§ 9 8 SCIENCE PROGRESS 



which is the measure of the obliquity, can no more be affected 

 by the changing inclination of the earth's orbit to the invariable 

 plane, than looping the loop can affect the distance separating 

 the pilot and observer. Even if Laplace's calculations regarding 

 the invariable plane had no such imperfections as have been 

 acknowledged by astronomers to exist, this fallacy would re- 

 main. No critics so far have touched on this crucial fact, but 

 have confined themselves to disputing about comparative 

 irrelevancies only. The decrease in obliquity is therefore an 

 outstanding fact per se, and can only be accounted for now 

 by admission that the pole of the ecliptic is not the centre of 

 the precession movement. 



During the last century astronomers felt it incumbent on 

 them to find some reason for the Glacial Epoch and procured 

 a spokesman in James Croll, who, basing his theory on the 

 varying eccentricity of the earth's orbit as enunciated by 

 Leverrier, produced a plausible explanation of the occurrence 

 of glacial epochs as being due to the increased distance of the 

 earth from the sun at diiferent periods in the past. This made 

 the last glaciation begin 240,000 years ago and end 80,000 

 years ago. This took the fancy of the geological world for 

 the time and became the accepted astronomical view, though 

 progress in geological knowledge has made the majority of 

 geologists grow increasingly sceptical as to its truth, so that 

 it is no longer a working hypothesis. 



When Sir R. Ball took up the subject in The Cause of an 

 Ice Age, he made the following commentary on the question 

 of whether glaciation affected both poles at the same period 

 or not : 



" They (geological evidences) assure us no doubt that ice 

 ages have occurred in both hemispheres, but they leave us 

 uninformed as to whether the ice ages were consecutive or 

 were concurrent. Now this is not a mere matter of ordinary 

 significance, as it involves an absolutely vital point in the 

 astronomical theory of the ice ages. So much so is this the 

 case, that if it could be shown that ice ages in the two hemi- 

 spheres were concurrent the astronomical doctrine would have 

 to be forthwith abandoned (italics in the original). 



" Of course there is no chance of such a contingency arising, 

 and I merely enunciate it to show the significance of the 

 doctrine that ice ages in the opposite hemispheres were not 

 concurrent, but were consecutive ; in fact we may feel con- 



