452 On the Length of the French Metre 
and on the other “ Fortin a Paris.” The terminating planes 
are supposed to be perfectly parallel, and the distance between 
them is the length of the métre. 
The métre a traits is the same width as the mé/re @ bouts, but 
only a quarter of an inch thick. The lines expressing the length 
of the métre are so fine that one of them is scarcely perceptible 
even with the assistance of a microscope, unless the light be very 
favourable. The situation of the lines may however be disco- 
vered by two strong black dots, made with a graver at the ex- 
tremities of each, and a fine line crosses them at right angles, to 
indicate the parts from which the measurements are to be taken. 
This métre, previous to being brought from Paris, was com- 
pared with a standard métre by M. Arago, with all that care and 
ability which he is so well knewn to possess, and which so de- 
licate an operation requires. The result was, that the distance 
between the lines was found to be less than a métre by +2722 of 
a millimétre, or ‘00069 of an inch. 
The same micrometer microscopes were used in the compari- 
sons which J am about to detail, as have been already described 
in my account of experiments on the length of the pendulum in 
the Philosophical Transactions of the present year; and as the 
length of the métre is nearly 39°4 inches, J was enabled to refer 
it to the same divisions of Sir George Shuckburgh’s scale as I 
had employed in the measurement of the pendulum. 
I commenced with the métre a traits. It was placed in con- 
tact with the standard scale, their surfaces being in the same 
plane. An excellent thermometer was laid upon the scale, and 
a piece of thick leather was placed upon its bulb in order to pre- 
vent its being affected by heat from the person of the observer. 
The whole was suffered to remain in this state for two or three 
days; after which the following observations were made at va- 
rious times, the microscopes being brought alternately over the 
métre and the scale. The value of each division of the micro- 
meter is 57355 of an inch*. 
* For the manner in which this value was obtained, see page 175 of the 
preceding paper. H 
Com- 
