Account Overdrawn - Four Revolutions in the Earth Sciences - page 20

point out, in his work on the age of the Earth and the Sun, Kelvin made three as-
sumptions: the Earth is rigid as steel, is homogeneous in its physical properties,
and has no undiscovered source of heat. 6 Kelvin clearly spelled out each assump-
tion, for example writing in his 1862 article in Macmillan's that his calculations
for the age of the Sun were reliable “unless sources now unknown to us are pre-
pared in the great storehouse of creation.” 7
Perry made a simple change to Kelvin's assumption of a homogeneous Earth:
he imagined that a thin, solid crust overlies a fluid interior that conducts heat by
convection. As the interior cooled, it would transfer heat to the surface. According
to Perry, that meant that Kelvin's figure of 100 million years is too low. If the in-
terior conducted heat ten times better than the surface, for example, Kelvin's result
would have to be multiplied by 56, Perry calculated, giving 5 billion years as the
age of the Earth.
Kelvin found Perry's theorem “clearly right.” Though he thought his original
range for the Earth of “20 to 40 millions probably wide enough,” it was possible
thathe“shouldhaveputthesuperiorlimitagooddealhigher,perhaps4000instead
of 400.” 8 Kelvin could afford to be generous with the possible maximum age of
the Earth, since he still had the limit of the Sun's age, which he now said was re-
stricted to a score or very few score of millions of years. The Earth could be no
older than the Sun. Kelvin stuck by his estimate of 100 million years: “That is all
Geikie wants; but I should be exceedingly frightened to meet him now with only
20 million in my mouth.” 9
In 1897, Kelvin had his last word on the subject of his lifelong obsession. He
had not changed his mind. He began his address to the Victoria Institute by noting
that “thirty or forty years ago” geologists had given “very little thought” to the age
of the Earth. This attitude, Kelvin said, “would have left geology much in the same
position as if it were impossible to ascertain whether the Battle of Hastings took
place 800 years ago, or 800 thousand years ago, or 800 million years ago.” 10 Such
a lax attitude was no longer acceptable, Kelvin said, for the analysis of the heat
escaping from the Earth “suffices to sweep away the whole system of geological
and biological speculation demanding an 'inconceivably' great vista of past time,
or even a few thousand million years, for the history of life on the earth.” 11 This
analysis “utterly refutedtheDoctrine ofUniformity astaughtbyHutton,Lyell,and
their followers” (20).
Kelvin cited his first estimate of 20 million to 400 million years from his 1862
paper, “On the Secular Cooling of the Earth.” Now, thirty-five years later, he said
that new knowledge of the thermal properties of rocks allowed a “closer estimate.”
Ignoring other new developments, such as Geikie's defection and Perry's logical