3^4 Meafure of the Mertdhn in frtmce, (fc. 



Every care was ttkcn to fupport and difpofe them properly in meafuring the tafes them- 

 fclves. Their cxtremiries were never brought into contaft 5 but an interval was left, 

 which was ineafured by a tongue of Platina, Aiding frofn the end of one of the rules, and 

 carrying a vernier and microfcopc. The correftions or allowances for differences of tem- 

 perature, for obliquities of the line adually meafured, and for the elevation above the level 

 of the fea, were alfo of neceility to be attended to and allowed for. 



-As the length of the bafes are exprefled in -modules, all the other refults are denoted by the 

 fame unity. But to give a proper notion of the comparative value of this unity, witli regard 

 to the ftandards employed in other great operations, it became neceflary to afcertain the 

 length of No. i, or the module from the toifc of the academy called the toife of Peru ; which 

 was done before the commencement of the meafuring the bafe. This comparifon was m de 

 with a degree of precifion fufficient to Afcertain the hundred thoufandth part of a toife. 

 The details of thefe experiments are given in the Memoir of Cit. Borda, already mentioned. 

 \After his return Cit. Delambre did not fail to compare the rules which had been ufcd for 

 the meafurement of the bafes, and did not find the flighteft change in their length. Laftly, 

 the commiffion charged feveral of its members to repeat the fame comparifon of the module 

 with that of the toife of Peru, that of the North, and that of Mairan, all three of whica have 

 become celebrated or important; the two firft by the great operations to which they have 

 been applied, and the third, becaufe it was in parts of that toife that Mairan has exprefltd the 

 refults of his valuable experiments on the length of the pendulum, and becaufe it is the 

 flandard of the toifcs which Were ufed to meafure the two terreftrial dej_rees in the neighbour- 

 hood of Rome, by the celebrated Bofcovich and Lemaire. This new/ comparifon ot the mo- 

 dule to the toife of Peru again afforded the fame refult, namely, that the fcales had undergone 

 no changes, and proved, moreover, that the module is exa£Hy twice the length of the toife 

 of Per\i, and confequently 12 ifeet in length, when the centigrade thermometer is at 12^ de- 

 grees: whence it is deducible, as well by a calculation from the dilatation of the metals as 

 from the dire£l: experiments of Borda, that at the temperature of 165^ degrees, which an- 

 fwers to 1 3 degrees of Reaumur's thermometer, the module is fliorter than the double toife 

 by two hundred parts of aline; that is to fay, about the eighty-fifth thoufandth part of the 

 whole. 



The obfervations of Azimuth were made on the fun, and on the pole ftar at Watten, at 

 Bourges, at Carcaffonne, and at Mountjouy; that is-to fay, at the two extremities of the me- 

 ridian, and two intermediate places. The obfervations of latitude, which were made with 

 the circle of Borda. From the great number of the obfervations, and their agreements 

 with each father, it is confidered as a certainly that the errbr cannot amount to any thing 

 near half a fecond in any of the latitudes obferved. 



Thefe obfervations were made at Dunkirk and at Evaux by Citizen Delambre; at Car- 

 caflbne and Monljouy by Citizen Mechain; and at Paris by Citizen Mechain, at the national 

 obfervatory, and by Citizen Delambre at his own private obfervatory. 



Four 



