# Gabriel Mouton

Gabriel Mouton
Born1618
Died28 September 1694 (aged about 76)
Known forProposing a natural standard of length based on the circumference of the Earth.
Scientific career
FieldsMathematics and astronomy

Gabriel Mouton (1618 – 28 September 1694) was a French abbot and scientist. He was a doctor of theology from Lyon, but was also interested in mathematics and astronomy. His 1670 book, the Observationes diametrorum solis et lunae apparentium, proposed a natural standard of length based on the circumference of the Earth, divided decimally. It was influential in the adoption of the metric system in 1799.

## The milliare

The system of units
Name Multiple of virga Approx. equivalents
Milliare 1000 1 minute of arc, 2 km, 1 nautical mile
Centuria 100 200 m
Decuria 10 20 m
Virga 1 2 m, 1 Parisian toise
Virgula 0.1 20 cm
Decima 0.01 2 cm
Centesima 0.001 2 mm
Millesima 0.0001 0.2 mm

Based on the measurements of the size of the Earth conducted by Riccioli of Bologna (at 321,815 Bologna feet to the degree), Mouton proposed a decimal system of measurement based on the circumference of the Earth, explaining the advantages of a system based on nature.

His suggestion was a unit, the milliare, that was defined as a minute of arc along a meridian arc, and a system of sub-units, dividing successively by factors of ten into the centuria, decuria, virga, virgula, decima, centesima, and millesima.[1] The virga, 1/1000 of a minute of arc, corresponding to 64.4 Bologna inches, or ~2.04 m, was reasonably close to the then current unit of length, the Parisian toise (~1.95 m) – a feature which was meant to make acceptance of the new unit easier.

As a practical implementation, Mouton suggested that the actual standard be based on pendulum movement, so that a pendulum located in Lyon of length one virgula (1/10 virga) would change direction 3959.2 times in half an hour. The resulting pendulum would have a length of ~20.54 cm.

His ideas attracted interest at the time, and were supported by Jean Picard as well as Huygens in 1673, and also studied at Royal Society in London.[clarification needed] In 1673, Leibniz independently made proposals similar to those of Mouton.

It would be over a century later, however, that the French Academy of Sciences weights and measures committee suggested the decimal metric system that defined the Metre as, at least initially, a division of the circumference of the Earth. The first official adoption of this system occurred in France in 1791.

By today's measures, his milliare corresponds directly to a nautical mile, and his virga would by definition have been 1.852 m.