THE GLACIAL PERIOD. 251 



that he had made a mistake if Iws attention had been suffi- 

 ciently given to the matter. The worst of it is that Croll 

 and other writers adopted Herschel's mistake as a scientific 

 verity, and reared on this untrustwortliy foundation no small 

 superstructure of figures and ai-guments. 



It is the object of this jmper to indicate clearly the 

 character of the error to which I have referred, and to sub- • 

 stitute for it the correct mathematical theory. It will not be 

 necessary for me to enter into the subject at any great 

 length, for I have fully develojied it in a little work which is 

 already througli the press, and which will perhaps have 

 appeared by the time this paper is read. I believe, iiowever, 

 that the subject will be of sufficient interest to justify me in 

 giving some account of it to your Association. I feel that I 

 am already in some degree known to those who may hear 

 tliis paper, at least if I may judge from the number of 

 readers that my little book Sfarland seems to have found in 

 the southern hemisphere. 



I will say once for all that the rectification of the eiTor to 

 which I have referred makes the astronomical theory a far 

 more potent factor in the explanation of ice ages than 

 appeared to be the case so long as the erroneous view was 

 entertained. Let me at once commence to explain the 

 matter. 



It is the radiation from the sun which prevents the surface 

 of the earth from having a temperature nearly as low as that 

 of space itself. We shall certainly not exaggerate if we 

 suppose that our earth when deprived of sunbeams would 

 have a temjjerature at least 300 deg. Fahr. l)elow tjuit which 

 it has at present. It is essential to bear this fact in mind, for 

 it thus becomes evident that a small relative fluctuation in 

 the total amount of heat received by the earth may cause a 

 large alteration in terrestrial tempei"ature. For the sake of 

 illustration we may assume the change of temperature to be 

 proportional to the change in the amount of heat received. 

 On this supposition an alteration of one-tenth in the heat 

 received would cause a change in temperature to the extent 

 of 30 deg. I do not, however, maintain that the connection 

 between temperature and the receipt of solar heat is at all so 

 simple as is supposed. My object will have been gained if 

 the illustration suffices to show that even so great a climatic 

 change as the advent of an ice age supposes does not neces- 

 sarily require any very large proportional alteration in the 

 daily receipt of heat from the sun. 



