270 S. Newcomh—CroUs Climate and Time. 



the absorption of heat, in the other, by contact with the earth ; 

 but the temperature of the region is substantially the same in 

 either case. 



We may now pass to the consideration of the author's views 

 of the cause of the Glacial epoch, or of glacial epochs in general, 

 as, according to his view, there must have been several of them. 

 He maintains that such a phenomenon may be fully accounted 

 for by the great eccentricity which the orbit of the earth is 

 known to assume at certain very long intervals. Using Le 

 Verrier's formulas for the secular variation of the planetary 

 orbits, he has computed this eccentricity for a number of 

 periods, extending back nearly 3,000,000 'years, and has thus 

 found a number of epochs at which it was three or four times 

 as great as at present. That, in the course of each million of 

 years, there are from time to time such periods of great eccen- 

 tricity is a well established result of the mutual gravitation of 

 the planets, but whether the particular epochs of great and 

 small eccentricity computed by Mr. Croll are reliable is a dif- 

 ferent question. The data for this computation are the for- 

 mulae of Le Verrier, worked out about 1845, without any 

 correction either for the later corrections to the masses of the 

 planets, or for the terms of the third order subsequently dis- 

 cussed by Le Verrier himself. The probable magnitude of these 

 corrections is such that reliance cannot be placed upon the 

 values of the eccentricity computed without reference to them 

 for epochs distant by nearly a million of years. This fact, of 

 itself^ does not militate against Mr. Croll's theory, since the 

 correct formulae would no doubt show other epochs of great 

 eccentricity which would entirely satisfy his conditions. The 

 proposition with which we are more especially concerned is the 

 general one that a great eccentricity, with the' perihelion in one 

 of the solstices, will give rise to a glacial epoch ' ' ' 

 ' ' ■ Mt is the 



s proposition which \ 

 next propose to examine. 



The difficulty which the sustainer of this proposition en- 

 counters at the outset is the demonstration of D'AIembert that, 

 whatever changes the earth's eccentricity and perihelion may 

 undergo, the total amount of heat received from the sun in the 

 course of a year is still the same for each hemisphere. Conse- 

 quently, if the mean temperature of the hemisphere depends on 

 the total amount of heat received, it must be the same for the 

 two hemispheres, and but 



temperature of the present t _ . 



clearly let us suppose the perihelion to coincide with the June 

 solstice, so that the earth is nearest the sun in our northern sum- 

 mer, and farthest from it in our northern winter. Then, in th 



