Geoscience Reference
In-Depth Information
at the present time is in direct opposition to the principles of natural philosophy.
There cannot be uniformity. The Earth is filled with evidences that it has not been
going on for ever in the present state, and that there is a progress of events towards
a state infinitely different from the present.”
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Kelvin set out to refute Lyell by showing that the Earth was born at some finite
time in the past and will not survive beyond some finite time in the future. The
Earth is neither eternal nor unchanging. As the second law dictates, like everything
else our planet is running down.
Kelvin knew that Fourier's mathematics and the second law showed that when
a body starts out with different internal temperatures at different places, heat flows
from hot to cold regions and eventually removes the differences. In his 1846 can-
didate's lecture, he had shown that if one knew how fast the Earth is losing heat
and how well rocks transmit heat, one could work the equations back to the point
when the process began and thereby estimate the age of the Earth.
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But before he took up that question, Kelvin began to puzzle over the age of
the Sun. It also had to obey natural law and therefore had to have an energy
source. Kelvin first attempted to explain the Sun's heat by appealing to the idea
that meteorites continually fall into the Sun and give up their gravitational energy
as heat. But calculations showed that the process was inadequate to explain the
Sun's abundant heat and light.
That theory having failed, the only apparent alternative was that the Sun's en-
ergy is left over from its birth. The second law then requires that the Sun be run-
ning down. If the Sun's energy is waning, it, too, must have been hotter in the past
and will one day be cooler, so cool that it will no longer warm and illuminate the
Earth.
In an 1862 article in the popular
Macmillan's Magazine
, Kelvin described the
results of his calculations of the age of the Sun:
It seems, therefore, on the whole most probable that the sun has not illuminated the earth for
100,000,000 years, and almost certain that he has not done so for 500,000,000 years. As for
the future, we may say, with equal certainty, that inhabitants of the earth can not continue to
enjoy the light and heat essential to their life for many million years longer
unless sources now
unknown to us are prepared in the great storehouse of creation
.
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To Kelvin, whether the Sun was 100 million years or 500 million years old
mattered little. The Earth could be no older. By limiting the age of the Sun, Kelvin
had refuted Lyell and uniformitarianism.
Now Kelvin turned to the Earth, using Fourier's mathematics to calculate its
maximum age. Fourier had attempted the same calculation and gotten 200 million
years, such a seemingly absurd figure that he did not even bother to write it down.
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