THEORIES OF TEMPERATURE CHANGES. 257 



they are likely to be attenrled, much is known; but when these interruptions 

 in the normal atmospheric conditions will take j)\sice cannot be predicted, 

 nor is it at all clearly understood why certain regions are peculiarly favorable 

 to their inception. Not the slightest progress has been made towards such 

 an insight into the phenomena of storms and weather fluctuations as would 

 justify the putting forth as a scientific statement of aii\' prophecy of such 

 occurrences, other than as indicated by the mean of the place and season, 

 for even so much .as a week in advance. 



We assume, then, that it is positively known that the climate of the earth 

 has become colder, in the coui-so of the geological ages ; and, moreover, that 

 there is evidence that this refrigeration has been continued in historic time 

 so as to have become sensible and a matter of importance in certain regions 

 where the conditions are such that the limit of endurance, on the part of the 

 inhabitants, had just been reached or even overpassed during the past few 

 centuries. We do not find that we can furnish positive proof, based on in- 

 strumental observations, of a perceptible change of climate in the countries 

 where such records have been kept, and for reasons which have been fully 

 explained in the preceding pages. As Arago remarks, a few centuries of 

 accurate observations will probably throw light on the question. 



Sectiox VII. — Theories of the Cause of Temperature Chanf/es. 



At the beginning of the present chapter there was suggested, as one of 

 the topics intended to be brought forward in it, an inquiry into the probable 

 cause of the climatic changes shown to have been taking place on the earth's 

 surface during the geological ages, and to be continuing in historic times. 



The geologist is not bound, however, to assign a cause for the phenomena 

 which he has proved to have taken place, for the true cause may very prob- 

 ably be one the investigation of which lies outside of his field of work. If 

 the problem discussed be, as is the case in the present work, a climatological 

 one, its solution will most naturally and reasonably be expected to be ob- 

 tained by the helj) of mathematics, or of astronomical or physical science. 

 In such a case, the geologist has simply to indicate the results at which he 

 has arrived, stating the facts by which his conclusions are supported, and 

 then to call on those whose proper business it is to throw light on the thoorv 

 of the causes by which the stated effects have been produced. Still, it seems 



