'rttE GLACIAL PERIOD. 2o3 



the elliptic. It will remain true even when one of the 

 seasons is 166 days and the other 199. 



There are two cases to be specially considered. First, 

 that in which the summer is 166 days and the winter 199; 

 second, that in which the summer is 199 days and the winter 

 166. There will be a profound difference between the 

 climates in the two cases supposed. Let us take the first, in 

 which the summer is as short as it is possible for that season 

 to become. The hemisphere must then receive 63 parts of 

 heat in the comparatively brief period of 166 days. The 

 result is a summer intensely hot so far as the direct radiation 

 from the sun is concerned. The winter which follows has 

 now its greatest length, and 37 parts of heat have to suffice 

 for 199 days. The temperature in that season must there- 

 fore sink extremely low. The hemisphere under considera- 

 tion will therefore have very hot and short summers, and 

 very long and cold winters. The circumstances are such as 

 would enable the winter deposit of ice and snow to increase 

 from year to year. Thus would be provided the ice sheet 

 characteristic of a glacial period. 



As the summer in the northern hemisphere is simultaneous 

 with the winter in the southern, and vice versa, it follows that 

 on the unglaciated half of the earth there will be a long 

 summer of 199 days, and a short winter of 166. According 

 to the invariable rule 63 per cent, of the total heat received 

 per annum by that hemisphere will arrive during the long- 

 summer of 199 days, while 37 per cent, will remain for the 

 winter of 166 days. It is evident that this hemisphere will 

 have a far more equable supply of heat than the o]i]30site 

 one, for the larger portion of the yearly supply of heat is 

 appropriated to the longer season, leaving the lesser portion 

 for the shorter season. Is not this exactly the climatic con- 

 dition that we should most desire ? In those latitudes which 

 we commonly call temperate we often experience too much 

 heat in the summer, and we would generally like a little 

 more in the winter. That is because our two seasons are of 

 nearly equal length. We get 63 per cent, of our total heat 

 supply in the six summer months, and 37 per cent, in the six 

 winter months. The climate would certainly be more 

 uniform if we could arrange to have the 63 per cent, distri- 

 buted over seven months, and the remaining 37 per cent, 

 over five months. So perfect an adjustment as this seems, 

 however, never to have been realised ; for the diflference 

 between the lengths of the seasons can never have been so 



