473 
the Density of the Earth . 
case. The pieces of wood are fastened to the wall of the build- 
mg ; and I find, that the weights may strike against them with 
considerable force, without sensibly shaking the instrument. 
In order to determine the situation of the arm, slips of ivory 
are placed within the case, as near to each end of the arm as 
can be done without danger of touching it, and are divided to 
eoths of an inch. Another small slip of ivory is placed at each 
end of the arm, serving as a vernier, and subdividing these 
divisions into 5 parts ; so that the position of the arm may be 
observed with ease to looths of an inch, and may be estimated 
to less. These divisions are viewed, by means of the short tele- 
scopes T and T, (fig. 1.) through slits cut in the end of the case, 
and stopped with glass ; they are enlightened by the lamps L 
and L, with convex glasses, placed so as to throw the light on 
the divisions ; no other light being admitted into the room. 
The divisions on the slips of ivory run in the direction Wze, 
(fig. 2.) so that, when the weights are placed in the positions 
w and w, represented by the dotted circles, the arm is drawn 
aside, in such direction as to make the index point to a higher 
number on the slips of ivory; for which reason, I call this the 
positive position of the weights. 
FK, (fig. 1.) is a wooden rod, which, by means of an endless 
screw, turns round the support to which the wire gl is fastened, 
and thereby enables the observer to turn round the wire, till the 
arm settles in the middle of the case, without danger of touch- 
ing either side. The wire gl is fastened to its support at top, 
and to the centre of the arm at bottom, by brass clips, in which 
it is pinched by screws. 
In these two figures, the different parts are drawn nearly in 
MDCCXCVIII. g P 
