220 
Accurate Balances. 
[July, 
each knife edge, which subjected them to continual and unequal wear ; this he has 
obviated by making the prisms bear on their whole length: Captain Kater thus 
describes the construction of some splendid balances made on this principle, for the 
standarding of the weights of Great Britain, duplicates of which were sent out to 
the new Calcutta Mint by the Hon’ble Court of Directors last year. 
“ The central knife-edge of the beam I am describing, was made 6 inches, and 
the two others 5 inches long. They were triangular prisms with equal sides 
of three-fourths of an inch, very carefully finished, and the edges ultimately 
formed to an angle of 120°. 
“ Each knife-edge was screwed to a thick plate of brass, the surfaces in contact 
having been previously ground together ; and these plates were screwed to the 
beam, the knife-edges being placed in the same plane, and as nearly equidistant 
and parallel to each other as could be done by construction. 
“ The support upon which the central knife-edge rested throughout its whole 
length, was formed of a plate of polished hard steel, screwed to a block of cast 
iron. This block was passed through the opening before mentioned in the centre 
of the beam, and properly attached to a frame of cast iron. 
“The stirrups to which the scales were hooked rested upon plates of polished steel, 
to which they were attached, and the under surfaces of which were formed by 
careful grinding into cylindrical segments. These were in contact with the knife- 
edges their whole length, and were known to be in their proper position by the 
correspondence of their extremities with those of the knife-edges. A well imagin- 
ed contrivance was applied by Mr. Bate for raising the beam when loaded, in order 
to prevent unnecessary wear of the knife-edge ; and for the purpose of adjusting 
the place of the centre of gravity, when the beam was loaded with the weight re- 
quired to be determined, a screw carrying a movable ball projected vertically from 
the middle of the beam. 
The performance of this balance fully equalled my expectations. With two 
hundred and fifty pounds in each scale, the addition of a single grain occasioned 
an immediate variation in the index of one-twentieth of an inch, the radius being 
fifty inches.” 
In the smallest balance of the set, the end supports ,}iave a spring adjustment, 
which will readily be understood by reference to fig. 17. In the balances just de- 
scribed the knife-edges are of steel, turniugupon agate planes; but for this country 
it might bare been better to have constructed the whole of agate, rounding off the 
edges slightly, to obviate the danger of their chipping under heavy pressure. 
3. The next point of importance in making a balance is the maintenance of h s 
good qualities, by relieving it from shocks and friction, as far as possible, both 
diu ing and after use. T. his object has been usually attained by lifting the beam 
off of its edges, by a sliding fork, and bringing up supports under the scale beam ■ 
Mr. Bate has introduced an important improvement in the mode ot effect big 
the leliet of the edges, which will be rendered intelligible by reference to the 
perspective view ot one of his balances, in fig. 6’ : a strong triangular frame, ha' 
a a eitical motion communicated by a lever and spiral segment below the stum , 
lifts off the hearings of the two end suspensions, so as to free the beam entirel) 
from connection with the scale pans, during the process of loading and unloading 
them ; by a further turn of the handle below, the beam itself is raised «> 
its fulcrum by two Y slides, which catch corresponding brass pins P ,otlU 
in & horn the beam above the pi'ism of steel, as is seen enlarged in figs- t an( ^ 
In causing the beam to descend, the supports are first disengaged, so that 1 ® 
weights can be nearly counterpoised by the suspension from the brass pics, be 
