222 
Accurate Balances. 
[July, 
crum was slightly lowered, and that of the centre of gravity of the beam conse- 
quently raised, both combining to give increased sensibility to its motion. 
The third set of bearings ad, were constructed in a similar manner, working 
outside of bb', and lifting the beam clear of them : the motion in both cases was 
communicated by cranks, to avoid any shock to the edge. The surfaces of ad, were 
ground perfectly plane, and were highly polished : the increase of sensibility due to 
the depression of the fulcrum may be estimated by the number of oscillations made 
by the beam, which were found to be respectively on cc' , 7 ; on bb' 4$ ; and in ad, 
only 3% per minute : but the alteration in the figure of the bearings, the perfection 
of the edge from being seldom used, and above all, the depression of the central 
fulcrum below the horizontal line, which is the condition exemplified in figure 2, 
combine in rendering the balance more than three times as sensible under the latter 
circumstances, i. e. the index travels over thrice the space upon the divided arc 
below. As it is convenient to measure very minute fractional weights at once upon 
this arc, it was necessary to provide for a change of the divisions with the change 
of bearings : it is needless to describe the mechanical contrivance for this simple 
object ; suffice it to say, that the divisions for ad, correspond to 3000ths of a troy 
grain, and that estimation may be made to a third of that quantity : a lens is fine 
in focus to read off the indications. The end supports, with their rings, arc entiu > 
of agate ; their prisms are longer than usual, to oppose the tendency ot the sea e* 
pans and silks to revolve, and thus wear the edge : instead of three silks also, t ie 
scales are suspended by a fine annealed silver wire, attached to a curved aim ot 
the scale pan, to obviate the accidents liable to occur from catching hold of t ie 
threads with the tongs, in placing or removing the load. 
2. — Of the Scale Box. 
It may not seem superfluous, after so minute a description of the balance itself 
to say a few words on the subject of the case which contains it ; for where sue 
an instrument is in constant use, a number of inconveniences are sure to strike 
the manipulator, which might be remedied by the communication of his experierc* 
to the artist. For instance, it may be remarked, tliat> glazed wooden cases, aie ^ 
suitable to this country: — the wood warps and admits dust into the box, and tie 
glass frequently cracks ; the strings of the sash frame are also a constant source^ 
annoyance, from their breaking ; especially when weighing operations are to 
conducted under a pankha : again, when the frame supporting the beam s a 
on the floor of the case, small weights are continually missed, and dust lodges^ 
every nook : many such particulars might be pointed out, but it will answer 
same purpose to describe the attempts made to remedy them in the instru 
ment now under review. 
The case, (fig. 10.) consists of two parts : — an outer ornamental stand of w° 01 ’ 
and an inner brass frame, glazed on all sides above and below, with plate g ,a5S » 
the front sash was counterpoised by weights in the back columns, with an 
nealed silver wires in lieu of strings, passing over pulleys of as large a diameter 
as the entablature would conceal: but further to prevent the necessity^ 
opening and closing this sash during work, a small sash was made to rise 
below, by pressing upon one of the knobs of the flower ornaments in the ce ^ 
compartment of the pedestal (e), it was fixed to a frame of iron, working 
parallel motion of a steam engine, and properly counterpoised, as will be n ^ 
stood from the section in fig. 14. By the side of the knob e , was another kn0 ^ 
connected with the supports under the scale-pans, and duly counterpoised, ‘ ^ 
without the intervention of strings or wheels ; the same motion, therefore, v JC 
the balance in action, served, when necessary, to close it from access ot air, by 1 ^ 
ing up the small window, and vice vers&. The central portion of the pedestal 
