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



temperature at a place and the sun-heat received is utterly 

 wide of the mark ; but first I will endeavour to trace 

 the origin of CrolPs mistake. In 1830 Herschel wrote 

 a paper " On the Astronomical causes which may influence 

 Geological Phenomena," and in it he dealt with the possible 

 changes of terrestrial temperature due to changes in the 

 earth s minor axis, making use of the argument that a 

 small percentage alteration above the absolute zero (which he 

 says some place at —1000° F., some at — 5000° F., and some 

 lower still) due to a small percentage change in the annual 

 quantity of sun-heat received would produce all the great 

 change in temperature required by geologists. In such a case 

 the method is perfectly valid, except that terrestrial radiation 

 should not be taken as simply proportional to absolute tem- 

 perature. In the centuries during which the earth receives 

 less annual heat from the sun we may fairly suppose that a 

 practically permanent state is reached in which the heat 

 annually received by the globe as a whole is equal to that 

 radiated by the globe as a whole. In that case we cannot 

 complain of Herschel in 1830 treating the radiation as pro- 

 portional to the temperature, and then treating the percentage 

 decrease in temperature of the globe as a whole as equal to the 

 percentage decrease in sun-heat received. Had he taken the 

 law of cooling as at present stated — i. e. radiation varies as the 

 fourth power of absolute temperature — the percentage decrease 

 in temperature would (for small changes) only be one fourth 

 of the percentage decrease in heat received. Subsequently 

 Herschel, in a rather confused paragraph in his i Outlines,' 

 369 a, referred to by Croll (p. 37) as his authority, appears 

 to apply the same method to calculating temperature differ- 

 ences between the northern and southern hemispheres in 

 the period of great eccentricity. But he rejects the calcula- 

 tion immediately after on the ground that loss of heat in 

 winter, through greater distance from the sun, is compensated 

 for by greater duration of winter. Hence the paragraph does 

 not really seem to justify Croll's application. For Croll applies 

 the method to the temperature of limited portions of the 

 earth's surface, in which, owing to the fact that heat is con- 

 tinually being transferred from one region to another, there 

 is never a time at which the gain by sun-heat is equal to the 

 loss by radiation. 



The statement just made shows that for Croll's argument 

 to have any practical validity as a method of calculating the 

 winter temperatures in the epoch of great eccentricity, it is 

 necessary not only that the temperature adjustments shall be 

 made with great rapidity but also that the direct effect of 



