Paris as It Was and as It Is / A Sketch Of The French Capital, Illustrative Of The Effects Of The Revolution

by Francis William Blagdon

NEW FRENCH WEIGHTS AND MEASURES.

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.