# December 10th, 1799 – France Goes Metric

The humble metre (spelt *meter* in the USA) was originally defined in 1793 as one ten-millionth of the distance from the equator to the North Pole. In 1889, it was redefined in terms of a prototype metre bar (the actual bar used was subsequently changed twice). In 1960, the metre was redefined in terms of a certain number of wavelengths of a certain emission line of krypton-86. In 1983, the current definition was adopted. In 1959, the imperial inch was re-defined as 0.0254 metres (2.54 centimetres or 25.4 millimetres). One metre is about 3^{3}⁄_{8} inches longer than a yard, i.e. about 39^{3}⁄_{8} inches, and on **December 10th, 1799**, France adopted the metre as its official unit of length.

In the 18th century, there were two approaches to the definition of the standard unit of length. One favoured Wilkins approach: to define the metre in terms of the length of a pendulum which produced a half-period of one second. The other approach was to define the metre as one ten-millionth (1/10 000 000) of the length of a quadrant along the Earth‘s meridian; that is, the distance from the Equator to the North Pole. This means that the quadrant (a section/distance ^{1}⁄_{4} of the Earth’s circumference) would have been defined as exactly 10 000 000 metres (10 000 km) at that time, with the total circumference of the Earth defined as 40 000 000 metres (40 000 km). In 1791, the French Academy of Sciences selected the meridional definition over the pendular definition because the force of gravity varies slightly over the surface of the Earth, which affects the period of a pendulum.

To establish a universally accepted foundation for the definition of the metre, more accurate measurements of this meridian were needed. The French Academy of Sciences commissioned an expedition led by Jean Baptiste Joseph Delambre and Pierre Méchain, lasting from 1792 to 1799, which attempted to accurately measure the distance between a belfry in Dunkerque and Montjuïc castle in Barcelona to estimate the length of the meridian arc through Dunkerque. This portion of the meridian, assumed to be the same length as the Paris meridian, was to serve as the basis for the length of the half meridian connecting the North Pole with the Equator. The problem with this approach is that the exact shape of the Earth is not a simple mathematical shape, such as a sphere or oblate spheroid, at the level of precision required for defining a standard of length. The irregular and particular shape of the Earth smoothed to sea level is called a geoid, which literally means “Earth-shaped”, but does not correspond to the actual shape of the earth, but rather is a mathematical model of its shape. Despite these issues, in 1793 France adopted this definition of the metre as its official unit of length based on provisional results from this expedition, and then six years later made it the official measurement of the nation.

*“Today in History” on The Pandora Society dot com is primarily focused on Victorian and Edwardian history and does not always have a direct connection to Steampunk, Dieselpunk, or whatever punk; in fact it rarely does, but it is our hope that in sharing these historical events they might serve as some inspiration to the writers in our community to create potential alternative history stories which we look forward to reading* 🙂