*. Although the heating-power of the Gulf- 

 stream had long been known, yet no one had imagined that 

 the warmth of our climate was due, to such an enormous extent, 

 to the heat conveyed by that stream. The amount of heat 

 received by an equatorial zone 64 miles in breadth repre- 

 sents, be it observed, merely the amount conveyed by one 

 current alone. There are several other great currents some 

 of which convey as much heat polewards as the Gulf-stream. 

 On taking into account the influence of the whole system of 

 oceanic circulation, it was not surprising that the difference of 

 temperature between the equator and the poles should be 

 reduced from 200° to 80°. 



From these considerations, the real cause of former com- 

 paratively mild climates in Arctic regions became now appa- 

 rent. All that was necessary to confer on, say, Greenland a 

 condition of climate which would admit of the growth f a 

 luxuriant vegetation was simply an increase in the amount of 

 heat transferred from equatorial to Arctic regions by means 

 of ocean-currents. And to effect this change of climate no 

 very great amount of increase was really required ; for it was 

 shown that the severity of the climate of that region was 

 about as much due to the cooling effect of the permanent snow 

 and ice as to an actual want of heat. An increase in the 

 amount of warm water entering the Arctic Ocean, just suffi- 

 * < Climate and Time/ pp. 34, 35 ; Phil. Mag., February 1870. 



