The modern metric system can trace its roots back to Gabriel Mouton, the vicar of St. Paul's Church in Lyon, France, and a notable astronomer and mathematician. In 1670, Mouton conceived of a system of measurement based on the length of one minute of longitude (remember that there are 60 minutes in each degree of longitude and latitude). This unit of length, he further proposed, should be based on decimal arithmetic, or on powers of ten. He also recommended the use of prefixes to make naming conventions less arbitrary.

French scientists continued to modify and refine Mouton's ideas, but they were never formally codified until the French Revolution. Upon its creation in 1790, the National Assembly requested the French Academy of Sciences to "deduce an invariable standard for all measures and all weights." The academy in turn appointed a commission to develop the system, with the stipulation that the final solution should be at once simple, yet scientific. Borrowing from Mouton, the commission established three basic principles:

- The unit of length would be equal to a portion of the Earth's circumference.
- Measures for volume and mass would be derived from length, thereby ensuring all units would have a relationship.
- Larger and smaller multiples of each unit would be created by multiplying and dividing by 10 and its powers.

The commission named the unit of length "metre" ("meter" in the U.S.), after the Greek word *metron*, which means "to measure." Next came the task of actually determining the exact length of a meter. This fell to two men, Pierre Mechain and Jean Delambre, who spent six painstaking years measuring the distance on the meridian from Barcelona, Spain, to Dunkirk in northern France. Their survey resulted in a value for the meter equal to "one ten-millionth part of a meridional quadrant of the earth." Other units came from the precisely defined meter. For example, the gram was made equal to the mass of a cubic centimeter of pure water at the temperature of its maximum density; the liter was made equal to the volume of a cube 10 centimeters (4 inches) on a side.

This was the first incarnation of the metric system, which France officially adopted in 1795. Four years later, scientists fashioned standards for the meter and kilogram out of platinum. These, too, were officially recognized by the French government and stored in a safe place so copies could be made as needed.

Next, the metric system takes the whole world by storm.

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