¢ 
i 
TRANSACTIONS OF SECTION D. 5 
physicist, who was looking about for arguments by which to revise what he con- 
ceived to be the hasty conclusions of the geologist as to the age of the earth, 
should have exposed himself to such an obvious retort in basing his own con- 
clusions as to its age on the assumption that the earth, which we know to be 
always changing in shape, has been unable to alter its equatorial radius by a few 
miles under the action of tremendous forces constantly tending to alter it, and 
having 1000 million years in which to do the work. 
With this flaw in the case it is hardly necessary to insist on our great uncer- 
tainty as to the rate at which the tides are lengthening the day. 
The spectacle presented by the geologist and biologist, deeply shocked at 
Lord Kelvin’s extreme uniformitarianism in the domain of astronomy and cosmic 
physics, is altogether too comforting to be passed by without remark; but in thus 
indulging in a friendly tu qguoque, I am quite sure that I am speaking for every 
member of this Section in saying that we are in no way behind the members of 
Section A in our pride and admiration at the noble work which he has done for 
science, and we are glad to take this opportunity of congratulating him on the 
half-century of work and teaching—both equally fruitful—which has reached its 
completion in the present year. 
The second argument is based upon the cooling of the earth, and this is the 
one brought forward and explained by Lord Salisbury in his Presidential Address. 
It has been the argument on which perhaps the chief reliance has been placed, and 
of which the data—so it was believed—were the least open to doubt. 
‘On the Sunday during the meeting of the British Association at Leeds (1890), 
I went for a walk with Professor Perry, and asked him to explain the physical 
reasons for limiting the age of the earth to a period which the students of other 
sciences considered to be very inadequate. He gave me an account of the data 
on which Lord Kelvin relied in constructing this second argument, and expressed 
the strong opinion that they were perfectly sound, while, as for the mathematics, 
it might be taken for granted, he said, that they were entirely correct. He did 
not attach much weight to the other arguments, which he regarded as merely 
offering support to the second. 
This little piece of personal history is of interest, inasmuch as Professor Perry 
has now provided us with a satisfactory answer to the line of reasoning which so 
fully satisfied him in 1890. And he was led to a critical examination of the sub- 
ject by the attitude taken up by Lord Salisbury in 1894, Professor Perry was 
not present at the meeting, but when he read the President’s address, and saw 
how other conclusions were ruled out of court, how the only theory of evolution 
which commands anything approaching universal assent was set on one side 
because of certain assumptions as to the way in which the earth was believed to 
have cooled, he was seized with a desire to sift these assumptions, and to inquire 
whether they would bear the weight of such far-reaching conclusions. Before 
giving the results of his examination, it is necessary to give a brief account of the 
argument on which so much has been built. 
Lord Kelvin assumed that the earth is a homogeneous mass of rock similar to 
that with which we are familiar on the surface. Assuming, further, that the tem- 
_ perature increases, on the average, 1° F. for every 50 feet of depth near the 
surface everywhere, he concluded that the earth would have occupied not less 
than twenty, nor more than four hundred, million years in reaching its present 
condition from the time when it first began to consolidate and possessed a uniform 
temperature of 7000° F. 
If, in the statement of the argument, we substitute for the assumption of a 
homogeneous earth an earth which conducts heat better internally than it does 
toward the surface, Professor Perry, whose calculations have been verified by Mr. O. 
Heaviside, finds that the time of cooling has to be lengthened to an extent which 
depends upon the value assigned to the internal conducting power. If, for 
instance, we assume that the deeper part of the earth conducts ten times as well 
as the outer part, Lord Kelvin’s age would require to be multiplied by fifty-six. 
Even if the conductivity be the same throughout, the increase of density in the 
