1875.) Variation in the Obliquity of the Ecliptic. 281 
known effects, and we cannot but consider the explanation 
feeble and inefficient. 
Secondly, we have the explanation afforded by the varia- 
tion of the obliquity to an extent of 12°, thus causing the 
arctic circle to reach to 54° latitude, and to actually bring 
this arctic circle down from its present boundary to that 
very parallel of latitude which the evidence of geology tends 
to prove was the boundary of glaciers in former times. 
Of the two explanations as supposed causes, there is no 
doubt that the variation in the obliquity alone affords any 
satisfactory reason for the observed effects, and the re- 
arrangement of land and water can only produce minor 
changes, which do not touch the vast problem requiring 
solution. 
The question as to a supposed variation to a great extent 
of the eccentricity of the earth’s orbit, is one which has 
been most ably dealt with by Mr. Belt in the October 
Number of this Journal. The investigation given by Mr. 
Belt to the facts of the glacial epoch seem to show that the 
glaciation of the two hemispheres was contemporaneous, 
and that there are other effects which are not such as would 
receive an explanation, even granting that there had occurred 
formerly a much greater eccentricity in the earth’s orbit than 
that which now occurs. 
As an explanation of the known climatic changes which 
have occurred, we can best test the value of the theory of a 
considerable variation in the obliquity by endeavouring to 
explain the observed facts, even of the Boulder period, by this 
theory, and then by aid of either an increased eccentricity 
in the orbit or an alteration in the arrangement of land and 
sea. We believe that it will be admitted that the variation 
in the obliquity will alone account for the facts in a satis- 
factory manner, and that other climatic changes, shown by 
geology to have occurred, will alone be explained by granting 
a considerable variation in the obliquity. Thus the evidence 
of geology is so strongly in favour of a variation, both 
greater and less than at present in the obliquity, that we 
believe it may be claimed that no other theory has yet been 
advanced which will account for what is known. 
If, as we believe will be the case, the future and present 
researches of geologists tend to prove that the northern and 
southern hemispheres were at the same time subjected to an 
arctic climate, it may be fairly claimed that no other theory 
will so completely and satisfactorily explain the phenomena 
as a great variation in the obliquity. 
We believe also it will be granted that, if it had been long 
