516 SCIENCE. 
the cosmos, has been brought forward of 
progress from a beginning which can be 
conceived, through successive stages to an 
end which can be foreseen. But the dis- 
proof leaves Hutton’s doctrine about the 
vastness of geological time exactly where it 
was. Surely it was no abuse of language 
to speak of periods as being vast, which can 
only be expressed in millions of years. 
It is easy to understand how the Uni- 
formitarian school, which sprang from the 
teaching of Hutton and Playfair, came to 
believe that the whole of eternity was at 
the disposal of geologists. In popular esti- 
mation, as the ancient science of astronomy 
was that of infinite distance, so the modern 
study of geology was the science of infinite 
time. It must be frankly conceded that 
geologists, believing themselves unfettered 
by any limits to their chronology, made 
ample use of their imagined liberty. Many 
of them, following the lead of Lyell, to 
whose writings in other respects modern 
geology owes so deep a debt of gratitude, 
became utterly reckless in their demands 
for time, demands which even the require- 
ments of their own science, if they had 
adequately realized them, did not warrant. 
The older geologists had not attempted to 
express their vast periods in terms of years. 
The indefiniteness of their language fitly 
denoted the absence of any ascertainable 
limits to the successive ages with which 
they had to deal. And until some evidence 
should be discovered whereby these limits 
might be fixed and measured by human 
standards, no reproach could justly be 
brought against the geological terminology 
It was far’more philosophical to be content , 
in the meanwhile, with indeterminate ex- 
pressions, than from data of the weakest or 
most speculative kind to attempt to meas- 
ure geological periods by a chronology of 
years or centuries. 
In the year 1862 a wholly new light was 
thrown on the question of the age of our 
[N. S. Von. X. No. 250. 
globe and the duration of geological time 
by the remarkable paper on the ‘Secular 
Cooling of the Earth,’ communicated by 
Lord Kelvin (then Sir William Thomson), 
to the Royal Society of Edinburgh.* In 
this memoir he first developed his now 
well-known argument from the observed 
rate of increase of temperature downwards 
from the surface of the land. He aston- 
ished geologists by announcing to them that 
some definite limits to the age of our planet. 
might be ascertained, and by declaring his 
belief that this age must be more than 20 
millions, but less than 400 millions of years. 
Nearly four years later he emphasized 
his dissent from what he considered to be 
the current geological opinions of the day 
by repeating the same argument in a more 
pointedly antagonistic form in a paper of 
only a few sentences, entitled, ‘The Doc- 
trine of Uniformity in Geology briefly re- 
futed.’+ 
Again, after a further lapse of about two 
years, when, as President of the Geological 
Society of Glasgow, it became his duty to 
give an address, he returned to the same 
topic and arraigned more boldly and ex- 
plicitly than ever the geology of the time. 
He then declared that ‘‘a great reform in 
geological speculation seems now to have 
become necessary,”’ and he went so far as to 
affirm that ‘it is quite certain that a great 
mistake has been made—that British popu- 
lar geology at the present time is in direct 
opposition to the principles of natural philos- 
ophy.”’{ In pressing once more the original 
argument derived from the downward in- 
crease of terrestrial temperature, he now 
reinforced it by two further arguments, 
the one based on the retardation of the 
earth's angular velocity by tidal friction, the 
* Trans. Roy. Soc. Edin., Vol. XXIII. (1862). 
t Proc. Roy. Soc. Edin., Vol. V., p. 512 (Dee. 18, 
1865). 
{ Trans. Geol. Soc., Glasgow, Vol. III. (February, 
1868), pp 1, 16. 
