VI. _The Thermal Solar Hypothesis._ The only other explanations of the climatic changes of glacial and historic times which now seem to have much standing are two distinct and almost antagonistic solar hypotheses. One is the idea that changes in the earth's climate are due to variations in the heat emitted by the sun and hence in the temperature of the earth. The other is the entirely different idea that climatic changes arise from solar conditions which cause a _redistribution of the earth's atmospheric pressure_ and hence produce changes in winds, ocean currents, and especially storms. This second, or "cyclonic," hypothesis is the subject of a book entitled _Earth and Sun_, which is to be published as a companion to the present volume. It will be outlined in the next chapter. The other, or thermal, hypothesis may be dismissed briefly. Unquestionably a permanent change in the amount of heat emitted by the sun would permanently alter the earth's climate. There is absolutely no evidence, however, of any such change during geologic time. The evidence as to the earth's cosmic uniformity and as to secular progression is all against it. Suppose that for thirty or forty thousand years the sun cooled off enough so that the earth was as cool as during a glacial epoch. As glaciation is soon succeeded by a mild climate, some agency would then be needed to raise the sun's temperature. The impact of a shower of meteorites might accomplish this, but that would mean a very sudden heating, such as there is no evidence of in geological history. In fact, there is far more evidence of sudden cooling than of sudden heating. Moreover, it is far beyond the bounds of probability that such an impact should be repeated again and again with just such force as to bring the climate back almost to where it started and yet to allow for the slight changes which cause secular progression. Another and equally cogent objection to the thermal form of solar hypothesis is stated by Humphreys as follows: "A change of the solar constant obviously alters all surface temperatures by a roughly constant percentage. Hence a decrease of the heat from the sun would in general cause a decrease of the interzonal temperature gradients; and this in turn a less vigorous atmospheric circulation, and a less copious rain or snowfall--exactly the reverse of the condition, namely, abundant precipitation, most favorable to extensive glaciation."