Metrology: the Science of Measurement, FROM THE FOOT TO THE METRE
Following on a discussion in the studio about Units of measurements, I would like to extend a little on the subject.
Firstly about the foot, foot as part of the body and foot as measuring tool. What a great tool. The ‘unit for measuring length equal to 12 inches or 30.48 centimetres’ comes in fifth position in the online Oxford English Dictionary, it is first and foremost a part of our body, one that we use to measure the land, or rather used to use. When we walk the land, we take measure of it, ultimately we experience its extent, how much of it we can cover in one hour, one day, by foot. We position ourselves and relate ourselves to the land. The Australian Aboriginals use both their feet and speech to measure and sing the land, in what we call songlines, told in the eponymous book by Bruce Chatwin, and what they call footprints of the ancestors.
The inch, about a twelfth of a foot, also happens to be about equal to the length of a phalanx. Or is it that the length of a phalanx (or a thumb, as it is known in other languages) happens to be about the twelfth of a foot? It was (is) a measuring unit of a phenomenological nature. One that we can relate to, and that we understand through our very own body.
But perhaps it is no surprise that this measurement standard, one based on walking the land, is called the imperial system.
In 1791, the metre was defined “as being equal to the ten millionth part of one-quarter of the terrestrial meridian“. In 1875 it was institutionalised and internationalised through the Bureau International des Poids et Mesures, still in action today, and the Metre Convention treaty, signed by 17 countries.
Although one is inclined to think of this definition of the metre as purely geometrical, requiring the most precise of astronomical observations to tell us the circumference of the earth, the metre was not in fact to be defined by looking at the stars. The ten millionth part of one-quarter of the terrestrial meridian was measured by walking the Earth. Between 1791 and 1799, two men, Pierre-François MECHAIN and Jean-Baptiste DELAMBRE, walked the meridian from Dunkirk to Barcelona, and arc of 9 and a half degree, in order to complete the triangulation.
The ten millionth of the quarter of the meridian was then made into a prototype of platinum-iridium, to be recently replaced by “the distance travelled by light in vacuum in 1/299 792 458 of a second”, admittedly a lot more difficult to remember that the length of my foot.
But ultimately, it all started with a walk from Dunkirk to Barcelona.
(illustration from Jules Verne: Aventures de trois russes et trois anglais en afrique australe, 1876)
some great information about the metric system here and about the ‘metre adventure’ here