8 E. P. Cukerwell — Theory of the Ice Age. 



now receive the same midwinter sun-heat; second, following Ball's 

 method, I find what latitudes now receive the same daily average 

 of sun-heat in winter, i.e. from equinox to equinox; third, I find 

 what latitudes now receive the same total amount of sun-heat in 

 their coldest 199 days as the others (40°, 50°, etc.) receive in the 

 199 days of the supposed Glacial winter. The results are : — 



First Comparison. 



Latitudes 43°, 52°, 61°, 70°, 80°, and 90° now receive on mid- 

 winter day, the same or slightly less heat than latitudes 40°, 50°, 

 60°, 70°, 80°, and 90° received on the midwinter day in the epoch 

 of great eccentricity. Hence, on CroU's hypothesis, we should 

 expect the midwinter temperatures of 40°, 50°, etc., in that epoch 

 to be about the same as those of 43°, 52°, etc., at present. 



Second Comparison. 



Latitudes 43-3°, 62-4°, 61-7°, 71-3°, 81°, and 90° now receive 

 in their winter of 179 days the same or slightly less daily average 

 of sun-heat as 40°, 50°, 60°, 70°, 80°, and 90° would receive in their 

 supposed Glacial winter of 199 days. 



Third Comparison. 



Latitudes 42 2°, 54°, 63-5°, 74°, and 84-5° now receive about 

 the same daily average of sun-heat in the 199 coldest days of the 

 present year as 40°, 50°, 60°, 70°, and 80° received in the 199 

 coldest days of the period of great eccentricity. 



A 



It is evident that in order to find those latitudes whose midwinter 

 temperatures are now the same, so far as direct sun-heat is con- 

 cerned, as those of 40°, 50°, 60°, 70°, and 80° in the period of great 

 eccentricity, we must take the latitudes somewhere about those given 

 by the second method. For though the midwinter sun-heat received 

 now by the latitudes thus obtained is decidedly less than that 

 received by 40°, 50°, etc., in the earlier period, yet as the daily 

 average remained at its small value during a slightly longer time 

 in the winter of great eccentricity, there might on that account be 

 a somewhat lower midwinter temperature (though as the lowest 

 temperature occurs in January all over the earth, it is not likely 

 that the extra time would produce much effect). 



As to the latitudes given by the third method, they receive so 

 much less sun-heat during the middle of the winter than 40°, 

 50°, etc., received in an equal time during the epoch of great 

 eccentricity that it seems quite evident that the present midwinter 

 temperatures of 44-2°, 54°, 63-5°, 74°, and 85° must be considerably 

 lower than those of 40°, 50°, 60°, 70°, and 80° were in the period of 

 great eccentricity, so that we are quite safe in taking them as giving 

 the extreme limit of possible change of climate, due directly to the 

 diminished sun-heat. 



Now to find the fall in temperature. As we go northwards from 

 lat. 50° to lat. 70° the decrease in midwinter temperature is 15° F. 



