248 



COMPARISON OF THE FRENCH METRE 



length without 

 graduation* 



Method by an 



intermediate 



piece. 



eafily be taken by the microfcopes ; nor could the Engliih. 

 fcaJe be meafured by the method employed for making new 

 flandard metres, which confifts in fixing one end againft a 

 firm fupport, and bringing the other into contact with the face 

 of a cock or Aider, adjufted fo as barely to admit the original 

 ftandard between it and the fixed furface. 



Mr. Lenoir attempted to overcome this difficulty by reducing 

 to a thin edge the terminations of a piece of brafs of the length 

 of a metre ; fo that it was compared with the flandard metre 

 in the ufual manner, and its extremities, when placed on the 

 Engliih fcale, confhtuted two lines parallel to thofe which 

 were really, engraved on the fcale, and capable of being viewed 

 by the microfcopes. 



The flandard metre of platina, and another flandard of iron, 

 belonging aifo to the Inflitute, were thus compared with the 

 Englifti foot ; each of thefe two meafures 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 Fahrenheit, the metre 

 of platina was equal to 39.3775 Englifh inches ; and that of 

 iron to 39.3788, meafured on M. Picket's fcale. 



Thefe firfl experiments, fhowed, however, that the method 

 employed was liable to fome uncertainty, arifing from the dif- 

 ficulty of placing the crofs wires precifely at the extremity of 

 the thin edge of the plate of brafs employed in the comparifon ; 

 a reflection or irradiation of light, which took place at that 

 extremity, prevented its being diftinctly obferved if the opti- 

 cal axis of the microfcope was precifely a tangent to the fur- 

 face exactly at the termination. 

 Accurate method I n order to remove this inconvenience, another arrange- 

 of comparifon ment was propofed by one of the Committee. (It was Mr. 

 a^ufe^cfrrving Pronv that frggefled this ingenious method, and M. Paul of 

 a line was caufrd Geneva, who happened to be prefent, that executed it. B.B.) 



the m0 r V ccife r0U8h A line was traced on a fma11 metalIic ruler > perpendicular to 

 length of the its length ; the end of the ruler was fixed againft a folid ob- 

 metre ;. un l cr the flacle, and the crofs wires made to coincide with the line: 

 the flandard metre was then interpofed between the fame ob- 

 flacle and the end of the piece, and the line traced on it, 

 which had now obvioufly advanced the length of the metre, 

 was fubjecled to the other microfcope. To the microfcopes 



thus 



Experimental 

 comparifon by 

 this method. 



Refult, 



in fome refpe£ts 

 uncertain. 



microfcopes. 



