262 G. W. Biilinan — Ri vised Theory of Glaeiation. 



Tlieory attributes glaciation solely to the lengthening of one season 

 at the expense of the other — in other words to purely astronomical 

 causes, viz. eccentricity and the precession of the equinoxes — Dr. 

 Croll attributed it to physical agencies brought into operation by 

 these astronomical causes. Among these physical agencies by far 

 the most important is the Gulf Stream. But according to the 

 Revised Theory the deflection of this great ocean current is not 

 required to produce glaciation, and Sir K. Ball supposes it to have 

 flowed much as it does now during the Glacial period. And there 

 are two considerations which seem to show that the Gulf Stream did 

 modify our glacial climate, and which were, therefore, difficulties on 

 Dr. Croll's view. The first of these is, that palseontological evidence 

 seems to show, that the western shores of our island were warmer 

 than the eastern — as they are to-day. And the second is, that in 

 North America glaciation extended fully 10° further south* than it 

 did here. 



Again, while, according to Dr. Croll, the astronomical conditions 

 might occur without leading to glaciation through secondary physical 

 causes, the Revised Theory requires a group of glacial periods 

 •whenever the eccentricity is sufficiently great. Dr. Croll has indeed 

 shown how the astronomical causes which under certain conditions 

 ■ — of distribution of land and watei', etc. — would produce a group 

 of glacial, separated by genial, periods, might, under other con- 

 ditions, lead to a group of genial periods separated by others a 

 little less genial. And thus — especially in his later writings — he 

 seems to have allowed considerable force to Sir Chas. Lyell's 

 views on the cause of glaciation, though only as adjuncts to the 

 Astronomical Theory. 



It is probably in the judicious combination of the two theories that 

 the true solution of the glacial problem will have to be sought. 



While, then, the essence of Dr. Croll's Theory consisted in the 

 continued increase in the cold, due initially to a lengthened winter, 

 by physical agencies thereby brought into action, the Revised Theory 

 attributes glaciation entirely to the reduction in temperature produced 

 by spreading the winter heat supply over a greater number of days. 



The question, then, of how far our present winter temperature 

 would be reduced if its heat supply were spread over 199 days — 

 the longest possible winter — is the critical point in the Revised 

 Theory. 



If it can be shown that the consequent reduction of its winter 

 temperature would place our climate on a level with those of 

 countries now glaciated ; and that the summer supply of heat — as 

 great as at present though concentrated into fewer days — would 

 be unable to melt the ice and snow, then glaciation follows as a 

 necessary result of the lengthening of the winter. But this has not 

 yet been done. 



Sir R. Ball (Appendix pp. 177, 178) gives data from which the 

 reduction in our average winter temperature during the winter of 

 199 days can be found. 



At present he calculates we receive a mean daily average of 0'75 



