In 1866 the law which made the metric system legal in the United States was passed. The same legislation directed that the 5-eent piece should weigh five grams and have a dianieter of two centimeters; that the unit for weighing letters in post-offiees should be the grain. Itut these details were imperfectly carried out. The use of units in electrical engineering, based on the metric system, was determined by the law of 1894. Bills have been introdueed into the House of Bepresentatives several times. proposing to make the metric system obligatory. The most recent bill of this kind was that of 1902. Al though compulsory legislation may not be imme diate, the adoption of the metric system is Con stantly extending. as shown by its use in weigh ing foreign mail matter, in weighing at the mints, in certain Government publications, in the Pharmacopeia and the Dispensatory. in cer tain measurements of the Coast and Geodetic Sur vey. and in much of the work of the arts and sciences.
The basal units of the metric system are for length the meter, for small surfaces the square meter, for area of land the square dekameter (or arc), for volume the cubic meter, for capacity the liter, for weight the gram, and for the monetary system the franc. The scale selected for the multiples and subdivisions of the various units is 10. The prefixes used to designate the multiples of the measuring unit are deka (10), hekto (100), kilo (1000), and myria (10,000), all from the Greek, and those used to designate subdivisions are deci (0.1), eenti (0.01), and milli (0.001), from the Latin.
The ratio between the successive denominations in the system of linear measure, of weight, of capacity, and of money is 10, the ratio between ; the successive denominations of surface measure is 100, and that of cubic measure i* 1000. The unit of capacity, the liter, is equal to a cubic decimeter, and approximately so to the volume of a kilogram of water at the maximum density. Its equivalent is 1.05671 liquid quarts (U. S. • standard) , or .SS036 liquid quart (British). The unit of weight, the gram. is the mass weight of one cubic centimeter of water, standard pure, at the maximum density. Its equivalent in the English system is 15.432+ troy grains.
The abbreviations of the following tables are those adopted by the International Commission of Weights and Measures: The advantages of the metric system over the English-American system are numerous. Al though, in both systems, the standard units of volume, capacity, and weight are directly con nected with the standard unit of measure, the relation in the French system is far more simple. Thus, in the English system one quart has a volume of 57r,t.i. eulde inches, while in the French system one liter has the volume of one cubic decimeter; in the English system the pound has the weight of about 0.015U of a cubic foot of standardized water, while in the French system one grain has the weight of one cubic centimeter. Although the names used in the metric system are generally longer than the names used in the English-American system, the nomenclature of the former has several advantages. Thus. the prefixes deci, cella, milli, deka, hekto, kilo have in point of derivation a numerical significance and have other applieations in the language, while the names inch, foot, yard. rod, and mile are devoid of numerical significance and are dis tinctive in their use. But the greatest advantage of all results from the use of a uniform scale of relation. 111 the English-American system seldom do more than two units in succession have the same scale. Thus, in the metric system, 10 centimeters = 1 decimeter. 10 decimeters = teeter, the ratio being always 10; while in the English system. 12 inches = 1 foot. 3 fest = 1 yard, nu, yards = I rod, . the ratio ehanging between every pair of units. The French Commission of 1790 reported in favor of the decimal scale for reasons of expediency. although admitting that the uniform scale of 12 possessed many advantages.
The metric system was once thought to he superior to all other systems of weights awl measures in being founded on an invariable magnitude, one ten-milibmth of a terrestrial quadrant. But science has dispelled this illusion by showing that this magnitude is not a constant and that. the distance originally taken as the basis of the meter was inaeeurately measured.
In 11310 the French Government conceived the idea of exelianging sets of the metrie units for of the emits of other nations in order to promote in international interest in the metric system. The international expositions at Loll
dm' IS31 ) and at. Paris (1S55) were, on account of the immense variety and confusion of metrical units. the first praetival demonstrations of the need of a universal deeimal system of weights and measures. At the Paris Exposition of 1667 a committee, representing several different na tions, was appointed to consider the question of uniformity, aml was called the Committee of Weights, Aleasures. and „Moneys. Mathieu was the president. of this commission. The Com mittee recommended instruction in the metric system in the public schools and its use in goV• ernmental departments and in scientific publies. tions. The aeodetie Association, whieh met at Berlin about this time, was also representative of several nations, and likewise favored the gen• oral adoption of the metric system. In the year 1869 a committee of the Academy of Scienees ut Paris and one of the Saint Petersburg Academy rceommended the convocation of an international commission. which should consider the means of providing all nations with sets of standard metric. units. Such a commission was invited by the French Government, and assembled at Paris in 1870. Twenty-four countries responded by send ing delegates. Joseph Henry and Julius E. llii gard representing the United States. This body was divided into committees, the most permanent ere being the French section, for the purpose of devising means for copying the standards lin SerVell in the archives. Some of the questions whirl' concerned the committee Were the coine position of the metal to be used in constructing the new unit of length, the most desirable form of cross-section, ways of expressing the length, as the distance between the ends, or between two tine lines made on the bar, means of comparing the new unit with the standard of the Archives, means for determining its variation due to changes in temperature, and other eonsiderations. The French section met in 1872 and proposed that an international bureau of weights and measures be located at Paris, the original con ception of tile international Bureau being later derided upon at the diplomatic conference of 1875. At time second meeting of the Inter national. C 'scion in 1872 it was decided to make the standard meter and the stand ard kilogram of the Archives the actual hasps for the new standards. In order to give the work of the emnmission the (diameter of an international act, its members so far being simply eitizens of their respective countries. the French Government invited plenipotentiaries and (Mega les from all of the nations interested. Dep• resentatives from twenty States assembled at Paris (18751 to constitute the Diplomatic Con• ferenee of the Meter. E. 13. Washburne acted as plenipotentiary and H. Vignand as delegate for the United States. The Observatory of the International Bureau, decided upon by this con ference, was completed in 1578. It stands at the entrance of the park of Saint. Cloud on a reserva lion presented by the French Government. The management of the Bureau reposes in the Inter. national Committee under the authority of the General Conference. In the Observatory are kept the instruments used in the determination of the international standards, the chief of which are the comparators. balances. and thermometers. The extent. of the demands upon this bureau may be inferred from time fact that in 1882 twenty three countries requested twenty-nine meters and thirty kilograms; in 1889 they requested thirty six meters and thirty kilograms. The meters are highly polished metal bars made of an alloy of platinum and iridium. and the kilograms are cylinders of the same material.
For a complete history of the metric system, with its present status, its equivalents, the Inter national Bureau of Weights and Moitsure.4, and the international abbreviations, consult Bigour dam Le systenle me'trique des poids et nneSnreS (Paris, 1901) ; Barnard, Metric System (3d ed., Boston, 1879) ; Fonvielle, Le metre 'international delinitif (Paris, 1875) ; Mendenhall, in Popular Science Monthly, vol. xlix. (1896) ; Potts, Ele mentary Arithmetic (London, 1886) ; Bassot, School of :Vines Quartrrly (November, 1901). See WEIGHTS AND MEASURES.