•ivith the Standard of the Engli/h Meafures. '233 



'of platina and iron of the fnfiiiiite have no divifion, the length 

 of the mtn\' lieiiio ci'cu bv the dillance between their ex- 

 trcuiities. This circimiliance prevented, on the one hand, 

 the length of a metre from being taken innnedia'.ely on the 

 ftandard? with the niicroleopes ; and, on the other, did not 

 permit the immediat<? eOimation of the metre of the Inftitute 

 in finglilb inehes, by the procefs employed fi)r fixing the 

 lianda'-d of the new (neifuve?; a procefs which conlifts in 

 relling the exirenntv of the metal rule fubjetted to examina- 

 tion againll the knee of a fixed guuoe, and np])lying to the 

 othei extremity a lliding knee, conftruded for eitimating the 

 diflTerencr between the meafures, the ratio of whieh is to be 

 determ.inLd, or the identity of which is to be afcertained. 



" The artift Lenoir tried to fuiinount thefe obflaeles by 

 eniploving a eopper rule, a metre in length, cut at its extre- 

 mities mto a very fharn bevel, in fuch a manner, that this 

 rule could be compared with the (tandard of the Inftitute 

 according to the ulual metliod of extreme conlaft, and which 

 being placed above the Knglilh rule, the edges of the bevel 

 produced on the furfaee of that rule the effeft of divifions- 

 parallel to thofe delineated upon it. By thefe rncans, the 

 microfcopes could be ufed f )f eliiniating in inches, and ten 

 thoiifandth parts of an inch, the J iilanee between thefe edges. 



" By this method, the (tandard metre of platina, e^kI an- 

 other metre of iron, both belonging to the Inftiiute, were 

 compared with the Englifli foot ; the two fornicr meafures 

 being conftru6lcd in fuch a n)anricr, that at the temperature 

 of melting ice they are equal to each other and to the ten 

 millionth part of a quarter of the meridian. It was found 

 that at the temperature of 15-3 degrees of the centigrade 

 therm'onieler the |)Iatina mi^tre was equal to 39'3775j aiid 

 the iron metre to 39'378<S Englifli inehes, nieaiured on the 

 rule of C. Picket. 



" But thefe rirfl obfervations made known to the commif- 

 (ioners that the pr.jcefs thev emi)loved might leave fomc un- 

 certainty, in eonfequenee of the great difficulty of placing the 

 interfection of the threads at the exail extremity of the bevel 

 of the rule, which ferved as a term of comparifon. A reflec- 

 tion or irradiation of light which took' place at that extremity, 

 prevented our clearly diftiuiiuifliing whether- the optic axis 

 of the n)ierofcope was cxaclly tangent to the fniall lurfaec 

 which teiininaied that bevel. • 



" To remedy this incon\ cnience, one of the eommiflioncrs * 



' Tliis method, as fniiuiL .ib in^oni'jus, svas propoitd liy C. Proiiv ; and 

 (.. PaiW c)F i.iencva, av'.io was aci.i kntally prcfciit, ciiiicj it into exicu- 

 tion ! it was attended witl: compl'itc fucccf$, 



propofed 



