Today, a Victorian scientist miscalculates the age
of the earth. The University of Houston's College
of Engineering presents this series about the
machines that make our civilization run, and the
people whose ingenuity created them.

More than a hundred biblical
scholars had estimated the age of the earth by the
mid 19th century. They found ages from 5400 to
almost 9000 years. By this time, geologists were
starting to insist that the Bible wasn't meant to
give us this sort of technical data -- that the
earth was, in fact, much older. But they didn't yet
have a basis for making their own estimates.

By now, Joseph Fourier had
developed a theory of heat conduction. It was based
on avant-garde mathematics that a lot of people
couldn't accept. Then, in 1862, a British
scientist, Lord Kelvin, used Fourier's theory to
calculate the age of the earth. He knew the earth's
temperature increased one degree Fahrenheit for
each 50 feet you went into the ground. He guessed
that the earth began as molten rock at 7000° F.
By solving Fourier's equation, Kelvin found that it
must have taken a hundred million years for the
earth's temperature to level out to one degree
every 50 feet.

Now the fat was in the fire! The deeply religious
and antievolutionist Kelvin had given an age that
was far too young to satisfy geologists and
Darwinists. But it was plenty old enough to waken
the ire of biblical literalists.

The major problem with Kelvin's estimate was that
he didn't know about radioactivity. Today, we know
that earth's temperature variation is sustained by
radioactive decay, and that Kelvin's cooling
calculation was worthless for telling us the age of
our planet. Its real value lay in the intellectual
stimulus it created.

The great Victorian scientists and mathematicians
knew something was wrong. So they formed ranks to
fight over questions of mathematical method and
biblical exegesis. Paul Nahin documents the debate,
which went on until the 20th century. He shows us
how it drew in Darwin,
Huxley, Heaviside, and many more. When
they were through fighting, at least mathematical
heat-conduction analysis had found a solid footing.

Today, modern chemical analysis tells us that the
earth is 4½ billion years old. But the
debate over Kelvin's calculation helped set up
techniques that let our students solve far nastier
heat-flow problems than he ever could -- techniques
for determining everything from how long it takes
to refrigerate fruit to how to cool a brake shoe.

And -- over and over again -- I'm dazzled by how
much more we seem to learn when the path to
understanding leads through briar patches like this
one.

I'm John Lienhard, at the University of Houston,
where we're interested in the way inventive minds
work.

(Theme music)

Nahin, P. J., Kelvin's Cooling Sphere: Heat Transfer
Theory in the 19th Century Debate over the
Age-of-the-Earth, *History of Heat Transfer: Essays
in Honor of the 50th Anniversary of the ASME Heat
Transfer Division* (E. T. Layton and J. H.
Lienhard, eds.). New York: ASME, 1988, pp. 65-85.
Thompson, Sir W. (Lord Kelvin), On the Secular
cooling of the Earth. *Mathematical and Physical
Papers, Vol III*, Elasticity, Heat,
Electro-Magnetism. London: C.J. Clay and sons,
1890, pp. 295-311. See also other articles on heat
flow in this volume.

This episode has been greatly revised as Episode 1568.

From Kelvin's collected papers,
1890

Lord Kelvin's calculation of Earth's age

From Light, Visible and Invisible,
1923

This odd photo turned up in an old book about
light.

It is an actual X-ray of Lord Kelvin's hand!

From *Electrivity in Everyday
Life*, 1904

And here is Kelvin himself. From left to right:

George Westinghouse, Lord Kelvin, and Charles
Merz.

They are observing a machine for changing ac to
dc.

The Engines of Our Ingenuity is
Copyright © 1988-1997 by John H.
Lienhard.

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