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No. 1506:
The First Mechanical Clocks

Today, we look for the first mechanical clock. 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.

In other programs, we talk about what a fool's errand it is to name the first inventor of anything. But since the exception really does prove the rule, let's raise yet another priority question; let's ask when the first mechanical clock was invented.

Mechanical clocks replaced the old water clocks, which, by the 13th century, had been around for millennia. Water flowed steadily into a vertical tank and the rising water level indicated the time of day. That's simple enough, but, like mechanical clocks, water clocks had become ornate structures with gears and dials. Like mechanical clocks, they tolled the hours and displayed the planets.

What makes a mechanical clock is a mechanism called an escapement -- the balance wheel on a watch or the pendulum on a grandfather's clock. An escapement ticks in a steady rhythm and lets the gears move forward in a series of little equal jumps.

Foliot mechanism in clockThe first escapement was the verge and foliot mechanism (see the full image below). The foliot is a horizontal bar with weights on either end. It sits on a vertical rod, called a verge. The verge has pallets to engage and release the main gear which is turned by a heavy stone on the end of a cable.

The verge nudges the foliot back and forth in an inertial rhythm, and that determines the pace of the gear train. It was complex and very creative, but when did it come about? We don't really know because its importance wasn't apparent at first. People who wrote about early clocks couldn't see that the escapement was not just an incremental improvement on the water clock. Rather, it was a whole new technology and a whole new metaphor.

Verge mechanism in clockFrench architect Villard de Honnecourt described the first escapement we know about in AD 1250; but he didn't yet use it to control a clock. Instead, he built a kind of almost-clock -- a gadget that steadily pointed at the sun as it moved across the sky.

After that, monastery records mention the bells, gearing, and towers that went with either kind of clock, while they ignore the heartbeat of the clock. The first clear drawing of an escapement was given by Jacopo di Dondi and his son in 1364. They'd probably been building clocks for twenty years by then. So we can only guess that the first mechanical clocks were made in the late 1200s.

It's strange that so great a change can be that invisible. The best water-clock accuracy was about fifteen minutes a day, and that's about as well as the first mechanical clocks did. But now, engineers began to cut that error in half every thirty years, right up into the 20th century. It wasn't long before mechanical clocks swept the imagination of the Western world and created new standards of precision in instruments and ultimately in thought itself.

The defining technology of an age might not be the most obvious one. Great changes often come in on little cat feet. That's what the mechanical clock did in the thirteenth century. And we might well wonder what technology is doing just that, today.

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

(Theme music)

Usher, A. P., A History of Mechanical Inventions. London: Oxford University Press, 1929, 1954, 1970, Chapter 7.

This is a greatly reworked version of Episode 72.

Replica of an early 17th-century foliot-and-verge clock
Replica of an early 17th-century foliot-and-verge clock,
generously provided by Mike Helfrich