CHAP. VIII THE CATJSES Of GLACIAL EPOCHS 



131 



The winter, moreover, would be long and the summer 

 short, the difference being twenty-six days. 



We have here certainly an amount of cold in winter 

 amply sufficient to produce a glacial period,^ especially as 

 this cold would be long continued ; but at the same time 

 we should have almost tropical heat in summer, although 

 that season would be somewhat shorter. How then, it 

 may be asked, could such a climate have the effect supposed ? 

 Would not the snow that fell in winter be all melted by 

 the excessively hot summer? In order to answer this 

 question we must take account of certain properties of water 

 and air, snow and ice, to which due weight has not been 

 given by writers on this subject. 



Properties of Air and Water, Snow and Ice, in Relation to 

 Climate. — The great aerial ocean which surrounds us has 

 the wonderful property of allowing the heat-rays from the 

 sun to pass through it without its being warmed by them ; 

 but when the earth is heated the air gets warmed by con- 

 tact with it, and also to a considerable extent by the heat 

 radiated from the warm earth, because, although pure dry 



1 In a letter to Nature of October 30th, 1879, the Rev. 0. Fisher calls 

 attention to a result arrived at by Poiiillet, that the temperature which the 

 surface of the ground would assume if the sun were extinguished would 

 be - 128° F. instead of - 239° F. If this corrected amount were used in 

 our calculations, the January temperature of England during the glacial 

 epoch would come out 17° F., and this Mr, Fisher thinks not low enough 

 to cause any extreme difference from the present climate. In this opinion, 

 however, I cannot agree with him. On the contrary, it would, I think, bo 

 a relief to the theory were the amounts of decrease of temperature in 

 winter and increase in summer rendered more moderate, since according 

 to the usual calculation (which I have adopted) the differences are un- 

 necessarily great. I cannot therefore think that this modification of the 

 temperatures, should it be ultimately proved to be correct (which is 

 altogether denied by Dr. Croll), would be any serious objection to the 

 adoption of Dr. CroU's theory of the Astronomical and Physical causes of 

 the Glacial Ejjoch. 



The reason of the theoretical increase of summer heat being greater than 

 the decrease of winter cold is because we are now nearest the sun in winter 

 and farthest in summer, whereas we calculate the temperatures of the 

 glacial epoch for the phase of precession when the aphelion was in winter, 

 A large part of the increase of temperature would no doubt be used up 

 in melting ice and evaporating water, so that there would be a much less 

 increase of sensible heat ; while only a portion of the theoretical lowering 

 of temperature in winter would be actually produced owing to equalising 

 effect of winds and currents, and the storing up of heat by the earth and 

 ocean. 



