Axioms

Jean Picard's proposed toise universelle was twice the length of a seconds pendulum. Unfortunately, if you set up a seconds pendulum in (say) Cayenne, French Guiana, you would find that it was 0.3% longer than one set up in the Paris Observatory, because the acceleration due to gravity at sea level varies with a number of factors including latitude.

The first formally adopted definition for the metre itself, proposed in 1791 by the French Academy of Sciences and adopted in 1793 by the National Assembly, was one ten-millionth of the distance from the North Pole to the Equator along the Paris meridian.