280 Variation in the Obliquity of the Ecliptic. [July, 
be judged of only by the most able mathematicians, and can 
be decided only by aid of the most complicated mathematical 
investigations ; consequently that it was not competent for a 
mere philosopher or enquirer to offer any opinion on the 
subject. 
With regard to the first of these two reasons, we trust 
we shall adduce sufficient evidence to show that the real 
question of a variation in the obliquity was not only not 
settled many years ago, but was not examined, except most 
partially and most superficially. With regard to the second 
reason, we believe it can be proved that the problem is one 
which must be solved long before it assumes the form of a 
mathematical problem; and, therefore, mathematics have 
little to do with the main point at issue. 
Under these conditions, we purpose in this paper investi- 
gating the question of a possible variation in the obliquity, 
and calling attention to various facts which seem to have 
been overlooked, neglected, or undervalued, when the problem 
was examined many years ago. 
In the first place, we will bring into notice the evidence 
in favour of a considerable variation having formerly 
occurred in the obliquity, and the general evidence in favour 
of a change being possible; secondly, the evidence or argu- 
ments that have been considered sufficient to prove that no 
greater change could occur than I 21’. 
The first evidence in favour of a considerable variation in 
the obliquity is the evidence of geology. Whatever effects 
may be supposed to have been produced as regards changes 
in climate by the alternate elevation and depression of land, 
yet the facts of the glacial epoch demand some more 
powerful cause than can be attributed to this particular 
one; that the glacial epoch was universal in both the 
northern and southern hemisphere, and reached only to 
latitudes higher than 50°, except in mountainous ranges, 
are two most important fa¢ts, and conclusively demonstrate, 
we believe, that the elevation or re-distribution of land will 
not account for such universal results. 
Let us fora moment compare the two explanations now 
before us, and note which appears the more satisfactorily to 
account for the conditions known to have prevailed during 
the past. 
First, we have the alternations of land and sea, as an 
explanation of why an arctic climate prevailed down to 
about 50° latitude in both hemispheres, and produced also 
glaciers on the mountains in localities within the present 
tropics. We have this supposed cause to explain these 
