Page:The American Cyclopædia (1879) Volume XVI.djvu/562

 542 WEIGHTS AND MEASURES extreme precision arises from the variability of temperature ; and this is greatly enhanced when the measures compared are of different volume, and still more when of different met- als. In comparisons of precision, it is there- fore necessary to insure a great uniformity of temperature, to prevent as much as possible the intiuence of the bodily heat of the observer upon the apparatus. In comparisons of weights, the accuracy attainable with the best balances at present in use is readily to the r.^nriirinr part, and by repetition, under favorable condi- tions, may be pushed considerably further, to the -fflVff of a grain in a pound, or at the ut- most to ,',-, of a milligramme in a kilogramme. But for such precision it is necessary that the weighing bo done in a vacuum, since minute errors will arise from movements of the air within the case of the balances, apart from the corrections due to the buoyancy of the air, which will affect differently weights of differ- ent volume. Even in France it was not found practicable at once to introduce the metric units into common use; and although they were at first made compulsory, it became ne- cessary to relax the law so as to permit the use of halves and quarters of the several units. Since 1840, however, the metric measures have been the only ones in common use in France ; and the system has found a very large accept- ance among other nations. This fact is chiefly due to the necessity existing in many coun- tries of making some change for the unification of standards used in their various provinces, in order to remedy the evil of varying local standards, and to the additional advantage of international uniformity presented by the ad op- tion of the metric system. For many years its use was limited to the nations of Latin origin cognate to the French, but the recent adoption of the metric system throughout the German empire has determined its prevalence on the European continent. The only standard which may contend with the metre for universal adoption is the English inch, which is the com- mon unit of length in the British empire, Rus- sia, and the United States. Sir John Herschel has pointed out that the polar axis of the earth is almost exactly 600,500,000 inches, and that the inch may therefore be considered quite as properly a natural standard as the metre ; and that the desirable correlation between volume and weight may be found in the fact that a cubic foot of distilled water weighs very nearly 1,000 ounces. By slight changes of the units this relation might be made exact, and the inch become equal to the Trnr.Trta.finr part of the earth's polar axis, 25 of such inches making a cubit, equal to the Ys.v^V.ffv P ar * f the polar radius. With a view to the construction of more perfect and uniform copies of the metre and kilogrammes, and their distribution to dif- ferent countries, an international standard com- mission, composed of scientific representatives from all civilized nations,was formed in 1869, meeting at Paris, and its work is now (1876) nearly completed. This movement has resulted in the establishment of a permanent interna- tional bureau of weights and measures, main- tained at the common charge of the contract- ing powers, and having for its object the pres- ervation of the international standards and their test copies ; the maintenance of the appa- ratus for comparison ; the periodic verification of the metres and other standards of different nations; the comparison of geodetic measur- ing bars, and scales used for scientific work ; and in general the maintenance of extreme precision and permanence in the fundamental units of measure. The new prototypes con- form as exactly as possible to those heretofore recognized as standards, and no attempt has been made to make them conform more nearly to their theoretical definitions. In regard to the metre, it has been found as the result of more extended geodetic measurements, up to 1875, that the meridional quadrant exceeds 10,000,000 metres by about 1,850 metres, and that consequently the metre falls short of its definition by its J^TT part. The kilogramme is in like manner found to differ from its pre- sumed value by some small fraction, in conse- quence of the great difficulty attending exact determinations of that kind. But it is now generally admitted that material standards, of which there are many exact copies, are all-suf- ficient for the preservation of the adopted units. The new standards are made of an al- loy of platinum and iridium, which possesses the most desirable mechanical and chemical properties, insuring their remaining unchanged for all time. See Panctou's Metrologie (Paris, 1780) ; Bessel on the Prussian unit of length (Berlin, 1889); the report of J. Q. Adams (Washington, 1821); Hassler's "Report on the Construction of Standards for the United States" (II. R. Doc. 299, 1882) ; the "Account of the Construction of the New National Stan- dard of Length, and of its Principal Copies," in the " Philosophical Transactions " (vol. clxvii., London, 1857); account of the restoration of the standard of weight, in the " Philosophi- cal Transactions " (1856) ; Bache's " Report on Standard Weights and Measures" (Sen. Ex. Doc. 27, 1857); J. II. Alexander, "Universal Dictionary of Weights and Measures " (Balti- more, 1850); Woolhouse, " Measures, Weights, and Moneys of all Nations" (Weale's series, London, 1856) ; F. W. Clarke, " Weights, Mea- sures, and Money of all Nations " (New York, 1875); Charles Davies, "The Metric System, &c." (New York, 1871) ; F. A. P. Barnard, "The Metric System" (New York, 1871); and reports of the warden of the standards (London), of which the 7th, 8th, and 9th (1872- '5) are very interesting and valuable. See also " Comparisons of Standards of Length made at the Ordnance Office, Southampton," by Capt. A. R. Clarke (1866), and " Report of the British Standards Commission on the Metric System " (1869). The following is- a summary of weights and measures in use among the