CATALOGUE. PHILOSOPHY AND ARTS, PRACTICAL JIECHAXICS. 14^ 



Prony's Report to the National Institute of Sciences and 

 Arts. 6 Nivose, year lo, (27th December, I801). Journ. 

 R. I., I. 123. 



A member read, in the name of a committee, the fol- 

 lowing report on the comparison of the standard metre of 

 the Institute, with the English foot. 



Mr. Pictet, Professor of Natural Philosophy at Geneva, 

 submitted to the inspection of the class in the month of 

 Vendc'miaire, an interesting collection of objects relative to 

 the sciences and arts, which he collected in his journey to 

 Englahd. 



Among them was a standard of the English linear mea- 

 sure, engraved on a scale of brass, of 4y inches in length, 

 divided by very fine and clear lines into tenths of an inch. 



It was made for Mr. Pictet by Troughton, an artist in 

 London, who has deservedly the reputation of dividing in- 

 struments with singular accuracy , it was compared with 

 another standard made by the same person for Sir George 

 Shuckburgh, and it was found that the difference between 

 the two was not greater than the difference between the di- 

 visions of each ; that is, it was a quantity absolutely insen- 

 sible. This standard may therefore be considered as iden- 

 tical with the standard described by Sir George Shuckburgh 

 in the Philosophical Transactions for 1798. 



M. Pictet also exhibited to the Institute a comparer, or 

 an instrument for ascertaining minute differences between 

 measures, constructed also by Mr. Troughton. It consists 

 of two microscopes with cross wires, placed in a vertical si- 

 tuation, the surface of the scale being horizontal, and fixed 

 at proper distances upon a metallic rod. One of them ic- 

 mains stationary at one end of the scale, the other is occa- 

 sionally fixed near to the other end ; and its cross wires are 

 moveable by means of a scrnw, desciibing in its revolution 

 yig of an inch, and furnished with a circular index, dividing 

 each mrn into 100 parts ; so that having two lengths which 

 differ only one tenth of an inch from each other, we may 

 determine their difference in ten thousandths of an inch. 

 The wires are placed obliquely with respect to the scale, so 

 that the line of division must bisect the acute angle that 

 they form, in order to coincide with their intersection. Ge- 

 neral Roy has described, in the 7ath volume of the Philo- 

 sophical Transactions, a similar instrument made by Rams- 

 den, for measuring the expansion of metals. 



M. Pictet offered to the class the use of the standard, 

 with the micrometer described, for the determination of the 

 comparative length of ths metre, and the English foot : 

 the offer was accepted with gratitude, and MM. Legendre, 

 Mcchain, and Prony, were appointed to cooperate with M. 

 Pictet in the comparison of the standard metre of platina and 

 the English foot, 



The first meeting was on the 28th Vendemiaire (21st of 

 October), at the house of Mr. Lenoir. 



At first a difficulty occurred from the different manner 

 in which the measures were defined : the English scale was 

 graduated by lines ; the French standards were simply cut 

 off to the length of a metre : hence the length of the metre 

 could not easily be taken by the microscopes ; nor could 

 the English scale be measured by the method employed for 

 making new standard metres, which consists in fixing one 

 end against a firm support, and bringing the otlier into con- 

 tact with the face of a cock or slider, adjusted so as barely 

 to admit the original standard between it and the fixed 

 surface. 



Mr. Lenoir attempted to overcome this difficulty by re- 

 ducing to a thin edge the terminations of a piece of brass of 

 the length of a metre ; so that it was compared with the 

 standard metre in the usual manner ; and its extremities, 

 when placed on the English scale, constituted two lines 

 parallel to those which were really engraved on the scale, 

 and capable of being viewed by the microscopes. 



The standard metre of platina, and another standard of 

 iron, belonging also to the Institute, were thus compared 

 with the English foot ; each of these two measures being 

 equal, at the temperature of melting ice, to the ten millionth 

 part of the quadrant of the meridian. At the temperature 

 of 15.3° of the decimal thermometer, or 59.5'^ of Fahren- 

 heit, the metre of platina was equal to 39.3775 English 

 inches ; and that of iron to 39.3788, measured on Mr. Pic- 

 tct's scale. 



These first experiments showed, however, that the me. 

 thod employed was liable to some uncertainty, arising from 

 the difficulty of placing the cross wires precisely at the ex- 

 tremity of the thin edge of the plate of brass employed in 

 the comiratison ; a reflection or irradiation of fight, which 

 took place at that extremity, prevented its being distinctly 

 observed if the optical axis of the microscope was precisely 

 a tangent to the surface exactly at the termination. 



In order to remove this inconvenience, another arranger 

 ment was proposed by one of the committee. (It was Mr. 

 Prony that suggested this ingenious method, and M. Paul 

 of Geneva, who happened to be present, that executed it. 

 B. B.) A line was traced on a small metallic ruler, per- 

 pendicular to its length ; the end of the ruler was fixed 

 against a solid obstacle, and the cross wires made to coin- 

 cide with the line : the standaid metre was then interposed 

 between the same obstacle and the end of the piece, and 

 the line traced on it, which had now obviously advanced 

 the length of the metre, was subjected to the other micro- 

 scope. The microscopes, thus fixed, were transferred to 

 the graduated scale ; one of them was placed exactly over 

 one of the divisions, and the micrometer screw was turned 



