178 Phillips— Geological Address. 
these effects can be calculated if we know at what rate in time, 
whether uniform or not, they were produced: if we know, not the 
true rate, but the “mits within which it must have operated, the 
result of the calculation will have a corresponding uncertainty; if we 
have no knowledge of the rate, calculations are out of the question. 
In applying this general view to the history of the earth, philoso- 
phers of eminence in physical science have employed different con- 
siderations and obtained a variety of results. The conclusions of two 
eminent mathematicians which have lately appeared may be cited 
with advantage. 
A. careful computation by Professor W. Thomson, on selected 
data, which determine the rate of cooling of earthy masses, assigns 
98,000,000 years for the whole period of the cooling of the earth’s 
crust from a state of fusion to its present condition; so that, in his 
judgment, within one hundred millions of years all our speculations 
regarding the solid earth must be limited.* 
On the other hand, Professor Haughton finds, from the data which 
he adopts, 1018 millions of years to have elapsed while the earth 
was cooled from 212° F. to 122° F., at which temperature we may 
suppose the waters to have become habitable; and 1280 millions of 
years more, in cooling from 122° to 77°, which is assumed to repre- 
sent the climate of the later Eocene period in Britain. Computations 
of this kind cannot be applied except on the large scale here exem- 
plified; and they lose all their value in the eyes of those who deny 
the general doctrine of a cooling globe.t Much as these periods 
exceed our conception, they appear to be in harmony with the 
results of astronomical research, which contemplates spaces, motions, 
and cycles of periods too vast for words to express, or numerals to 
count, or symbols to represent. i 
The greatest difficulty in obtaining trustworthy results as to elapsed 
time is found where it was least expected—among the later Cenozoic 
deposits from rivers and lakes, and on the variable shores of the sea. 
This is the more disappointing because within this period falls the 
history of the human race. ‘Taking as its earlier limit the latest 
wide prevalence of glaciers in Europe, attempts have been made to 
measure its duration by several processes. Quite recently Mr. Croll} 
recalls attention to an astronomical cause of change of temperature— 
the varying excentricity of the earth’s orbit—by which in a small 
degree the total guwantity of heat received in the earth in a year, and 
in a much greater degree the distribution of this heat on the opposite 
circumpolar spaces, are altered.§ The effect of this at particular 
epochs would be, on one hemisphere an approximate equality of sum- 
mer and winter heat, on the other an augmented difference between 
them. If at the epoch of maximum excentricity the earth was in 
* Phil. Mag., Jan. 1863. 
+ Appendix to a Lecture on Geology, in the ‘ Reader,’ Feb. 1864. 
t Phil. Mag., Aug. 1864. 
§ Consult on this subject generally the valuable communication of Sir J, Her- 
schel to the Geological Society, Proc. vol, i. p. 244, for Dec. 1830. 
