to ascertain a Standard of Weight and Measure. 137 
{§. 7.) Description of the Beam Compass , or divided Scale of 
equal Parts. 
a b, (Tab. V. fig. 1.) is a block or beam of mahogany, 
6 feet 3 inches long, 6 inches deep, and 5 wide, upon which 
are laid two brass rulers, c d e, and f g, each divided into 
60 inches, and tenths. The former of these, called the Scale, 
is, for a time, kept immoveable by the finger-screws c e d, and 
is furnished with very fine hair-line divisions, intended to be 
viewed only by the microscopes h, i : the latter, called the 
Beam, has no motion but by means of the screw g, and bears 
stronger divisions upon it, with which the sliding pieces or in- 
dexes, at k and m, may readily be compared by the naked eye, 
and is intended only to set the microscopes, or rather the wires, 
in their focus, to the required distance nearly; viz. tc within T ~ 
or -i-Q of an inch. The microscopes are compound, and similar 
to those described by the late General Roy, in his account of 
his large theodolite. (See Phil. Trans. Vol. LXXX.) The one 
at h, contains only cross wires fixed in its focus ; the other at 
i, has a micrometer also, by means of which its cross wires 
may be moved to the right or left, over the image of the divi- 
sions of the scale, any given space, not exceeding T % inch ; 
and the quantity so moved may be measured by the divisions 
on the screw head, passing under the index at 0. The divisions 
on these rules have been called inches and tenths : it was not 
necessary that they should be more than equal parts ; but they 
were in fact laid down by Mr. Troughtqn, from a scale of the 
late excellent artist Mr. J. Bird, who had divided into inches 
several scales of different lengths ; one of which, 42 inches 
long, belonged to the late General Roy; a second, of 5 feet, 
was purchased by Alexander Aubert, Esq. and a third, of 
MDCCXCVIII, T 
