CLTMATE A<SD THE POLAK ICE-CAP. 401 



The sun's aphelion distance occurs at present near the mid- 

 summer of the northern hemisphere ; so that if, as I maintain, 

 glaciation is the result of a cold summer due to the remoteness 

 of the sun, the glaciation has to be accounted for by the heat 

 received by the earth at the northern midsummer being less than 

 at present in the ratio of 0-875 to 0-967, equal to 0-905 to 1-000, 

 or nearly a tenth part less. 



The following attempt to estimate the effect of this difference on 

 terrestrial temperatures is by the method adopted by Mr. Croll from 

 Sir John Hcrschel Q Climate and Time,' p. 37). 



The temperature of space is estimated by Pouillet and Sir John 

 Herschel at — 239° Fahr. The mean temperature of the entire 

 northern hemisphere for July is estimated by Dove (' British- Asso- 

 ciation Report, 1848) at 71°. The mean July temperature of the 

 northern hemisphere is therefore 310° above that of space — in 

 other words, 310° degrees warmer than it would be in the absence 

 of the sun. If, then, the amount of solar heat received by the 

 earth is diminished in the ratio of 0-905 to 1-000, the temperature 

 due to the sun should be diminished in about the same ratio, or 

 from 310° to 280°-5, say by 30 degrees. 



This estimate makes, of course, not the slightest pretension to 

 accuracy ; but it is probably nearly enough true to give an idea of 

 the scale of the effect. In one way it is much too high. There 

 would not be time to produce the result: the remoteness of the 

 sun when at aphelion distance would not have so great an effect 

 on climate as if his distance were permanently increased. But on 

 the other hand, we have every reason to believe that Herschel's and 

 Pouillet's estimate of the temperature of space is much too high ; 

 and the greater the difference between the temperature of the 

 earth's surface and that of space, the greater will be the effect of 

 any variation in the sun's distance. 



The hemisphere which has its summer in aphelion has its winter 

 in perihelion ; and the winter temperature will be raised by the 

 nearness of the sun about as much as the summer temperature 

 is lowered by his remoteness ; so that the annual range of tem- 

 perature will be greatly diminished, without necessarily altering 

 the mean temperature at all. The mean annual temperature will 

 no doubt be lowered, from causes to be stated further on; but 

 for the present we will suppose it unchanged. If, then, the mid- 

 summer temperature is lowered by 30° and that of midwinter raised 

 by as much, the effect on the annual range will be 60° ; and this will 

 be enough to destroy the present difference between summer and 

 winter in all except the most extreme climates. According to Mr. 

 Keith Johnston, jun. ('Proceedings of the Royal Society of Edin- 

 burgh,' 1868-69), there are only three regions of the earth where 

 the range is more than 60° : these may be roughly denned as Siberia 

 with Central Asia, part of North America with Baffin's Bay and 

 part of Greenland, and Lapland. 



In the following tabular statement I assume the eccentricity to 

 be at its maximum, and the perihelion to occur at the midwinter of 



2f2 



