THE ART OF WEIGHING AND MEASURING. 607 



toises used by the Academy in the eighteenth century, were compared 

 with it, and ultimately it was made the legal standard of France by 

 an order of Louis XV^, dated May IG, 176G. As the toise of Peru is the 

 oldest authentic copy of the toise of the Chatelet, the effect of this order 

 was simply to perpetuate the earliest known state of that ancient 

 standard. 



The metric system originated from a motion made by Talleyrand in 

 the National Assembly of France, in 1790, referring the question of the 

 formation of an improved system of weights and measures, based upon 

 a natural constant, to the French Academy of Sciences; and the prelim- 

 inary work was intrusted to five of the most eminent members of that 

 Academy, — namely, Lagrange, Laplace, Borda, Mouge, and Condorcet. 

 On March 19, 1791, these gentlemen, together with Lalande, presented 

 to the Academy a report containing the complete scheme of the metric 

 system. In pursuance of the recommendations in that report the law 

 of March 20, 1791, was enacted for the construction of the new system, 

 and the Academy of Sciences was charged with the direction of the 

 necessary operations. Those requisite for the construction of a stan<lard 

 of length were: 



(1) The determination of the difference of latitude between Dunkirk 

 and Barcelona. 



(2) The re-measurement of the ancient bases which had served for 

 the measurement of a degree at the latitude of Paris, and for making 

 the map of France. 



(3) The verification by new observations of the series of triangles era- 

 ployed for measuring the meridan, and the prolongation of them as far 

 as Barcelona. 



This \york was intrusted to Mechain and Delambre, who carried it 

 on during the seven years from 1791 to 1798, notwithstanding many 

 great difficulties and dangers. The unit of length adopted in their 

 operations was the toise of Peru, and from the arc of 9*^ 40' 45" actually 

 measured, they inferred the length of an arc of the meridian extending 

 from the equator to the pole to be 5,130,740 toises. As the meter was 

 to be one ten millionth of that distance, its length was made 0.5130740 

 of a toise, or, in the language of the committee, 443.290 lines of the toise 

 of Peru at a temperature of 13° Reaumur (lOjo C. or 61:^^ Fahr.).* 



Before attempting to estimate how accurately the standards we have 

 been considering were inter-compared it will be well to describe briefly 

 the methods by which the comparisons were effected. In 1742 Graham 

 used the only instruments then known for the purpose, — namely, very 

 exact beam compasses of various kinds, one having parallel jaws for 

 taking the lengtiis of the standard rods, another with rounded ends for 

 faking the lengths of the hollow beds, and still another having fine 

 points in the usual manner. The jaws, or points, of all these instru- 

 menf s were movable by micrometer screws having heads divided to show 



* 22, pp. 4:>2, 4•^'^, and (;42. 



