MEASURE. 



proposed comparison of their standard 

 metre of platina and the measure just 

 mentioned. The first assembling of this 

 committee was on the 21st of October, of 

 the same year, at the mansion of M. Le- 

 noir. Upon commencing their operations, 

 they found some difficulty arising from 

 the different manner in which the mea- 

 sures were defined: the French stan- 

 dards were merely cut off to the length 

 of a metre ; but the English scale was 

 graduated by lines ; consequently, the 

 length of the former could not readily be 

 taken by the microscopes, neither could 

 the English scale be measured by the 

 usual method adopted for making new 

 standard metres, which is accomplished 

 by fixing one extremity against a firm 

 support, ** and bringing the other into 

 contact with the face of a cock, or slider, 

 adjusted so as barely to admit the original 

 standard between it and the fixed sur- 

 face." 



M. Lenoir endeavoured to remove this 

 unfortunate impediment, by taking a piece 

 of brass of the length of a metre, and re- 

 ducing the terminations to a thin edge, 

 which was compared by the committee 

 with the standard metre as usual ; when 

 placed on the English scale, the extremi- 

 ties of the brass made two parallel lines 

 to those engraved on the scale, an<J thus 

 the apparatus was capable of being seen 

 through the microscope : by these means 

 the standard metre of platina, and ano- 

 ther belonging to the Institute, made of 

 iron, were compared with the English 

 foot ; the two measures each 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 English inches, and that of 

 iron to 39.3788, measured on M. Pictet's 

 scale. 



It was discovered, however, that the 

 manner employed produced results not 

 quite satisfactory, as an uncertainty oc- 

 curred through the difficulty of placing 

 the cross wires exactly at the extreme of 

 the brass plate, where a reflection of light 

 took place which precluded a dislinct ob- 

 servation, whether the optical axis of the 

 microscope was decidedly a tangent to 

 the surface precisely at the termination. 

 M. Prony, a member of the committee, 

 suggested another arrangement as a reme- 

 dy for this obstacle, and M. Paul, of Gene- 

 va, who was present, carried it into execu- 

 tion : this latter gentleman traced a per- 

 pendicular line to its length, on a small 



metallic ruler, the end of which he placed 

 against a firm resistance, and the cross 

 wires were made to agree with the line ; 

 they then interposed the standard metre 

 between the end of the piece and the re- 

 sisting substance, " and the line traced on 

 it, which had now obviously advanced 

 the length of the metre, was subjected to 

 the other microscope. The microscopes, 

 thus fixed, were transferred to the gra- 

 duated scale ; one of them was placed ex- 

 actly over one of the divisions, and the 

 micrometer screw was turned in order to 

 measure the fraction, expressing the dis- 

 tance of the other microscope from ano- 

 ther division.*' 



A second comparison took place on the 

 26th of October, at the residence of a 

 member of the committee ; and after se- 

 veral satisfactory experiments, it was dis- 

 covered, that at the temperature 12.75, 

 or 55 of Fahrenheit, the standard of pla- 

 tina was 39.3781, and that of iron 39.3795 

 English inches. The different metres be- 

 ing intended to be equal at the tempera- 

 ture ot melting ice, the preceding experi* 

 ments may be tried by bringing their re- 

 sults to the same temperature. To deter- 

 mine this, we have Borda's accurate trials, 

 and the report of the committee of weights 

 asid measures on the dilatation of platina, 

 brass, and iron, whence it appears, " that 

 for each degree of the decimal thermo- 

 meter, platina expands .00000856 ; iron, 

 00001156; and brass, 00001783 : for Fah- 

 renheit's scale these quantities become 

 476; 642, and 990 parts in a hundred 

 millions. From these data we find, that, 

 at the freezing point, the standard metre 

 of platina was equal to 39.38280, and that 

 of iron to 39,38265 English inches of M. 

 Pictet's scale. The difference is less than 

 the 500th of a line, or the 200,000th of the 

 whole metre " 



The facts obtained by all the compari- 

 sons amount to this conclusion, taking 

 each of the measures at the temperature 

 of melting ice, the individual standard 

 metres are equal to the 10,000,000th part 

 of the quadrant of the meridian, and to 

 39.38272 English inches of M. Pictet's 

 scale. 



It is found, upon examination of the 

 reduction of the standards of platina and 

 iron to the freezing point, that they vary 

 rather less than is asserted in the report, 

 and that they agree *' within a unit in the 

 last place of the decimals expressing their 

 magnitudes, or one ten thousandth of an 

 inch." At the freezing- point, the standard. 

 of platina becomes equal to 39.37380, and 

 that of iron to 39.37370 English inches on 



