$J2 Mtthod employed in France 



each rule of the platina*. The four rules were placed a* 

 the end of each other, fupported by iron tripods mounted on? 

 rhree fcrews, to make the extremities correfpond at equaF 

 heights, and brought into a line by marks to direct the fight, 

 placed fuccefhvely on the wooden wedges before mentioned. 

 Had each rule been placed in immediate contact with itsneigh- 

 bour, the operator in placing one rule might have deranged 

 the preceding, and the contact befides would never have 

 been perfect. To prevent this inconvenience, care was taken 

 to leave a fuSicient interval from one rule to another; but 

 the- extremity of each rule was furnilhed with a fmall rule, 

 or flip of platina, which could be pufhed into a groove to fill 

 up the interval, and to form a perfect contact wkh the fol- 

 lowing rule : a fcale fitted with a nonius, viewed through a 

 encrofcope, mcafured the length of the fmall rule, to nearly 

 *^^_ T of a toife. Eut in thus meafuring a line on ground, 

 which often rifes or falls inferifibly, it was necefJary to pay 

 great attention to the differences of the level. For this pur- 

 pofe the cafe of each rule had, at equal diftances from its- 

 extremities,^ two fmall cubes made of copper, which rofe to 

 equ;d heights on- the phne of the rules; and upon which 

 were laid the two branches of a fquare, bearing a fpirit level. 

 This level being twice placed on each rule, in the two con- 

 trary directions, gave the mean inclination ahnoft to a 

 minute; fo that with a very fimple formula erch meafure 

 was found yeduced to the horizontal linr, and the feiies of 

 obfervations gave at the fame time the cuiiiplete level of the 

 bafe. In commencing the operation, the firft rule was 

 placed in fuch a manner that the centre of a plummet, fuf- 

 pended at its anterior extremity, fhoula fall exactly on the 

 centre of the copper cylinder, already mentioned, which 



» Al'ufion is made here not to common rhermtimete'rs, but to newly 

 invented metallic thermometers, divided according to a new dec ; mal f. ?le, 

 of which z.jKS parts correfpond to one degree of Reaumur. To P*prt r s; 

 therefore, that degree of expanfion of the 1 1 ;. t i n a , o 00000924, muft be 

 multiplied by 2,31 i, which gives a little more than 0.00002. 



formed 



