User:Rhialto/french units

In France, before the decimalised metric system of 1799, a well-defined old system existed, however with some local variants. For instance, the lieue could vary from 3.268 km in Beauce to 5.849 km in Provence. Between 1812 and 1839, many of the traditional units continued in metrified adaptations as the mesures usuelles.

Length
The French law for the definitive metre of 1799 states that one decimal metre is exactly 443.296 French lines, or 3 French feet, 0 French inches and 11.296 French lines. The French royal foot is exactly 9000/27706 metres, or about 0.3248 metres. 

In Quebec, the surveys in French units were converted using the relationship 1 pied (of the French variety; the same word is used for English feet as well) = 12.789 English inches. This makes the Quebec pied very slightly smaller (about 4 parts in one million) than the pied used in France.

This page uses the Paris definitions, although the difference is not significant for the level of precision available at the time. In addition, the changing definitions of the metric units since 1799 means that even the precise relationship between French feet and metres may no longer be so accurate.


 * fr:Unités de mesure de l'Ancien Régime
 * The French typographic point, the Didot point, was 1/72 French inches, i.e. two royal points. The French pica, called "Cicéro", measured 12 points Didot.

Mass
The law of 19 Frimaire An VIII (December 10, 1799) defines:


 * The kilogramme is equal to 18,827.15 grains.
 * The kilogramme is, in addition, defined as the weight of 1 dm³ of distilled water at 4 degrees centigrade, i.e. at maximum density.

Traditionally, the French pound (livre) was defined as exactly 1/70 of a French cubic foot. When the kilogramme was defined, knowledge that a pied du roi cube filled with water masses exactly 70 pounds was apparently lost. According to the traditional (cubic foot) definition, one livre was about 489.675 grammes. According to the kilogramme definition, one livre was about 489.506 grammes; The difference is about 0.035%. However, a small difference in salinity - for example: the difference between distilled water and very good quality drinking water - is enough to explain this difference.

The units in the following table are (except for the talent) calculated based on the kilogramme definition of the livre.