measure. 
for the purpose of determining the compa- 
rative length of the metre and the English 
foot : the oft’er was gratefully accepted by 
the Society, and Messieurs Legendre, Bla- 
chain, and Prony, were appointed to assist 
M. Pictet in making the proposed compari- 
son of their standard metre of platina and 
the measure just mentioned. The first as- 
sembling of this committee was on the 2ist 
of October, of the same year, at the man- 
sion of M. Lenoir. Upon commencing 
their operations, they found some difficulty 
arising from the different manner in which 
the measures were defined : the French 
standards were merely cut off to the length 
of a metre ; but the English scale was gra- 
duated by lines; consequently the length 
of the former could not readily be taken by 
the microscopeSj neitlier could the English 
scale be measured by the usual method 
adopted for making new standard metres, 
which is accomplished by fixing one extre- 
mity against a firm support, “ and bringing 
the other into contact with the face of a 
cock, or slider, adjusted so as barely to ad- 
mit the original standard between it and 
the fixed surface.’’ 
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 extremities 
of the brass made two parallel lines to those 
engraved on the scale, and thus the appa- 
ratus was capable of being seen through the 
microscope : by these means the standard 
metre of platina, and another 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 mjllionth part of the 
quadrant of the meridian. “ At the tem- 
perature of 15.3° of the decimal thermome- 
ter, or 69.5“ 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 occur- 
red through the difficulty of placing ffie 
cross wires exactly at the extreme of the 
brass plate, where a reflection of ligjit took 
place which precluJed a distinct observa- 
tion whether the optical axis of the micros- 
cope was decidedly a tangent to the sur- 
Ikce precisely at the termination. M. Pro- 
ny, a member of the committee, suggested 
another arrangement as a remedy for this 
obstacle, and M. Paul, of Geneva, who was 
present, carried it into execution : this latter 
gentleman traced a perpendicular luie to its 
length, on a small metallic rider, the end of 
which he placed against a firm resistance, 
and the cross wires were made to agree 
with the line ; tliey then interposed the 
standard metre between the end of the • 
piece and the resisting substance, “ and the 
line traced on it, which had now obviously 
advanced the length of the metre, was sub- 
jected to the other microscope. The mi- 
croscopes, 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 in order to 
measure the fraction, expressing the distance 
of the other microscope from another divi- 
sion.” 
A second comparison took place on the 
26 th of October, at the residence of a mem- 
ber of the committee ; and after several sa- 
tisfactory experiments, it was discovered, 
that at the temperature t2.73“, or 55 of 
Fahrenheit, the standard of platina was 
39.3781, and that of iron 39.3795 English 
inches. The different metres being intended 
to be equal at the temperature of melting 
ice, the preceding experiments may be tried 
by bringing their results to the same tem- 
perature. To determine this, we have 
Borda’s accurate trials, and the report of 
the committee of weights and measures on 
the dilatation of platina, brass, and iroti, 
whence it appears, “ that for each degree 
of the decimal thermometer, platina ex- 
pands .00000856; iron, 00001156; and 
brass, 00001783: for Fahrenheit’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 tliat of iron tp 39.38265 
Epglish 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 melt- 
ing ice, the individual standard metres aye 
equal to the 10,000,000th part of the qua- 
drant of the meridian, and to 39.38272 
English inches of M. Pictet’s scale. 
It is found, upon examination of the re- 
duction of the standards of platina and iron 
to the freezing point, that they vaiy rather 
less than is asserted in the report, and that 
