242 
POPULAR SCIENCE REVIEW. 
him, supposed the Earth to have passed through hotter and 
colder zones in space. Others have adopted the view that the 
Sun is a variable star, and that the glacial epoch corresponds 
to the periods of decrease in the solar heat. Again, it has been 
a favourite hypothesis with some astronomers that changes in 
the obliquity of the ecliptic, explains the variations of geo- 
logical climate, but it can be shown from celestial mechanics, 
that the variations in the obliquity of the ecliptic must always 
have been so small, that they could not materially affect the 
climatic conditions of the globe. The only other view which 
requires consideration (the former ones being unsupported by 
any satisfactory evidence) is the change in the eccentricity of the 
Earth’s orbit, the influences of which on temperature have been 
examined by Herschel and others. Upon the authority of Mr. 
Croll’s philosophical work already quoted, it will appear 
that if the secular variations of climate which we have been 
considering be either directly or indirectly the result of changes 
in the eccentricity of the Earth’s orbit, we have the means of 
determining when these variations took place. If the glacial 
epoch was influenced by the causes here referred to, Mr. Croll 
shows that we have the means of ascertaining with tolerable 
accuracy, not merely the date of its commencement, but the 
length of its duration. M. Leverrier determined the superior 
limit of the eccentricity of the Earth’s orbit, and he has given 
formulae by means of which the extent of the eccentricity for 
any period, past or future, may be computed. From tables 
constructed by that eminent astronomer, it has been computed 
by Mr. Croll — supposing it to be admitted that the glacial epoch 
was in any way dependent upon the alterations of eccentricity 
— “that it extended over a period of 160,000 years. But as 
the glaciation was only on one hemisphere at a time, 80,000 
years or so, would represent the united length of the cold periods.” 
By a careful examination of the amount of denudation 
effected during the glacial epoch, it has been supposed that the 
surface of the country was ground down by the ice, to the 
extent of one-tenth of an inch annually, which gives upwards of 
650 feet of matter removed from the surface during the whole of 
the glacial epoch. By determining the rate of subserial denuda- 
tion, which is ascertainable by learning the quantity of matter 
which is carried off the land at the present time — that is, by 
ascertaining the amount of sediment annually carried into the 
sea by the river systems — we have an approximate measure of 
time since the close of the glacial epoch. The facts of geology 
appear to prove that the close of the Great Ice Age does not 
date back beyond 80,000 years. 
^ It has been thought by some geologists that alterations in 
the distribution of land and water would produce the changes 
