CAUSE OF GLACIAL CLIMATE 649 



un<l popular form, the hypothesis would seem to require a great 

 i-k'vution of a large part of two continents for each ice epoch, and a 

 grrat depression for each interglacial epoch, an extremely improb- 

 able sequence of events. This hypothesis has lost rather than 

 gained favor, as evidence has accumulated. 



Astronomic hypotheses. An ingenious semi-astronomical hy- 

 pothesis \\as advanced by Croll l in the latter part of the last cen- 

 tury, and for a time it was widely accepted. It is founded pri- 

 marily on variations in the eccentricity of the earth's orbit, combined 

 with the precession of the equinoxes. Plausible as the hypothesis 

 seemed at the outset, prolonged study has tended to weaken, rather 

 than strengthen it. 



The orbit of the earth is slightly elliptical, and this ellipticity is subject to con- 

 siderable variation. This does not alter the total amount of heat received from 

 the sun by the earth, or by either hemisphere; but it affects the distribution of heat 

 within tlu- year, shortening or lengthening the cooler and warmer seasons, according 

 as they fall in the perihelion or the aphelion part of the earth's orbit. Thus the 

 hemisphere which has summer in perihelion has a short summer with much heat 

 per hour; the other hemisphere has a long summer with less heat per hour. The 

 precession of the equinoxes reverses the seasonal relations of the hemispheres every 

 10,500 years. At present the earth is nearest the sun in winter in the northern 

 hemisphere (summer in the southern hemisphere). In 10,500 years (owing to the 

 precession of the equinoxes) the earth will be nearest the sun in the summer of the 

 northern hemisphere (winter of the southern hemisphere). We shall then have a 

 shorter summer with more solar heat per hour than now, and a longer winter with 

 less heat per hour. Croll's hypothesis is built upon the belief that snow-accumula- 

 tion would be favored by long winters, and snow-melting reduced by short summers. 

 The hypothesis is that the glacial epochs occurred during the period of aphelion 

 winters in times of great eccentricity. 



It is admitted that these astronomical relations are insufficient in themselves 

 to produce the observed glaciation, and so certain terrestrial conditions were made 

 important elements in the working force of the hypothesis. It was held that the 

 zone of the trade-winds and the thermal equator would be shifted from the glaciated 

 hemisphere toward the warmer one, and that this shifting would turn a large 

 part of the warm equatorial waters away from the cooler hemisphere. Croll held 

 that if the trade-wind belts were shifted southward a few degrees, a large part of 

 the equatorial current would be south of Cape St. Roque, and so turned into the 

 South Atlantic, greatly lowering the temperature of the northern hemisphere. 

 When the southern hemisphere was passing through its cold period, nearly all the 

 equatorial current would be north of St. Roque, and this would give the northern 

 hemisphere a moist interglacial epoch. 



If the hypothesis were correct, (i) glacial epochs should alternate between the 

 northern and the southern hemispheres, and (2) their duration should be limited 



1 Climate and Time in their Gtehfical Relations; James Croll, pp. 312-328; 

 Climate and Cosmology and Tlie Cause of the Ice Age, Sir Robt. Ball. 



