64 E. P. Ciilverwell — Theory of the Ice Age. 



to the entry into those regions of the Gulf Stream, so that the 

 glaciation of Greenland is the counterpart of our genial climate. 

 Into these geographical conditions I do not, however, propose to 

 enter here ; I will content myself with a general argument for the 

 geographical theory of glaciation as opposed to the astronomical, 

 namely, that whereas the change in the winter isotherinals due to 

 sun-heat in times of great eccentricity amounts at most to about 

 4° of latitude, the present changes in winter isotherraals amount to 

 over 33°, as where the 0°C. winter isothermal passes from south of 

 New York to north of the North Cape, a shift due to geographical 

 conditions about eight times as great as any shift due to astronomical 

 conditions could be. 



Is it possible that there may have been any considerable 

 interchanges of atmosphere between the earth and the regions 

 of space through which it has passed ? It is certain that there 

 must have been some such interchange. Whether the atmospheric 

 pressure is increasing or diminishing depends on whether, in the 

 course of the earth's motion through space, more of the interstellar 

 molecules get entangled in the earth's atmosphere than, leaving 

 that atmosphere, get entangled in the interstellar gases through 

 which the earth happens to be passing, and are thus dragged away 

 from the earth. If once we were allowed to assume the magnitude 

 of these changes, the whole difficulty of explaining Glacial or genial 

 ages, so far as temperature changes are concerned, would vanish. 

 For instance, if due to some gaseous conditions of space, or perhaps 

 to the absorption into the atmosphere of the gaseous components of 

 meteorites or shooting stars, there be an addition to the atmospheric 

 pressure of one millimetre in three centui'ies, and if this process 

 has been continued for 20,000 or 25,000 years, then it follows that, 

 some 25,000 years ago the atmospheric pressure would have been 

 less by about one-tenth part than it is at present. This would be 

 equivalent to raising land and sea by about 2500 feet, for the 

 blanketing effect of the decreased atmosphere would be about the 

 same as that which now lies above a mountain 2500 feet high. 

 Thus a Glacial epoch might be produced without any alteration in 

 the geographical conditions. And if, on the other hand, either 

 through the earth plunging into a more gaseated region of space, 

 or through some catastrophe, the atmospheric pressure were to be 

 much increased, the resulting increase of temperature might be very 

 great, and a genial age might be the result — a rise of 50° F. might 

 readily be got. 



No doubt there is much that may fairly be called " vague specu- 

 lation " in this suggestion ; but there is an advantage even in that, 

 for the more vague it is the more difficult to refute it. Certainly 

 I do not think it can be refuted by calculations such as those of 

 Mr. G. H. Bryan, p. 682 of the 1893 Keport of the Brit. Assoc, 

 on the supposition that the temperature is constant and that the 

 gases have established themselves in a permanent state. Thus he 

 finds that at any given instant there are very few particles ready to 

 leave the atmosphere of a large planet. But the very essence of 



