WITH THE ENGLISH STANDARD. 24 , T 



It was made for M. Pi&etby Troughton, an artiit in Lon- made by Trough- 

 don, who has defervedly the reputation of dividing inftru- Mru 

 ments with fingular accuracy ; it was compared with another 

 ftandard made by the fame perfon for Sir George Shuckburgh, 

 aad it was found that the difference between the two was not 

 greater than the difference between the divifions of each ; that 

 is, it was a quantity abfolutely infenfible. This ftandard may 

 therefore be confidered as identical with the ftandard defcribed 

 by Sir George Shuckburgh in the Philofophical Tranfactions 

 for 1798. 



M. Pi&et alfo exhibited to the Inftitute a comparer, or an Inftrument of 

 inftrument for afcertaining minute differences between mea- j^^J^ m ] C rof- 

 fures, conftrucied alfo by Mr. Troughton. It confifts of two copes, 

 microfcopes with crofs wires, placed in a vertical (ituation, 

 ihe furface of the fcale being horizontal, and fixed at proper 

 diftances upon a metallic rod. One of them remains ftation- 

 ary at one end of the fcale, the other is occafionally fixed near 

 to the other end ; and its crofs wires are moveable by means 

 of a fcrew, defcribing in its revolution -j-^ of an inch, and fur- 

 niihed with a circular index, dividing each turn into 100 

 parts ; fo that having two lengths which differ only one tenth 

 of an inch from each other, we may determine their difference 

 in ten thoufandths of an inch. The wires are placed obliquely 

 with refpecl to the fcale, fo that the line of divifion muft bifect 

 the acute angle that they form, in order to coincide with their 

 interfeciion. General Roy has defcribed, in the 75th volume 

 of the Philofophical Tranfactions, a fimilar inftrument made by 

 Ramfden, for meafuring the expanfion of metals. 



M. Pi&et offered to the clafs the ufe of the ftandard, with 

 the micrometer defcribed, for the determination of the com- 

 parative length of the metre, and the Englifh foot : the offer 

 was accepted with gratitude, and M M. Legendre, M^chain, 

 and Prony, were appointed to co-operate with M. Piclet in 

 the comparifon of the ftandard metre of platina and the Englifh 

 foot. 



The firft meeting was on the 28th Vende'miaire (21ft of Firft meeting of 

 Oftober,) at the houfe of Mr. Lenoir. the commiflion. 



At firft a difficulty occurred from the different manner in Difficulty to 

 which the meafures were defined : the Englifh fcale was g ra . compare the 

 duated by lines ; the French ftandards were fimply formed to gra duated by 

 the length of a metre ; hence the length of the metre could not h^es, and the 



ri French, which 

 ea % is of the prscife 



