6 REPORT—1896. 
deeper part, by augmenting the capacity for heat of unit volume, implies a longer 
age than that conceded by Lord Kelvin. If the interior of the earth be fluid or 
cantain fluid in a honeycomb structure, the rate at which heat can travel would 
be immensely increased by convection currents, and the age would have to be 
correspondingly lengthened. If, furthermore, such conditions, although not 
obtaining now, did obtain in past times, they will have operated in the same 
direction. 
Professor Tait, in his letter to Professor Perry (published in ‘Nature’ of 
January 3, 1895), takes the entirely indefensible position that the latter is bound 
to prove the higher internal conductivity. The obligation is all on the other side, 
and rests with those who have pressed their conclusions hard and carried them 
far. These conclusions have been, as Darwin found them, one of our ‘ sorest 
troubles’; but when it is admitted that there is just as much to be said for another 
set of assumptions leading to entirely different conclusions, our troubles are at an 
end, and we cease to be terrified by an array of symbols, however unintelligible to 
us. It would seem that Professor Tait, without, as far as I can learn, publishing 
any independent calculation of the age of the earth, has lent the weight of his 
authority to a period of 10 million years, or half of Lord Kelvin’s minimum. But 
in making this suggestion he apparently feels neither intérest nor responsibility in 
establishing the data of the calculations which he borrowed to obtain therefrom a 
very ditferent result from that obtained by their author. 
Professor Perry’s object was not to substitute a more correct age for that 
obtained by Lord Kelvin, but rather to show that the data from which the true 
age could be calculated are not really available. We obtain different results by 
making different assumptions, and there is no sufficient evidence for accepting one 
assumption rather than another. Nevertheless, there is some evidence which 
indicates that the interior of the earth in all probability conducts better than the 
surface. Its far higher density is consistent with the belief that it is rich in 
metals, free or combined. Professor Schuster concludes that the internal electric 
conductivity must be considerably greater than the external. Geologists have 
argued from the amount of folding to which the crust has been subjected that 
cooling must have taken place to a greater depth than 120 miles, as assumed in 
Lord Kelvin’s argument. Professor Perry’s assumption would involve cooling to 
a much greater depth. 
Professor Perry’s conclusion that the age of the habitable earth is lengthened 
by increased conductivity is the very reverse of that to which we should be led 
by a superficial examination of the case. Professor Tait, indeed, in the letter 
to which I have already alluded, has said: ‘ Why, then, drag in mathematics 
at all, since it is absolutely obvious that the better conductor the interior in 
comparison with the skin, the longer ago must it have been when the whole was 
at 7000 F., the state of the skin being as at present?’ Professor Perry, in reply, 
pointed out that one mathematician who had refuted the tidal retardation 
argument 1 had assumed that the conditions described by Professor Tait would 
have involved a shorter period of time. And it is probable that Lord Kelvin 
thought the same; for he had assumed conditions which would give the result— 
so he believed at the time—most acceptable to the geologist and _ biologist. 
Professor Perry’s conclusion is very far from obvious, and without the mathematical 
reasoning would not be arrived at by the vast majority of thinking men. 
The ‘natural man’ without mathematics would say, so far from this being 
‘absolutely obvious,’ it is quite clear that increased conductivity, favouring escape 
of heat, would lead to more rapid cooling, and would make Lord Kelvin’s age even 
shorter, 
The argument can, however, be put clearly without mathematics, and, with 
Professor Perry’s help, I am able to state it in a few words. Lord Kelvin’s 
assumption of an earth resembling the surface rock in its relations to heat leads to 
the present condition of things, namely, a surface gradient of 1° F. for every 
50 feet, in 100,000,000 years, more or less. Deeper than 150 miles he imagines 
1 Rev. M. H. Close in R. Dublin Soc., February 1878. 
