Among the great ideas realized during the first period of the
revolution, must be reckoned that of a uniform system of weights and
measures. From all parts of France remonstrances were sent against
the great variety of those in use. Several kings had endeavoured to
remedy this evil, which was so hurtful to lawful trade, and
favourable only to fraud and double-dealing. Yet what even _they_ had
not been able to effect, was undertaken by the Constituent Assembly.
It declared that there ought to be but one standard of weights and
measures, in a country subject to the same laws. The _Academy of
Sciences_ was charged to seek and present the best mode of carrying
this decree into execution. That society proposed the adoption of the
decimal division, by taking for a fundamental unit the ten-millionth
part of the quarter of the terrestrial meridian. The motives which
determined this choice were the extreme simplicity of decimal
calculation, and the advantage of having a measure taken from nature.
The latter condition would, in truth, have been accomplished, had
there been taken, as a fundamental unit, the length of the pendulum
marking seconds for a given latitude; but the measure of an arc of
the meridian, executed with the precision to be obtained by the
methods and instruments of the present day, was extremely interesting
in regard to the theory of the figure of the earth. This influenced
the decision of the Academy, and if the motives which it presented to
the Constituent Assembly were not exactly the real ones, it is
because the sciences have also their policy: it sometimes happens
that to serve mankind, one must resolve to deceive them.
All the measures of the metrical system, adopted by the Republic, are
deduced from a base taken from nature, the fourth part of the
terrestrial meridian; and the divisions of those measures are all
subjected to the decimal order employed in arithmetic.
In order to establish this base, the grand and important work of
taking a new measure of the terrestrial meridian, from Dunkirk to
Barcelona, was begun in 1792. At the expiration of seven years, it
was terminated; and the Institute presented the result to the
Legislative Body with the original table of the new measures.
MÉCHAIN and DELAMBRE measured the angles of ninety triangles with the
new reflecting circles; imagined by MAYER, and which BORDA had caused
to be constructed. With these instruments, they made four
observations of latitude at Dunkirk, Paris, Évaux, Carcassonne, and
Barcelona; two bases measured near Melun and Perpignan, with rules of
platina and copper, forming metallic thermometers, were connected
with the triangles of the meridian line: the total interval, which
was 9°.6738, was found to be 551584.72 toises. As the degrees
progressively diminished towards the south, but much more towards the
middle than towards the extremities, the middle of the whole arc was
taken; and, on comparing it with the degrees measured at Peru,
between the years 1737 and 1741, the ellipticity of the earth was
concluded to be 1/334 the mean degree, 57008 toises; and the MÈTRE,
which is the ten-millionth part of the quarter of the meridian,
443.296 lines of the old French toise which had been used at Peru.
The Commissioners, sent from foreign countries, verified all the
calculations, and sanctioned the results. The experiments of the
pendulum made at the observatory, with extreme care, by BORDA,
MÉCHAIN, and CASSINI, with a new apparatus, constructed by LENOIR,
shewed the pendulum to be 0.99385 of the _mètre_, on reducing it to
the freezing point, and in _vacuo_: this would be sufficient for
finding again the _mètre_, though all the standards were changed or
lost.
Exact experiments, made by LEFÈVRE-GINEAU, with instruments
constructed by FORTIN, shewed the weight of the cubic decimetre of
distilled water, at the point of the greatest condensation to be
18827.15 grains of the pile of 50 marcs, which is preserved here in
the _Hôtel de la Monnaie_, and is called _Le poids de Charlemagne_;
the toise being supposed at 13 degrees of the thermometer of 80
degrees. The scales of FORTIN might give a millionth part and more;
and LEFÈVRE-GINEAU employed in all these experiments and calculations
the most scrupulous degree of exactness.
Thus the MÈTRE or principal unit of the French linear measures has
furnished those of the weights; and all this grand system, taken from
nature, is connected with the base the most invariable, the size of
the earth itself.
The unit of the measures of capacity is a cube whose side is the
tenth part of the _mètre_, to which has been given the name of LITRE;
the unit of measures of solidity, relative to wood, a cube whose side
is the _mètre_, which is called STÈRE. In short, the thousandth part
of a _litre_ of distilled water, weighed in _vacuo_ and at the
temperature of melting ice, has been chosen for the unit of weights,
which is called GRAMME.
The following TABLE presents the nomenclature of these different
Measures, their divisions, and multiples, together with the new
Weights, as decreed by the Legislative Body, and to it is annexed
their correspondence both with the old French Measures and Weights,
and those of England.
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