194 Mr. J. Croll on Ocean-currents. 



earth is greater in June, when the earth is in aphelion, than in 

 December, when it is in perihelion. This, I venture to affirm, 

 is also what ought to follow according to theory, although this 

 very fact has been adduced as a proof that eccentricity has 

 at present but little effect on the climatic condition of our 

 globe. 



That the mean temperature of the whole earth would, during 

 the glacial epoch, be greater when the earth was in aphelion 

 than when in perihelion will, I think, be apparent from the fol- 

 lowing considerations : — When the earth was in the perihelion, 

 the sun would be over the hemisphere nearly covered with snow 

 and ice. The great strength of the sun's rays would in this 

 case have little effect in raising the temperature ; it would be 

 spent in melting the snow and ice. But when the earth was in 

 the aphelion, the sun would be over the hemisphere compara- 

 tively free, or perhaps wholly free, from snow and ice. Conse- 

 quently, though the intensity of the sun's rays would be less 

 than when the earth was in perihelion, still it ought to pro- 

 duce a higher temperature, because it would be chiefly em- 

 ployed in heating the ground, and not consumed in melting 

 snow and ice. 



The present difference between the climatic condition of the 

 two hemispheres is exactly what it ought to be on the supposi- 

 tion that it is produced by eccentricity. From what has already 

 been stated, I presume that the low temperature of the southern 

 hemisphere cannot be referred to the mere distribution of sea 

 and land, unless we admit that an enormous amount of heat is 

 constantly being transferred by means of ocean-currents from 

 the southern hemisphere to the northern. For, as we have seen, 

 the physical properties of water, statically considered, ought to 

 produce a higher mean temperature than that of land. And if 

 it be admitted that the present lower mean temperature of the 

 southern hemisphere is owing to the transference of heat by 

 ocean- currents, the question arises, to what extent is this trans- 

 ference due to eccentricity ? 



An objection answered. — A very common misconception re- 

 garding the physical cause of the motion of ocean-currents will 

 no doubt suggest to the minds of some readers an objection to 

 the conclusion arrived at in reference to the displacement of the 

 currents from the cold to the warm hemisphere. In order to 

 meet this objection, it will be necessary to consider at some length 

 the misconception to which I refer. 



[To be continued.] 



