﻿Astronomical Theory of Ice Ages and Genial Ages. 541 



An alternative form is got by performing the multiplications 

 and rearranging. It is 



2 A 2 B 2 C 2 - 2£ A 3 BC - % A 3 BC D + 4£ A 2 B 2 C - 6§ A 2 BCD 



+ SA 4 -42A 3 B + 42A 2 BC + 62A 2 B 2 -40ABCD = 0. 



The symmetry with respect to A, B, C, D is evident a priori. 



Mowbray Hall, near Capetown, S.A., 

 September 5, 1894. 



LXIII. A Mode of Calculating a Limit to the Direct Effect of 

 Great Eccentricity of the Earth 9 s Orbit on Terrestrial 

 Temperatures, showing the Inadequacy of the Astronomical 

 Theory of Ice Ages and Genial Ages. By Edward P. 

 Culverwell, M.A., Fellow of Trinity College, Dublin*. 



THE fundamental assumption made by Dr. James Croll 

 in his well-known writings on the Glacial Periods, and 

 subsequently adopted by Sir Robert S. Ball in ' The Cause 

 of an Ice Age,' is that we may attain to some approxi- 

 mate idea of the lowering of terrestrial temperatures, due to 

 greater winter distance from the sun, by the following con- 

 siderations : — 



Were it not for solar heat the earth would sink to what 

 Ball calls its natural zero, which must be nearly the absolute 

 zero of temperature. Hence the effect of the sun-heat is to 

 maintain it at its present excess above that temperature, and 

 any decrease in sun-heat will be accompanied by a more or 

 less proportionate decrease in the excess of the earth's tempera- 

 ture above the natural zero. To be on the safe side, however, 

 Croll takes this natural zero as a temperature of — 239° F. 

 (Pouillet's temperature of space), and Ball as —300° F. Croll, 

 in chap. xix. of ' Climate and Time,' which he devotes to this 

 subject, supposes the midwinter temperature in an epoch of 

 great eccentricity to be proportional to the sun-heat received on 

 midwinter day — a supposition open to the obvious criticism 

 that the adjustment of temperature to sun-heat can hardly bo 

 instantaneous. This is probably the reason why Ball modified 

 the argument and takes the average winter temperature as 

 proportional to the average daily winter heat from equinox to 

 equinox. 



I shall show, by actual comparison with terrestrial tempera- 

 tures, that this assumption of the proportion between the 



* Communicated by the Author. 



