﻿Chemistry and Physics. 391 



It is hardly to be hoped, however, that we shall ever attain to 

 any very accurate knowledge of the geological time scale from 

 this kind of argument. 



But there is another theory which is precise in its estimate, and 

 which, if acceptable from other points of view, will furnish ex- 

 actly what is requisite. Mr. Croll claims to prove that great 

 .changes of climate must be brought about by astronomical events 

 of which the dates are known or ascertainable (" Climate and 

 Time"). The perturbation of the planets causes a secular vari- 

 ability in the eccentricity of the earth's orbit, and we are able 

 confidently to compute the eccentricity of many thousands of 

 years forward and backward from to-day, although it appears 

 that, in the opinion of Newcomb and Adams, no great reliance 

 can be placed on the values deduced from the formulae at dates 

 so remote as those of which Mr. Croll speaks. According to Mr. 

 Croll, when the eccentricity of the earth's orbit is at its maxi- 

 mum, that hemisphere which has its winter in aphelion would 

 undergo a glacial period. Now, as the date of great eccentricity 

 is ascertainable, this would explain the great Ice Age and give us 

 its date. 



The theory has met with a cordial acceptance on many sides, 

 probably to a great extent from the charm of the complete an- 

 swer it affords to one of the great riddles of geology. 



Adequate criticism of Mr. Croll's views is a matter of great 

 difficulty on account of the diversity of causes which are said to 

 cooperate in the glaciation. In the case of an effect arising from 

 a number of causes, each of which contributes its share, it is obvi- 

 ous that if the amount of each cause and of each effect is largely 

 conjectural, the uncertainty of the total result is by no means to 

 be measured by the uncertainty of each item, but is enormously 

 augmeuted. Without going far into details it may be said that 

 these various concurrent causes result in one fundamental propo- 

 sition with regard to climate, which must be regarded as the key- 

 stone of the whole argument. That proposition amounts to this — 

 that climate is unstable. 



Mr. Croll holds that the various causes of change of climate 

 operate inter se in such a way as to augment their several efficien- 

 cies. Thus the trade-winds are driven by the difference of tem- 

 perature between the frigid and torrid zones, and if from the 

 astronomical cause the northern hemisphere becomes cooler the 

 trade-winds on that hemisphere encroach on those of the other, 

 and the part of the warm oceanic current which formerly flowed 

 into the cold north zone, will be diverted into the southern hemis- 

 phere. Thus the cold of the northern hemisphere is augmented, 

 and this in its turn displaces the trade-winds further, and this 

 again acts on the ocean currents, and so on; and this is neither 

 more nor less than instability. 



But if climate be unstable, and if, from some of those tempo- 

 rary causes for which no reasons can as yet be assigned, there 

 occurs a short period of cold, then surely some even infinitesimal 



