VARIATIONS IN CLIMATE. 809 



it is usually agreed tliat they may be disregarded in accounting 

 for such clianges as may have taken place within man's memory. 

 Mr. James Croll, who has discussed this question with consider- 

 able fullness, and is inclined to allow them all the force they are 

 entitled to, ascribes less importance to their direct operation than 

 to the secondary effects they induce through their influence on 

 the currents of the ocean and air and upon features of the earth's 

 surface. M. Woeikoff allows them still less agency in the mat- 

 ter than Mr. Croll, and ascribes the greatest influence upon cli- 

 mate to the elevation and configuration of the land, as Mr. Lyell 

 did in the earlier days of geology ; and M. A. Blytt, of Chris- 

 tiania, has shown, by pertinent contemporaneous examples, how 

 climate in Scandinavia may be influenced by slight differences in 

 situation, soil, and exposure. 



The whole subject has just been reviewed by Sir Robert S. 

 Ball, Astronomer Royal of Ireland, one of the most eminent living 

 mathematicians, in his book on The Cause of an Ice Age. Speak- 

 ing particularly of Glacial periods, he shows that changes in the 

 intensity of solar radiation, relatively unimjiortant to the sun, 

 may produce enormous climatic effects on the earth. By an exact 

 calculation he finds that, with the present obliquity of the ecliptic, 

 while the earth as a whole receives equal amounts of heat from 

 the sun during the two halves of the year, the distribution as to 

 a single hemisphere is extremely unequal — a fact which previous 

 writers seem to have overlooked — the exact distribution being 

 sixty-three per cent of the whole amount of heat during the sum- 

 mer and thirty-seven per cent during the winter half. When 

 the line of the equinoxes is perpendicular to the major axis of the 

 earth's orbit and the eccentricity is at its maximum — the condi- 

 tions establishing the greatest possible difference in the length of 

 the seasons — the sixty-three per cent of heat is distributed over a 

 very short and therefore intense summer, and the thirty-seven 

 per cent over a long and therefore cold winter. The northern 

 hemisphere, when placed in such a condition, will have a summer 

 of one hundred and sixty-six days, during which the sun is at its 

 least possible distance, and a winter of one hundred and ninety- 

 nine days, with the sun at its greatest possible distance. This 

 Prof. Ball regards as a condition favorable to glaciation. The 

 ice and snow will accumulate during the rigors of the long 

 winter, while the succeeding brief summer has not power enough 

 to thaw as much water as has been solidified in the winter, and 

 the ice will grow from year to year. All this time the southern 

 hemisphere would be enjoying a widely different condition. Its 

 summer would contain as great a number of days as it is possible 

 for that season to possess, while the fierce heat of the sun would 

 be abated from its average amount, because the sun would be at 



