8io THE POPULAR SCIENCE MONTHLY. 



the greatest distance from the earth it is possible for it to attain. 

 The winter would be short and warm. The present difference in 

 the length of the seasons is seven days, and the position of the 

 perihelion is such that it is near its maximum for the present 

 eccentricity. The directions in which the precession of the equi- 

 noxes and the variation of the obliquity of the ecliptic are tend- 

 ing are for reduction of the inequality, and ice ages are not to be 

 expected from vicissitudes such as are now possible. 



It is not denied that climates have been, and are, changing ; 

 but the changes are believed to be special, local, temporary, and 

 oscillatory, and most largely determined by causes that may be 

 found on the surface of the earth. M. Arago thought they 

 might all be attributed to agricultural works, to the clearing 

 of woods from plains and mountains, to the drying up of 

 marshes ; and he doubted if it could be proved that the cli- 

 mate had become warmer or colder in any place the physical 

 aspect of which had not been perceptibly changed during a series 

 of ages. 



The present drift of the opinion of many careful students of 

 the subject seems to be that exaggerated ideas have been held 

 of the extent of climatic variations, both in the present and the 

 past. M. Woeikoif, whose opportunities for studying climato- 

 logical phenomena over a large extent of territory have not been 

 surpassed, believes that this is so, even when the application is 

 made to the Glacial period ; that not intense cold, but those con- 

 ditions of temperature and moisture most conducive to the pre- 

 cipitation and accumulation of snow, formed the chief factors of 

 its characteristic phenomena. Chief among these were proximity 

 of the sea and a temperature of the surface-water rather below 

 than above the freezing-point. The effect on glacial accumula- 

 tion of the conditions commonly supposed to correspond with the 

 combination of high eccentricity and an aphelion winter would, 

 in his opinion, be the opposite to what is attributed to it ; for the 

 greater cold assumed to j^revail in winter would not be conducive 

 to the precipitation of snow, while the more intense heat of mid- 

 summer would probably melt the snow at heights where the pres- 

 ent temperature rises but little above the melting-point. Hence 

 the conditions in the interior and eastern part of a continent like 

 Asia would be less favorable than they are now to marked glacia- 

 tion. The western parts of continents and islands would be more 

 fully under the influence of the sea; and as there is no reason to 

 suppose that its surface temperature would be lower than now, 

 it follows that there would not, all other things being equal, be 

 more snow than now in countries where rain is the rule, even in 

 winter. The effect of the combination would be in any case but 

 slight, and not by far, in M. Woeikoff's opinion, to be compared 



