Today, a story about 19th-century science and 21st-century technology. 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.

Here comes a wild new idea for meeting our energy needs. And it leads us right back to the Victorian scientist Lord Kelvin. In 1851, Kelvin wrote our modern system of thermodynamics. Up to then, the things we knew about heat seemed to disagree. He made sense of it all. Once he did, we could design engines that'd draw power from any natural temperature gradient.

So we look for natural gradients. The Gulf Stream provides one. Engineers have worked on a floating engine to generate electric power from warm water flowing over colder water below.

There's a much larger temperature gradient right under our feet. In many places, Earth's temperature rises 150 degrees each mile you drill downward. And here we run into Lord Kelvin again.

In 1862 the deeply religious and anti-evolutionist Kelvin shocked both fundamentalists and geologists. He calculated that Earth was a hundred million years old. If we began as molten lava, he said, it would've taken that long to cool down and establish Earth's temperature gradient.

Kelvin didn't know that radioactivity sustains the gradient. His result was low by a factor of 50. And it kicked off a fight that lasted into this century. Yet the fight itself finally gave us better mathematics as well as better physics.

Now engineers want to tap into Earth's temperature gradient. They're drilling test holes into the earth. They mean to pump cold water down into the rock, 12,000 feet below. They should be able to bring it back under pressure at 460 degrees. The hot water can then supply a power plant on the surface.

We Americans are so hungry for energy. We use 80 quadrillion BTUs a year. But Earth is vast. The energy stored in subsurface rock could supply that energy for a hundred thousand years.

Still, it's a new technology. And new technologies always harbor troubles. We'll have to force water down through cracks in the hot, dry rock. Then we have to find the water after it's heated and pump it back up. And, like coal or oil, Earth's heat can be used up. When it's gone, it's gone for a long, long time.

As we drill downward I hope we do the right thing. And I look back at quiet, scholarly Lord Kelvin. There he sits behind it all. He wasn't thinking about 21st-century power. He was thinking about Watt's steam engine -- and about a debate between geologists and fundamentalists that's now grown old.

So honest 19th-century thought is still feeding 21st century life. Kelvin's lucid mind set forces in motion that've reached beyond his comprehension -- and beyond his dreams.

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

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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.

Wald, M.W., Mining Deep Underground for Energy. The New York Times, Sunday, November 3, 1991, pg. 16 F.

Some might object when I credit Kelvin for setting up the means for drawing power from a temperature gradient. Carnot had already given us the underlying idea in 1824. But he did so without understanding what heat was. It wasn't until 1851 that Kelvin made a proper separation of the first and second laws of thermodynamics. Only then could we really relate the power output of a heat engine to a temperature gradient, quantitatively.