4 E. P. CuhenveU — Theory of the Ice Age. 



Chap. IV. Outline of the Physical Agencies which lead to Secular 



Changes in Climate (pp. 54-80). 

 Chap. XIV. Five pages of this chapter on The Wind Theory 

 of Oceanic Circulation in relation to Changes of Climate 

 (pp. 226-235). 

 Chap. XIX. Geological Time — Probable Date of the Glacial 



Epoch (pp. 311-328) ; and 

 Chap. XXV. The Influence of the Obliquity of the Ecliptic on 



Climate, and on the Level of the Sea (pp. 398-419). 

 There are two other Chapters, XXIII. and XXIV. (pp. 368-397), 

 on " The Physical Causes of the Submergence and Emergence of 

 the Land during the Glacial epoch," which may, perhaps, be con- 

 sidered to be more or less closely related to the astronomical theory, 

 but, as they are quite independent of the origin of the glaciation, 

 I have not included them, nor do I intend to discuss them here, 

 neither do I intend here to discuss the position taken up by Dr. 

 Croll in dealing with the obliquity of the ecliptic. I have given 

 a short criticism of it in a paper which appeared in the Phil. Mag. 

 of December, 1894. 



I. — Statement of Ckoll's Position. 



Croll's reasoning consists of a chain of arguments, and care is 

 necessary to distinguish them one from the other. In his view the 

 direct effect of the asti-onomical cause is to lower the terrestrial 

 temperature considerably ; this lowering sets up other agencies, the 

 indirect ones, and these operate chiefly to produce an Ice Age. In 

 other words, Croll sets himself to answer three questions : — 



1. What was the direct effect of the increase in eccentricity upon 

 the climatic and physical condition of the earth ? 



2. How did this direct effect set up the indirect agencies ? 



3. How did these indirect agencies operate to produce a Glacial 

 epoch ? 



As Croll has nowhere classified the causes he assigns in relation 

 to these questions, I will now endeavour to exhibit each in its proper 

 place. 



CrolVs Ansioer to Question 1. 



His answer to this question resolves itself into a chain of five 

 links, denoted here by A, B, C, D, E, and F. 



(A) In a period of great eccentricity, the hemisphere which has 

 its winter in the part of the orbit remote from the sun will have a 

 long winter and a short summer, the greatest length of the winter 

 being 199 days as against our 179 days, and the least length of 

 summer being 166 days as against our 186 days (Croll, p. 57). 



(B) Owing to the greater distance of the sun in this long winter 

 much less daily sun-heat will be received in the long winter on any 

 latitude than is now received in an equal time on the same latitude; 

 on midwinter day the decrease will be about 16 per cent, of the 

 present amount (pp. 55-57, 323). 



Hence — 



(C) The winter temperature, as far as directly affected by sun- 



