254 DISCUSSION OF THE DESICCATION QUESTION. 



of colder and warmer periods during the geological ages seems, however, to be 

 supported only by evidence of tlie most vague and unsatisflictory kind : such 

 as it is, it will be noticed, and comments made upon it, in the next chapter, 

 where, as it mostly concerns the so-called Glacial and Inter-glacial epochs, it 

 will find its .appropriate place. 



When we seek to ascertain whether the prevalent idea of the improvement 

 of the climate in certain countries, as manifested by the lessening severity of 

 the winters and diminution of the intense heat of the summers, has a 2iosi- 

 tive basis of fact, we find it extremely difficult to arrive at a positive decision 

 in the matter. It is not unreasonable to suppose that a decreased emission 

 of heat from the sun would make itself felt, not so much, at first, in any 

 material decrease of the average annual temperature, as in a decreased evapo- 

 ration from the ocean surfiice, bringing about the phenomena of desiccation 

 which have been described. The diminished rain-fall thus produced would, 

 very probably, be accompanied by a decreased tendency to atmospheric dis- 

 turbances and barometric fluctuations. Thus we might have, as a result, 

 that tendency to an equalizing of the seasons of which Arago speaks so 

 positively as having taken place in this country and, to a certain extent, in 

 Europe, and which might be more perceptible to mankind in general than 

 would be the slight diminution in the mean temperature, by wdiich the 

 described change was brouglit about. 



A considerable portion of Arago's investigation, to which reference has in 

 the preceding pages been so frequently made, is occupied by chronologically 

 arranged tables of such natural phenomena as might be expected, from a 

 comparison of the frequency of their recurrence or in other ways, to throw 

 light on the question here suggested. These tables, which were compiled 

 and drawn up by Arago with the assistance of Barral, comprise statements 

 of the years in which the great rivers of Central Europe have been frozen 

 over, as also historical accounts of the occurrence of winters of unusual sever- 

 ity, or of exceptional mildness. Similar facts are presented with reference to 

 the summers, the whole going back, with many breaks and imperfections, 

 to the time of the earliest historical records. 



From these tables it is extremely difficult to draw any positive conclusions; 

 indeed Arago is unwilhng to commit himself to such. He evidently inclines, 

 liowever, to the opinion that the deterioration of the climate of France dur- 

 ing the historic period, which — as he says — is generally believed to have 

 occurred, does not find a support in the facts cited. He remarks that it is 



