62 E. P. Ouherwell — Theory of the Ice Age. 



but eX^QwTien, i.e. "in summer on the same hemisphere." The 

 " more favoured regions " are enjoying a cooler summer than usual,- 

 for, in the Glacial winter the whole earth is further from the sun- 

 than usual, and every region receives less heat. Hence it follows 

 that if the extra heat of the short Glacial summer is to produce the 

 inci'eased evaporation, and if the snowfall is to be in winter, it is 

 necessary that the moisture thus evaporated in summer shall be held- 

 in the atmosphere till precipitated by the cold of winter. Croll, 

 who saw well the difficulty here involved, boldly asserts that the 

 chief snowfall will be in summer. 



I have now disposed of every argument of importance put 

 forward by Ball, and shown that the theory as presented by him 

 does not even claim to account for at all as great an effect aS' 

 that asserted by Croll. For, as has been shown above, Croll claims' 

 a lowering of 45 '3° F. in the mid- winter temperature of Great 

 Britain, while Ball's theory only gives about 25°. Hence it is quite^ 

 a mistake to suppose that though Croll's mode of presenting the- 

 astronomical part of the theory may require to be strengthened by 

 considering the changes produced by ocean currents, such auxiliary 

 causes are rendered unnecessary by the changes Ball has introduced. 

 Evidently they would be more necessary on Ball's theory than on 

 Croll's. 



Hence the net result of the discussion is that the theory remains 

 where Croll left it. 



In concluding a long and somewhat dry criticism, I may be 

 allowed to express my satisfaction in finding that the conclusion 

 I have reached on purely physical grounds, negative though it be, 

 is so far in accord with the views of the modern school of geologists 

 that it permits of the date of the prehistoric glaciations being fixed* 

 in accordance with the geological evidence, and, as I venture to 

 think, finally removes a restriction as to the date of the Ice Age, 

 vphich many geologists had felt to be an unreal one. 



[Since writing this paper, my attention has been directed to Sir 

 H. H. Howorth's Glacial Nightmare, in which there is a long and 

 able criticism of the Astronomical Theory of the Ice Age, in which 

 a summary of the criticisms advanced by previous writers is included. 

 The chief feature of this paper, i.e. the mode of calculating a limit 

 to the probable alteration in terrestrial temperatures due to direct 

 sun-heat, is not of course referred to, not having been published 

 until last year.] 



As to the genial age, we have the great authority of Lord Kelvin 

 for thinking that it might be due to a more open Arctic Sea in which 

 a great influx of warm water was maintained. See Geological 

 Climate in his recently republished Lectures and Addresses. 



After saying that a sinking of 1000 feet would submerge vast 

 areas of land, and leave the North Pole very free and open to access 

 by sea, he continues : " Just think of a current of three-quarters of a 

 nautical mile per hour, or 70 miles in 4 days, 40 fathoms deep, flowing 

 across the Arctic circle to the Pole. This would cover the whole 

 area by one fathom, and if 7"1° C. above the freezing-point would 



