24 Scientific Proceedings, Royal Dublin Society. 
objection pointed out by Professor Huxley seems to be sufficiently 
obvious to make it common property. Sir William, in order to 
apply his mathematical calculation to the cooling earth, was 
obliged, as we have said, to make some assumption as to the 
difference of temperature between the interior and the surface of 
the globe, and to suppose that of the surface to be constant. But 
physical considerations show that there must have been a great 
cloud covering round the earth during the time of her own higher 
temperature, and caused thereby. (Professor Tait mentions that 
such would have been caused by the sun’s greater radiation.) 
Geological phenomena lead to the conclusion that there must have 
been, formerly, a much greater uniformity of climate over the 
globe than what now obtains; this also points to the universal 
cloud covering as its easiest explanation. It is unnecessary to 
observe what a very great effect this must have had in retarding 
radiation from the earth. We constantly have most striking 
evidence of this on nights which are at one hour clear, and at 
another clouded. Therefore, then we have various and strong 
reasons for believing that the rate of cooling of the globe has 
been considerably slower than Sir William estimates, and that 
therefore the restriction of the length of geological time, on 
that ground, is probably very much less than he contemplates ; 
how much less it is impossible to conjecture. 
May we not, then, venture to maintain with the utmost 
deference that the argument for what we regard as the very 
inconvenient restriction of geological time is not proved? From 
the nature of the case, this is sufficient for our purpose. We 
have strong positive reasons for believing in the great extent of 
the geological period; and therefore we are not called upon to 
absolutely disprove arguments against it which, of necessity, rest 
largely upon assumption. We contend that we have a logical 
right even to invert the order of ratiocination, and to argue from 
the strength of our conclusions, founded upon observation, to the 
weakness of largely hypothetical considerations, if they be 
opposed to them. 
Nore.—On March 8, shortly after this paper was read, Professor Tait communicated a 
paper to the Royal Society of Edinburgh, in which he showed that, contrary to the usual 
belief no pure metal on which he has experimented (except iron) diminishes in 
thermal conductivity with rise of temperature. But our argument from thermal caj.a- 
city is quite unaffected by this, 
