In the nineteenth century there was a confusing multiplicity of units of measurement. In England, for example, length was measured in inches, feet, yards, furlongs, rods, chains, poles, perches, miles, and more.
In 1791 the French National Assembly instructed the Academy of Sciences to design a simple decimal system. In 1793 the unit of length, the meter, was chosen to be one ten-millionth of the distance between the north pole of Earth and the equator, the specific meridian that was chosen being the one that passes though Paris. Unfortunately, the length of the meridian had not been measured at the time and this Job was carried out by Jean-Baptiste-Joseph Delambre (1749-1822) and Pierre Mechain (1744-1804).
A platinum bar engraved with two marks separated by the new "meter" was then placed in the International Bureau of Weights and Measures in Paris. Decimal divisions, such as the centimeter and kilometer were then used, the circumference of the Earth being about 40,000 kilometers (25,000 miles).
The word "about" has to be used because the French did not take into account the fact that the Earth is a slightly flattwed sphere, not a perfect one. The meter also led to the metric definition of the unit of mass, the kilogram. The reforms instituted by the French Revolution defined the kilogram as being the mass of a cube, each side 10 centimeters (3.9 in) square, of pure, air-free water at the temperature of 4°C (39.2°F), the temperature at which the density of liquid water is at its maximum.
The British, being somewhat suspicious of the French, did not adopt the system.
