364 Mr. J. Croll on Geological Time, and the probable 



very nearly the same as those which were afterwards obtained 

 by Leverrier. 



Before the beginning of the present century the effects which 

 the excentricity of the earth's orbit may have on climate were 

 discussed. But about thirty-five years ago the question as to 

 how far a change in the amount of excentricity might affect the 

 climate of our globe was considered by Sir John Herschel ; and 

 his results were published in the Transactions of the Geological 

 Society of London, vol. iii. (2nd series). But he does not ap- 

 pear at the time to have been aware of the conclusions arrived at 

 by Lagrange regarding the superior limit of the excentricity of 

 the earth's orbit. He came, however, to the conclusion that 

 possibly the climate of our globe may have been affected by va- 

 riations in the excentricity of its orbit. "An amount of varia- 

 tion/' he says, " which we need not hesitate to admit (at least 

 provisionally) as a possible one, may be productive of consider- 

 able diversity of climate, and may operate during great periods 

 of time either to mitigate or to exaggerate the difference of winter 

 and summer temperatures, so as to produce alternately in the 

 same latitude of either hemisphere a perpetual spring, or the 

 extreme vicissitudes of a burniug summer and a rigorous 

 winter." 



Unfortunately, however, this opinion was to a great extent 

 nullified by a statement which afterwards appeared in his ' Trea- 

 tise on Astronomy,' to which reference will be presently made. 

 Shortly after the appearance of Herschel' s paper, the subject 

 was investigated by Arago, Poisson, Humboldt, and other as- 

 tronomers. The general conclusion arrived at, however, was 

 that the climate of our globe could not be much affected by any 

 change which could take place in the ellipticity of its orbit. The 

 reason which induced astronomers to come to that conclusion 

 seems chiefly to be this : whatever be the extent of the excen- 

 tricity, the total amount of heat falling on both hemispheres must 

 be the same. The sun, for example, is much nearer the earth 

 when he is over the southern hemisphere than he is when over 

 the northern ; but the southern hemisphere does not on this ac- 

 count receive more heat than the northern ; for, owing to the 

 greater velocity of the earth when nearest the sun, the sun does 

 not remain so long on the southern hemisphere as he does 

 on the northern. These two effects so exactly counterbalance 

 each other that, whatever be the extent of the excentricity, 

 the total amount of heat reaching both hemispheres is the 

 same. And it was considered that this beautiful compensa- 

 ting principle would protect the climate of our globe from being 

 seriously affected by an increase in the excentricity of its orbit, 



