78 Captain Kater on the comparison of 
cutting point. This, I consider, to be an essential improve- 
ment, as no accidental derangement of the cutting frame can 
take place without its being immediately perceptible ; and 
the apparatus may be conveniently applied to the division of 
straight lines or circles, in the manner I have described in 
the Philosophical Transactions for 1814. 
The micrometer microscopes, used in the comparison of 
the different standards, were those employed in the determi- 
nation of the length of the seconds pendulum, the description 
of which may be seen in the Philosophical Transactions for 
1818. But as the arrangement of Mr. Ramsden's bar, re- 
quired that the support to which the microscopes were at- 
tached should rest on its surface, some other form of the 
beam carrying them became necessary for this purpose. 
A board was prepared of well seasoned mahogany, 36 inches 
long, 3 inches wide, and ^ thick, and an edge bar of maho- 
gany 3^ inches wide and thick, was firmly fixed along the 
middle of it lengthwise, which most effectually prevented 
the possibility of flexure. To the extremities of this edge 
bar, and projecting beyond them, the microscopes were 
fixed, their cross wires being about 40 inches asunder. By 
this arrangement, the very important advantage was ensured, 
that the apparatus being laid on a plain surface, such as a 
scale, and the microscopes adjusted to distinct vision, on 
placing it on another plane scale, the object glasses of the 
microscopes would be precisely at the same distance from 
this last surface as they were from that to which they were 
applied in the first instance, and consequently, no error could 
arise from parallax. 
A piece of very thin brass, usually called latin brass, was 
