G. W. Bulman — Revised Theory of Glaciafion. 261 



erroneous statement, that, of the total heat received by a hemisphere 

 in a year, one-half is received during the summer, and the other half 

 during winter. 



Sir E. Ball has worked out the true distribution of the yearly 

 heat between the seasons, and arrived at figures about which he 

 claims there can be no question. His result is, that of the total 

 yearly heat I'eceived by a hemisphere, 37 per cent, is received 

 during winter, and the remaining 63 per cent, during summer. 

 Granting these figures, it is maintained that glaciation follows as 

 a necessary consequence, whenever the difference in the lengths of 

 summer and winter is sufficiently great. 



This diffei-ence in length between the two seasons formed the 

 starting point of Dr. Croll's Theory. It was supposed by him that 

 the winter supply of heat being spread over a greater number of 

 days, the temperature would be so reduced, and the snowfall so 

 increased thereby, that finally — with the aid of ocean currents, etc. 

 — glaciation would result. 



And in showing that only 37, instead of 50, per cent, of the 

 yearly heat is received during winter, Sir E. Ball appears to have 

 removed a difficulty ; 37 per cent, of the yearly heat would certainly 

 not maintain so high a temperature as 50 per cent, during the 

 lengthened glacial winter. But a little consideration will show that 

 the removal of the difficulty is only apparent. 



Our present average winter temperature forms the starting point 

 for the calculation of what it would be when the winter was at its 

 longest, and this calculation can be made quite independently of the 

 figures 37 and 50, The restdt will be the same whether we assume 

 37 or 50 per cent, of the yearly heat to be received during winter, 

 for these figures do not necessarily enter into the calculation at all. 

 They did not, as far as I am aware, enter into that of Dr. CroU, 

 who reasoned correctly, that our present winter supply of heat, 

 when spread over 199 days instead of 179, would produce an 

 average temperature lower in proportion to the lengthening of the 

 season. 



The difflctdty in connexion with the supposition of half the yearly 

 heat suppl}' coming in winter is rather that of understanding how 

 a winter climate could be produced at all ; for our winter being 

 shorter, if it received the same amount of heat, would be actually 

 hotter than summer. 



But granted that 50 per cent, of the annual beat supply permits 

 a winter climate during a portion of the year at present, the 

 lengthening of the season might be supposed to reduce the temperature 

 as easily as on the supposition that the percentage was 37 per cent. 



Sir E. Ball considers that Dr. Croll was unacquainted with the 

 unequal distribution of heat between the seasons, but he has not 

 shown that any of the latter's reasonings or calculations depend on 

 the assumption of its equal distribution. 



The theory set forth in " The Cause of an Ice Age " is regarded 

 as a revised form of Dr. Croll's, but the two theories differ in certain 

 essential particulars. For, in the first place, while the Eevised 



