Today, we find out how long an inch is. 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.

Units of measure were fairly standard in 18th-century England. Henry VIII had defined the yard as the distance from his nose to the tip of his outstretched hand. His daughter, Queen Elizabeth, rather liked the Roman mile -- a distance of about 5000 English feet. But she wanted it to be exactly eight English furlongs, so she standardized it at 5280 feet. Units like these were the same everywhere in England by the late 1700s.

But in France, measurements were a mess. Every province had different standards, and that made scientific discourse very difficult. Finally, in 1791, the French Academy of Science was asked to set up national standards, and it did so from scratch. What England did arbitrarily over hundreds of years, France vowed to do rationally. France determined to let the immutable laws of nature set her weights and measures.

At first, they wanted to standardize the length of a pendulum that would give a one-second swing. But gravity isn't exactly the same everywhere, so they gave that up. Instead, they used one ten-millionth of the distance from the north pole to the equator as the standard meter. They took the mass of liquid in a one-centimeter cube as the standard gram, and so on.

So the metric system was born, and French scientists gloated over its beauty. Antoine Lavoisier was anything but detached when he praised this product of detached science. "Never has anything more grand and simple, more coherent in all its parts, issued from the hand of man," he cried. (Three years later, the not-so-detached revolutionary government detached Lavoisier's head, but they kept the metric system for another century and a half.)

The metric system finally gave way to the International System of units. The calorie -- the energy needed to heat a gram of water one degree -- was replaced with the Joule. A Joule is defined in terms of work, not heat. The Centigrade temperature scale was shifted a little and renamed after Celsius.

We're less sure of science's detachment today. We realize that no system of units will ever be completely coherent because science itself isn't complete. Yet we have accepted the brilliance of the French idea -- the notion that nature itself should guarantee the consistency of our units.

By the way, do you know how the inch is defined today? It's not the 12th part of a standard foot. The inch is defined as precisely 2.54 centimeters. We quit trying to base length on a dead man's arm. We abandoned the old English standard -- a long time ago.

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

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I have recast this episode as Episode 2528.