of Secular Changes of Climate. 83 



Doubtless this is perfectly correct ; but on page 502 he states 

 that he has pointed out with more precision than has, he 

 believes, hitherto been done, the different effects on climate of 

 water in the liquid and solid states. This is a somewhat 

 doubtful statement; for in chapter iv. l Climate and Time/ in 

 Phil. Mag. March 1870, and in other places, will, I think, 

 be found all that this section contains. In fact the influence 

 of snow and ice as a permanent source of cold is one of the 

 main factors of my theory. The three great factors are 

 (1) the influence of snow and ice, (2) the influence of aqueous 

 vapour, and (3) the influence of ocean-currents. How per- 

 sistently has it been urged as an objection to my theory that, 

 during the glacial epoch, the great heat of the perihelion 

 summer would more than counterbalance the effect of the 

 aphelion winter. But I have maintained that the summers, 

 notwithstanding the intensity of the sun's rays, instead of 

 being warmer than at present, would in reality be far colder; 

 for this reason, that the temperature of a snow-and-ice covered 

 country can never rise much above the freezing-point. As an 

 example of this I pointed out that, i were it not for ice, the 

 summers of North Greenland would be as warm as those of 

 England (whereas hi point of fact they are colder than our 

 winters) ; and that were India covered with an ice-sheet, its 

 summers would be colder than those of England.' 



" Another point," he says, " of great importance in con- 

 nexion with this subject is the fact, that this permanent 

 storing-up of cold depends entirely on the annual amount of 

 snowfall in proportion to that of the sun- and air-heat, and 

 not on the actual cold of winter, or even on the average 

 cold of the year/'' This, I have shown (American Journal 

 of Science, Oct. 1883 ; Phil. Mag. Oct. 1883) at considerable 

 length, is one of the most widespread and fundamental errors 

 within the whole range of geological climatology. Perpetual 

 snow, instead of being due " entirely " to the annual amount 

 of snowfall in proportion to the quantity of heat received by 

 the snow, is in most cases not even mainly due to this cause. 

 Overlooking the fact that in the conservation of snow the 

 temperature of the snow is one of the main factors has been 

 a fruitful source of error. 



High Land and Heavy Snowfall in relation to the Glacial 

 Epoch. — According to Mr. Wallace, " high land and great 

 moisture" are essential to the initiation of a glacial epoch. 

 Undoubtedly high land and great moisture are the most favour- 

 able conditions for bringing about a glacial state of things ; 

 but I can hardly agree with him that they are necessary and 

 indispensable. 



G2 



