Measures for Progress: A History of the National Bureau of Standards

528
APPENDIX B
As the more practicable of the two earth circles, the committee proposed to
measure an arc of meridian between Dunkirk, on the northern coast of France, and
Barcelona, on the Mediterranean Sea. From the distance determined, computation
would be made of the length of the entire quadrantal arc from the pole to the equator,
allowing for the deviation of the earth's form from a true sphere. The ten-millionth
part of the total computed length was then to be taken as the base or fundamental unit
of length and accurately marked off on a suitable number of specially constructed metal
bars, copies of which would provide working standards for science and commerce.
The plan was adopted and the Academy of Sciences assigned the term metre
(meter) from the Greek metron, a measure, to the one ten-millionth part of the quadrant,
fixing the new unit provisionally at 3 pieds 11.44 lignes, based on calculations made of
a meridian in France by Lacaille in 1740.2 This unit was roughly similar to the Dutch
eli, the English yard, the Italian braccio, and other standard lengths in the nations of
Europe.
From the concept of this single length standard, all other weights and measures
were to be derived. Decimal multiples and submultiples of the meter were to express
its macro and micro versions. A new, single unit of weight or mass, the gram, with
similar decimal multiples and submultiples, was to replace existing weights, the new
standard corresponding to the mass of 1 cubic centimeter (that is, a cube one-hundredth
of a meter on a side) of pure water. The unit of capacity, the liter, would be a
volume of pure water equal to 1 cubic decimeter. The measure of volume, especially for
cord wood, the stere, was to be a meter cubed. And the unit of land area, the are, was to
be a square 10 meters on a side or 100 square meters.
The concept had and still has merit and profundity. But as we shall see, the
theoretical perfection of the metric system could not be realized.
Working with the greatest precision attainable with the instruments and knowl-
edge available, members of the Academy measured by triangulation the meridional dis-
tance through Paris of the arc from Dunkirk to Barcelona, a strip of country made up of
mountainous and inaccessible districts. The work was not only arduous but hazardous,
since it was carried out during the Reign of Terror and was subject to repeated
harassment.
In April 1795, even before a definitive meter was derived from the astronomical
and geodetic measurements along the Dunkirk-Barcelona meridian, the revolutionary
government instituted the metric system in France, using the provisional meter as
standard and fixing the nomenclature of the new units of measure.
In June 1798, 6 years after beginning the fieldwork, the observations of the parties
of geodecists under Méchain and Delambre were completed and at the invitation of the
government a committee of delegates from the republics of Europe studied the assembled
computations. While one section of the committee examined the measurements of the
arc of meridian and the actual length of the meter, another, which included Johann
Georg Tralles, deputy of the Helvetic Republic,3 undertook determination of the unit of
mass, the gram. When so small a mass could not be realized with sufficient accuracy,
its multiple, the kilogram, was selected for construction of the standard of mass.
2 Interestingly, the provisional meter in brass, constructed by Lenoir of Paris in 1795,
proved to differ from the meter finally determined by only about 0.33 millimeter.
Before the establishment of the metric system, the principal units in use in France were
the pied du Roi (0325 meter) for lengths and the livre poids de mare (489.5 grams) for
weights. The pied du Roi was divided into 144 lignes, and 6 pieds du Roi made a
toise (1.949 meters), the common unit of length.
See Hassler appendix for note on Tralles.