3.^2 Companfon of the Definitive 'Metre 



the above rule, divided ir.lo tenths of an inch, a comparer or 

 inftriinient proper for eftimaling the fmall difll'rencts between 

 the mcafiires, conUriidec! bv the fame artiit, and ot" which 

 the iollowing is a fliort dei'cription : 



*' The comparer is conipolcd of two niicrofcopes with 

 threads, which are j laced in a vertical fituation. The faces 

 of the rule to !)e examined beinsr horizontal, and at a con- 

 venient dilhuH;e from e^c b other, by niakino- them move 

 along a n^etai rod with divilions, one of the nncroicopes re- 

 mains fixed near One (,f the cxtren>e lengths of the point to 

 be comparecl, and ierves to alfrre the imniobdity of that 

 point ; the fecond miirofcope is towards the other extremity 

 of the fame Itngth, and fixed alfo; but the frame which bears 

 thcfe threads may 1>" moved bv means of a micrometer fcrew 

 the threads of which are equal to th of an inch EngHfh ; 

 and the plate palled over bv the index being divided into an- 

 other lOo parts, gives --^ th of an inch Englifli for every 

 divifion of the plate. By meams ot tins arrangement, if the 

 njicrofcopes are firil: placed in fuch a manner that the inter- 

 fc«ftiuns of their threads correipond to the extremities of any 

 length intended to be compared with another wliich difi'crs 

 from it by a quantity lef> than / loth of an inch, the dif- 

 ference of the two lengths, by emplning the njicrometer, 

 may be cRimuted in lo.oocth pari- of an inch. 

 i.-j '' The threads placed iii the focus are difpofed obliqi'tely 

 ■jo regard to the line-; ofdivition, io that a correfpondence is 

 .known to. have taKen phice when a line of divifion divides 

 into two equal parts the -acute angle formed by the two 

 threads. :; ' i; ■■ 



" We fliall here ;:bferve, that, in the vear 1785, Rnmfdcn 

 employed, in tlie lit;e manner, tw:o nncroicopes. adapted to 

 an inUruihent he had invented to meafure the diialalion of 

 ri:ctals. General K iv gave a.defcription of this inltrunient 

 in the 75th vriUime of the Philofophical TranlaiMions. 



"• C. Piclet oHt-red to entrull the clals with the EngliHi 

 flandard and the comparer above delcribed,' to enable them 

 16 determine the ratio of th& metre to the Enghlli foot: the 

 clafs accepted (hi-; offer with cratitude, and cliarged C. Le- 

 gendre, JVlechain, and mvTeU, (Prony,) to join with 

 C. Pi£let in order to dotormifie the ratio between the 

 ftandara of. the platina metre and the Englith foot. 



" The followrng is a {lu)rt account: of the refult of the 

 ilabour undertaken on this fi/ibjeft at the honfe ot Lenoir for 

 a firrt tomparifon : r '' 



" The. 49 inches of the Englifli rule terminate at divifion-s 



traced out on one of the faces of that rule 5 and the Itandards 



6 cf 



