Matters of Tolerance

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From The New York Review of Books:

Scientists and engineers recognize an elusive but profound difference between precision and accuracy. The two qualities often go hand in hand, of course, but precision involves an ideal of meticulousness and consistency, while accuracy implies real-world truth. When a sharpshooter fires at a target, if the bullets strike close together—clustered, rather than spread out—that is precise shooting. But the shots are only accurate if they hit the bull’s eye. A clock is precise when it marks the seconds exactly and unvaryingly but may still be inaccurate if it shows the wrong time. Perversely, we sometimes value precision at the expense of accuracy.

Simon Winchester, whose The Perfectionists ventures a history of this abstract concept, offers another way of looking at the distinction: a Rolls-Royce automobile, the 1984 Camargue model. In the course of a story filled with wonderful machines of every type, Winchester reveals himself to be something of a Rolls-Royce fanboy, but he declares this one to have been an ugly behemoth:

While the engineers had lovingly made yet another model of a car that enjoyed great precision in every aspect of its manufacture, those who had commissioned and designed and marketed and sold it had no feel for the accuracy of their decisions.

Winchester is a longtime journalist turned author, a meticulous researcher and catholic thinker who has written superb books about The Oxford English Dictionary, the Krakatoa eruption, the birth of modern geology, and (separately) the Atlantic and Pacific Oceans. Compared with topics like those, precision may seem an odd choice. What does it mean to write a history of so abstract a concept? Where does it even begin?

First Winchester needs to convince us that precision is a thing. It is, he tells us, a component of machines, and for that matter “an essential component of the modern world,…invisible, hidden in plain sight.” Besides being a component, it is a “phenomenon” that has transformed human society. We take it for granted, like the air we breathe, though we are suckers for precision snow tires and precision beard trimmers and we aspire to precision medicine and precision tattoo removal. It is “an essential aspect of modernity that makes the modern possible,” Winchester writes:

Precision is an integral, unchallenged, and seemingly essential component of our modern social, mercantile, scientific, mechanical, and intellectual landscapes. It pervades our lives entirely, comprehensively, wholly.

Which of the sciences are the most precise? Biology is messy, a science of divergence and variation, of creatures in all shapes and sizes. “Astronomical precision” is an oxymoron, astronomy being full of approximations and guesses piled atop one another—although the instruments of astronomy are tools of increasing and, lately, astounding precision. Mathematical precision trumps astronomical precision; mathematics is precise by definition. Winchester is not exploring the world of abstractions, though, but the real world, where people make things. His father was a precision engineer who turned metal into the most perfect machinery possible. Wood is nice but imprecise. The story of precision begins with metal.

. . . .

In Scotland, James Watt was designing a new engine to pump water by means of the power of steam. In England, John “Iron-Mad” Wilkinson was improving the manufacture of cannons, which were prone to exploding, with notorious consequences for the sailors manning the gun decks of the navy’s ships. Rather than casting cannons as hollow tubes, Wilkinson invented a machine that took solid blocks of iron and bored cylindrical holes into them: straight and precise, one after another, each cannon identical to the last. His boring machine, which he patented, made him a rich man.

Watt, meanwhile, had patented his steam engine, a giant machine, tall as a house, at its heart a four-foot-wide cylinder in which blasts of steam forced a piston up and down. His first engines were hugely powerful and yet frustratingly inefficient. They leaked. Steam gushed everywhere. Winchester, a master of detail, lists the ways the inventor tried to plug the gaps between cylinder and piston: rubber, linseed oil–soaked leather, paste of soaked paper and flour, corkboard shims, and half-dried horse dung—until finally John Wilkinson came along. He wanted a Watt engine to power one of his bellows. He saw the problem and had the solution ready-made. He could bore steam-engine cylinders from solid iron just as he had naval cannons, and on a larger scale. He made a massive boring tool of ultrahard iron and, with huge iron rods and iron sleighs and chains and blocks and “searing heat and grinding din,” achieved a cylinder, four feet in diameter, which as Watt later wrote “does not err the thickness of an old shilling at any part.”

By “an old shilling” he meant a tenth of an inch, which is a reminder that measurement itself—the science and the terminology—was in its infancy. An engineer today would say a tolerance of 0.1 inches.

Link to the rest at The New York Review of Books