Today, let's ride the Sforza Horse into the 20th century. 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.

Some while ago I went to a special exhibit of Leonardo da Vinci's sketches at the Art Museum. I saw row upon row of drawings of a heroic horse -- a great beast pawing the air from every angle. This went beyond art. This was obsession.

These were sketches for a monument, never built, to Francesco Sforza. Sforza's son, Ludovico, commissioned the statue. He meant to build up his own political stock by making his father three times larger than life.

But Leonardo got no further than the horse Francesco was to ride -- a huge bronze horse, three times life size. The sketches don't show us much of Francesco. But that great snorting, pawing beast is the stuff bad dreams are made of.

It was to have been hollow inside -- an inch-and-a-half-thick shell. It would've weighed 65 tons and gone far beyond the foundry capabilities of Leonardo's age. Worse yet, it would still outreach foundry technique even today.

Leonardo made a clay model of the horse, and he began amassing bronze. But Ludovico lost confidence in the plan. He finally decided all that bronze would serve his political fortunes better in other ways. He had it melted down to make cannons for his war with France. A few years later, French soldiers wrecked Leonardo's clay model. They used it for target practice.

Recently 150 art historians and engineers met to see whether they might actually build that great beast after all. What they found wasn't encouraging. Even if all four legs were on the ground, they couldn't possibly hold that much weight.

For that matter, such a large mold could never stand up to the hydrostatic pressure of the molten metal. And a casting that large wouldn't cool uniformly. Thermal stresses would crack it. So Ludovico was perfectly right to doubt Leonardo's plan. Like many of Leonardo's engineering works, his Sforza Horse was doomed to live only in the imagination.

Still, Japanese engineers have already built a full-scale Sforza Horse, complete with rider. But not of bronze! Theirs was made of plastic, reinforced with fiberglass. Somehow that's not the same. It just isn't the same.

An American foundry proposes to make a Sforza Horse of bronze. They offer to cast a thin bronze skin in ten pieces and assemble it over a stainless steel skeleton.

So we probably shall see a Sforza Horse one day. But I'd rather let that monster live where he came from -- in the place where he has the greatest effect. Better this horse should continue to live -- inside the human head.

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

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Schwarz, F.D., Leonardo's Horse. American Heritage of Technology & Invention, Vol. 7, No. 2, Fall 1991, pp. 6-7.