THE PLEISTOCENE OR GLACIAL PERIOD. 427 



the upper limit being an eccentricity of 0.07. It is not claimed that 

 this alters the total amount of heat received by the earth, or by 

 either hemisphere, or even the proportions received during the periods 

 between the equinoxes, which, according to Ball, are in the ratio of 

 63 for the summer to 37 for the winter, but that the distribution of 

 heat within these periods is markedly affected by the shortening or 

 lengthening of the two seasons, according as they fall in the peri- 

 helion or the aphelion portion of the orbit. In the perihelion 

 portion there is a short season with much heat per hour, and in 

 the aphelion portion a long season with less heat per hour. The 

 precession of the equinoxes causes the seasons to shift relative to the 

 perihelial and aphelial points. At present the earth is nearest the 

 sun in our early winter, or in the early summer of the southern hemi- 

 sphere. In 10,500 years the earth will be nearest the sun in our early 

 summer, or the early winter of the southern hemisphere. We shall 

 then have a shorter summer with more solar heat per hour, and a 

 longer winter with less heat per hour. There have been differences 

 of opinion as to how this change in the distribution of heat would 

 affect glaciation. The Crollian hypothesis is built upon the belief 

 that snow accumulation would be favored by the long winters, and 

 snow-melting reduced by the short summers, notwithstanding their 

 greater heat per diem. 



It is conceded that the amount of eccentricity at present is too 

 small to produce a very appreciable effect, otherwise we would have 

 a glacial epoch now in the southern hemisphere. The eccentricity 

 fluctuates in a very complicated way because of the varying attraction 

 of the other planets on the earth, whose lines of attraction are con- 

 stantly shifting, and are usually diverse and more or less mutually 

 neutralizing. At long intervals, the planets pull measurably together and 

 give relatively high eccentricity, but this never exceeds about four 

 times the present amount. The hypothesis assumes that the rela- 

 tively high eccentricity that is attained at these periods is sufficient 

 to produce the essential conditions of the glacial period. 



It is admitted that these astronomical relations are insufficient in 

 themselves to produce the glacial effects observed, and so certain ter- 

 restrial conditions are made important elements in the working phase of 

 the hypothesis. Prominent among these, it is held that the zone of the 

 trade-winds and the thermal equator would be shifted from the gla- 



