﻿546 Mr. E. P. Culverwell on the Inadequacy of the 



Croll's hypothesis which tends to leave more freedom to the 

 somewhat fluctuating demands of their science. 



Is there, then, any physical method of estimating the 

 changes in midwinter temperature due to eccentricity ? The 

 problem is of course far too complicated to be rigorously 

 solved ; but happily, in respect of the direct effect of sun- 

 heat, we have what seems a fair and rational method of 

 calculation. When, in the long winter of the supposed glacial 

 epoch, the earth was much farther from the sun than at present, 

 each parallel of latitude of course received less daily winter 

 heat than the same parallel now receives. But let us find 

 what parallels of latitude now receive the same sun-heat in 

 winter as the parallels 40°, 50°, 60°, 70°, 80°, and the pole, 

 90°, which I will call the standard parallels, received in the 

 long winter of great eccentricity. I take three bases of 

 comparison — 



First, following Croll's method, I find that latitudes 43°, 

 52°, 61°, 70°, 80°, and 90° receive on midwinter day at present 

 the same sun-heat as the standard latitudes received on their 

 midwinter day in the period of great eccentricity. 



Now as to the temperatures. I think it is evident that the 

 present midwinter temperatures of 43°, 52°, &c. must, so far 

 as direct sun-heat is concerned, be somewhat higher than those 

 of the standard latitudes in the former epoch. For though 

 the midwinter sun-heat is the same, the standard latitudes 

 have had a longer period of cooling, i. e. the radiation from 

 them in the earlier epoch has exceeded the sun-heat received 

 for a few days longer than in the case of latitudes 43°, 52°, 

 &c. at present. 



Second. I find that latitudes 43°*3, 52°-4, 61 c '7, 71°-3, 

 81°, and 90° now receive in their winter of 179 days just the 

 same daily average of sun-heat as the standard latitudes received 

 in their supposed glacial winter of 199 days. 



In this case, though the period over which the daily supply 

 is taken is shorter in the case of 43°*3, 52°*4, &c. than for the 

 standard latitudes, yet the former receive appreciably more 

 midwinter sun-heat than the latter ; so that whatever differ- 

 ence of temperature there is due to the first cause will be 

 more or less counterbalanced by the other, and we are probably 

 not far wrong in saying that, so far as direct sun-heat is con- 

 cerned, the present midwinter temperatures of latitudes 43°'3, 

 52°*4, 61°-7, 71°-3, 81°, and 90° must be about the same 

 as those of 40°, 50°, 60°, 70°, 80°, and 90° were in the epoch 

 of great eccentricity. 



Third. I find that latitudes 44°-2, 54°, 63°'5, 74°, and 

 84°*5 now receive the same total sun-heat in the 199 coldest days 



