﻿550 Mr. E. P. Culver well on the Inadequacy of the 



this suggestion of an additional cause of the Genial Age may 

 require an answer, I leave Dr. OrolFs earlier pages to answer 

 his later ones. 



The mode in which the calculations were made must now 

 be explained. Originally I had intended to evaluate the 

 integrals which give the winter sun-heat on each latitude, but 

 the calculations appeared so long that I thought of using a 

 graphic method by photographing a terrestrial globe in various 

 positions. But having met with Mr. Meech's paper in vol. ix. 

 of the ' Smithsonian Contributions to Knowledge/ I used his 

 values for the daily heat on each 15th day of the year for 

 intervals of 10 degrees of latitude as sufficiently close approxi- 

 mations to the quantities of sun-heat received on that day and 

 the seven preceding and succeeding days, except in a few 

 cases where the rate of change varied considerably, when I 

 used a graphic method of interpolation by plotting the curve 

 on millimetre-paper. Thus I was able to get the total quantity 

 of sun-heat received (a) for winter and summer, using the 

 words in their technical sense of the intervals between the 

 equinoxes ; (6) for our coldest 199 days ; and (e) for our 

 coldest 166 days, these being the lengths of the supposed 

 glacial and genial winters. Of course, since the total annual 

 sun-heat on any latitude is independent of the length of the 

 seasons, it was easy to get the summer sun-heats for our 166 

 and 199 hottest days from these figures. 



For the purposes of this paper I reduced the numerical 

 values I had obtained from Mr. Meech's tables, so that the 

 quantity of winter or summer sun-heat falling on the equator 

 should be represented by 380, that being the excess of tem- 

 perature of the equator above Ball's " natural zero." The 

 numbers thus obtained are : — 



Latitudes 40° 



Summer sun -heat 399 



Winter sun-heat 199 



coldest 199 days... 229 



coldest 166 days ... 179 121 66 23 3 



I have plotted these numbers to scale in fig. 2. 



The " summer " and " winter " sun-heats are of course the 

 same whatever be the relative lengths of those seasons. Hence 

 the shift of the isothermals is shown by lengths of the inter- 

 cepts made by the sun-heat curves in fig. 2 on a horizontal 

 line. The actual amount of the shift of the winter isothermals 

 in the supposed Glacial epoch, as obtained from this figure, is, 

 40° to 44°'2, 50° to 54°, 60° to 63°*5, 70° to 74°, and 80° to 



50° 



60° 



70° 



80° 



90° 



379 



347 



329 



317 



306 



138 



79 



32 



7 







161 



101 



48 



16 



5 



