it is usually agreed that they may be disregarded in accounting for such changes as may have taken place within man's memory. Mr. James Croll, who has discussed this question with considerable 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 matter than Mr. Croll, and ascribes the greatest influence upon climate to the elevation and configuration of the land, as Mr. Lyell did in the earlier days of geology; and M. A. Blytt, of Christiania, 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. Speaking particularly of Glacial periods, he shows that changes in the intensity of solar radiation, relatively unimportant 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 summer 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 conditions 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
