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No. 422:
Shot Tower

Today, we let nature do our work for us. 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.

What's the shape of a falling raindrop? We like to sketch raindrops with a tear-drop shape. Actually, they're spherical. Once they've fallen far enough, surface tension has pulled them into the shape with the least surface. That's a sphere.

In 1782 an English plumber named William Watts saw possibility in that. He realized that if he dropped molten lead far enough through the air, it, too, would form into spheres. The surface tension of lead is a lot higher than that of water, so it forms very perfect spheres indeed. He saw that he had a new way to make buckshot.

Watts went back to his brick row house in Bristol and began adding floors to it. It was already three stories high. He doubled that. He put some castle-like trim on the top and called the design Gothic. He wanted his neighbors to like the addition, but the real action was inside his strange new home.

He knocked holes through each of the floors inside and put a water tank at the bottom. At the top, he poured lead into a sieve. The lead formed into spheres as it fell six floors. By the time the drops hit the water below, they'd started to solidify. The water caught and cooled them the rest of the way.

Up to then, most shot was cast. That was very labor-intensive. Shot was also made by pouring lead into a sieve over a barrel. That really did give tear-shaped drops. Before Watts, no one had yet realized that a much longer fall would give spheres.

Watts saw how he might greatly cut the cost of making high-quality shot. Then he gambled his home that it would work. And it did. Shot towers like his sprouted all over England and Europe. In 1808 Jefferson imposed the Embargo Act. That ended our shot supply. So we, too, began making shot towers.

Yet the process changed little. Shot makers added an upflow of air, and they invented ways to sort out deformed shot. Yet Watts's old patent still gave a pretty good description of 20th-century shot-making. In fact Watts's old house -- his original shot tower -- kept producing shot until 1968.

Watts's invention teaches us the two essential elements of good invention. The first is perception. Watts gazed more closely at nature and saw what other people had missed. The other element is simplicity. Others had labored to control the process with their own hands. Watts had the grace to stand aside and let nature do the work for him. The real beauty of this process is that, in the end, there is no human process at all.

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

(Theme music)

Minchinton, W., The Shot Tower. American Heritage of Invention and Technology, Spring/Summer 1990, pp. 52-55.