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The cycle of paucity and plenty was now primed to repeat. But the next energy shortage wouldn't arise just yet. Depopulation actually meant a better life for survivors. And so it was for almost three centuries. The Renaissance — that "great awakening" — began with a European population of around 50 million. Compare that with 73 million just before the Plague. Not until the 17th century, did Europe reach — then surpass — its pre-Plague population.

The use of coal in place of wood kept rising in post-plague Europe. Never mind urban dirt and grime; at least Europe had domestic and process heat. Fueled by coal, it surged ahead into a future that saw no limits. And the population rose once more.

Surface coal had soon run out. Then deep mines provided even better coal — at least until shafts began reaching down into the water table. Beyond that, mining would need pumps to drain the water that seeped in. And pumps that could drain those mines would need far more power than animals could provide. Mines were not generally located near streams to power water wheels. And a windmill might go slack with people working in the wet tunnels below.

So Europe began running low on coal just as its population finally surpassed that of the 14th century. And desperation once more laid its hand on the land. Desperation — extreme necessity — is said to be the mother of invention. Well, I could not disagree more. Invention's parents are freedom and the pleasure of inventing. We humans do very strange things when we're in dire straits.

At the heart of the problem was the question: What is energy? If we want a supply of energy, we really should know what it is. Well the question had only begun to attract attention in the 17th century. And it hadn't even occurred to medieval engineers. Supposed you'd asked a medieval millwright to explain, say, the mechanical power generated by a windmill or a waterwheel. Well, power didn't run out for either one. It just kept coming. Energy was obviously something provided by God — not for us to understand.

Center Wheel Perpetual Motion Machine
J. H. Lienhard with his own over-centered wheel type of perpetual motion machine. (Alas, it failed to work!)

By the middle 13th century, the expanded use of water and wind power had created in the medieval mind a kind of mania for such power. The power at the disposal of the average person had roughly quadrupled. And there was no obvious limit. People didn't know what energy was, but when they saw all it could do for them, they wanted more — and more.

So where they turned can hardly be a great surprise — in the 13th century and again as the energy supply became tight in the 17th. In Latin it was perpetua mobile. People looked for means of harnessing perpetual motion. Now, when you and I talk about a perpetual motion machine we mean one that produces power without being fed an even more power in a different form. Say, an engine that produces electrical energy without taking in even more of the energy liberated from coal.

Since 1850, we've all agreed on thermodynamic laws that tell us such machines cannot exist. (Well, almost all of us. I still get dozens of perpetual motion machine proposals every year. I have to go through the unhappy task of telling people that their very clever ideas will fail if they ever get built.)

But that's now. Let us try to turn our minds back to the thinking of that medieval engineer, hungry for more power. For years, he's watched his water wheels turn and turn and turn. Windmills turn, and stop for a while, but then turn some more. His eyes tell him that perpetual motion obviously is possible for the Breath of God is always there.

As early as 1150 A.D., Hindu mathematician, Bhaskara, proposed a machine that would produce continuous power. It was simple enough — a wheel with weights mounted around its rim so they swung radially outward on one side and inward on the other. The wheel was supposed to remain out of balance and turn forever. Medieval engineers knew nothing about the conservation of either energy or of angular momentum. They had no way of understanding how such a machine was doomed to fail.

The over-centered wheel reached the Moslems in 1200 and France by 1235. For the next 500 years, countless writers recommended this ingenious, if impossible, little device. We wonder, did they ever try to make one? They did, of course. And the machines always failed. Yet to minds that believed perpetual motion possible, failure merely meant they did not yet have the proportions quite right. Failure did not dampen hope. (In fact, I still get mail from inventors with new variations on this old idea.)

two perpetual motion machines
Left: Villard de Honnecourt's idea of an over-centered wheel perpetual motion machine, 1235 AD. Right: A late 17th-C notion of a pump driven by an over-centered-wheel perpetual motion machine, as shown in Böckler's Theatre of New Machines

Scientists finally began recognizing the impossibility of perpetual motion, but not 'til the late 17th century. And only in the 18th century did engineers have the math to show that the over-centered wheel could never work. It's been a scant century and a half or so since textbooks have included thermodynamic laws that deny perpetual motion.

Hydrostatic Perpetual Motion
Robert Boyle's idea of hydrostatic perpetual motion

But each new physical phenomenon that we discover awakens new perpetual motion hopes. Each new force of nature has driven, and still drives, people to look for ways of exploiting it to produce power without consuming energy. And the opposite is also true.

The search for perpetual motion led to the invention of the mechanical clock. It finally led to a better understanding of static electricity, surface tension, magnetism, hydrostatic forces, and so on. Some people still look for it in the face of the physics that says it is impossible. Others simply look for as-yet-unthought-of ways to keep getting around those laws to produce power — dare I say, for new means of tapping into the Breath of God.

So two ideas collided in the late 17th century. One was that perpetual motion was impossible. The other was that perpetual motion would save us by powering the pumps needed to take coal mining on down into seams below the water table. Well, they clearly couldn't do that. And we're still burning coal today. So let's next see how we dodged the bullet at this particular 11th hour?


For excellent treatments of perpetual motion machines see: A. W. J. G. Ord-Hume, Perpetual Motion: the History of an Obsession. (London: George Allen & Unwin Ltd., 1977.) and L. White, Jr., Medieval Technology and Social Change. (New York: Oxford University Press, 1966): Chapter III.

A thermodynamic point here: We speak of perpetual motion machines of the first kind, PMM-I, and the second kind PMM-II. A PMM-I violates the First Law of Thermodynamics by continuously producing more energy as useful work than it receives in other forms. A PMM-II need not violate the First Law, but its efficiency is greater than allowed by the Second Law of Thermodynamics.