In 1582, Galileo noticed something quite mundane. It may or may not be legend: While sitting in his pew in the cathedral at Pisa, he watched the lantern over the nave swinging back and forth, and doing so at a regular rate. He experimented with a pendulum and found out that the rate of the swing depended not on the weight of the pendulum bob, but on the length of the pendulum itself. The longer the pendulum arm, the slower and more languid the back-and-forth interval. A short pendulum would result in a more rapid tick-tock, tick-tock. By way of Galileo’s simple observation, length and time were seen to be linked—a linkage that made it possible that a length could be derived not simply from the dimensions of limbs and knuckles and strides, but by the hitherto quite unanticipated observation of the passage of time.

From the book The Perfectionists: How Precision Engineers Created the Modern World by Simon Winchester. Harper

A century later an English divine, John Wilkins, proposed employing Galileo’s discovery to create an entirely new fundamental unit, one that had nothing to do with the then-traditional standard in England, which was a rod that was more or less officially declared to be the length of a yard. In a paper published in 1668, Wilkins proposed quite simply making a pendulum that had a beat of exactly one second—and then, whatever the length of the pendulum arm that resulted would be the new unit. He took his concept further: a unit of volume could be created from this length; and a unit of mass could be made by filling the resulting volume with distilled water. All three of these new proposed units, of length, volume, and mass, could then be divided or multiplied by 10—a proposal which made the Reverend Wilkins, at least nominally, the inventor of the idea of a metric system. Sad to say, the committee set up to investigate the plan of this remarkable figure1 never reported, and his proposal faded into oblivion.

1 Wilkins, who was variously warden of Wadham College, Oxford, and master of Trinity College, Cambridge, was a polymath the like of which is impressive even today. Not only was he a practicing priest and college administrator, but he had a great interest in science. He suspected there might be life on the moon, imagined the existence of new planets, devised plans for submarines and aircraft and perpetual motion machines, and in the same book that proposed a metric system based on the pendulum, proposed the establishment of a new universal language, because of the deficiencies of Latin. Also, during his time at Wadham, he created transparent beehives so that honey could be harvested without disturbing the bees.

Except that one aspect of the Wilkins proposal did resonate—albeit a century later—across the English Channel in Paris, and with the support of the powerful cleric and diplomat Charles Maurice de Talleyrand-Périgord. The formal proposal, which Talleyrand put to the National Assembly two years after the French Revolution in 1791, exactly duplicated Wilkins’ ideas, refining them only to the extent that the one-second beating pendulum be suspended at a known location along the latitude of 45 degrees north. (Varying gravitational fields cause pendulums to behave in varying ways; sticking to one latitude would help mitigate that problem.)

But Talleyrand’s proposal fell afoul of the post-revolutionary zeal of the times. The Republican Calendar had been introduced by some of the firebrands of the day, and for a while France was gripped by a mad confusion of new-named months (Fructidor, Pluviôse, and Vendémiaire among them), 10-day weeks (beginning on primidi and ending on décadi), and 10-hour days—with each hour being divided into 100 hundred minutes and each minute into 100 seconds. Since Talleyrand’s proposed second did not match the Revolutionary Second (which was 13.6 percent shorter than a conventional second of the Ancien Régime), the National Assembly, gripped by the new orthodoxy, rejected the idea wholesale.

It would be more than two centuries out before the fundamental importance of the second was fully accepted. For now, in the minds of 18th-century French assemblymen, length was a concept vastly preferable to time.

about the author Simon Winchester is the author of multiple New York Times bestsellers, including The Professor and the Madman and A Crack in the Edge of the World.

For in dismissing Talleyrand so they turned instead to another idea, brand-new, which was linked to a natural aspect of the Earth, and so in their view more suitably revolutionary. Either the meridian of the Earth or its equator should be measured, they said, and divided into 40 million equal parts, with each one of these parts being the new fundamental measure of length. After some vigorous debate, the parliamentarians opted for the meridian, in part because it passed through Paris; they then decreed to make the project manageable that the meridian be measured not in its entirety, but only in the quarter of it that ran from the North Pole to the equator—a quarter of the way around, in other words. This quarter should then be divided into 10 million parts—with the length of the fractional part then being named the meter (from the Greek noun μέτρον, a measure).

A great survey was promptly commissioned by the French parliament to determine the exact length of the chosen meridian—or a tenth part of it, an arc subtending about 9 degrees (a tenth of the 90 degrees of a quarter-meridian), and which, using today’s measurement, would be about 1,000 kilometers long. It would necessarily be measured in the length units of 18th-century France: the toise (about 6 feet long), divided into 6 pieds﻿du﻿roi, each pied divided into 12 pouces, and these further divided into 12 lignes. But these units were of no consequence—because all that mattered was that the total length be known and then be divided by 10 million—with whatever resulted becoming the measure that was now desired, a creation of France to be eventually gifted to the world.

The proposed survey line ran from Dunkirk in the north to Barcelona in the south, each port city self-evidently at sea level. Since this 9-odd-degree arc was located around the middle of the meridian—Dunkirk is at 51 degrees north and Barcelona 41 degrees north, with the midpoint of 46 degrees north being the village of Saint-Médard-de-Guizières in the Gironde—it was thought likely the oblate nature of the Earth’s shape, the bulge that afflicts its sphericity and makes it resemble more of an orange than a football, would be most evident and so easier to counter with calculation. (To further confirm the Earth’s shape, the French Academy of Sciences sent out two more expeditions, one to Peru and the other to Lapland, to see how long a degree of high latitude was. All confirmed the orange shape that Isaac Newton had predicted centuries before.)