io6 Capt. Kater on the length of the French metre 
Four rectangular pieces of brass were prepared precisely 
similar to those described in the account of experiments on 
the pendulum in the Philosophical Transactions before 
referred to. These were marked C, c } D and d. The per- 
fectly plane rectangular edges of the pieces C and c, being 
placed in contact, and kept thus by means of a spring, the 
distance of the fine lines drawn on their surfaces, parallel 
and very near to the rectangular edges, was found to be 
500,5 divisions of the micrometer, and the pieces D and d 
being placed in like manner in contact, the distance of the 
lines on their surfaces estimated in the same divisions was- 
456 , 7 - 
The metre a houts being placed by the side of the brass 
scale and in contact with it, the pieces D and d were applied 
to its extremities, the surfaces of the brass pieces being a 
little below the surface of the metre in order to preclude any 
error which might have arisen from the edges of the metre 
projecting beyond its terminating planes. Each of the brass 
pieces was supported in this position upon a piece of lead of 
a sufficient thickness, and kept in close contact with the end 
of the metre by means of a slight spring bearing against a 
pin driven perpendicularly into the lead. 
In order to ensure a perfect contact between each brass 
piece and the terminating plane of the metre, a flat ruler of 
brass was laid upon the surface of the metre so as to project 
beyond its extremity, and the end of the lead was elevated or 
depressed so that the line of light seen between the piece of 
brass and the ruler, the eye being level with the surface of 
the brass, appeared to be equal in every part, when it was 
inferred that the surfaces of the metre and of the piece of 
