Accurate Balances. 
219 
1831 .] 
When the outer knife edges or supports are above the fulcrum, as in fig. 2, the 
slightest deviation from the horizontal position will cause the descending arm to 
preponderate, until counterbalanced by the displacement of the centre of gravity ; 
since the lever of measurement is estimated on the horizontal line, or is as the 
cosine of the angle made therewith by either arm : when the supports are below the 
fulcrum, as in fig. 3, it is obvious that the ascending arm becomes elongated, and 
consequently the angular movement, with a given addition of weight, will be pro- 
portionally diminished. The Hindustani balance is a sample ot this eironeous 
construction, and with all the appearance ol great delicacy, is in reality much in 
ferior to our commonest apothecary’s scales. It consists of a very slight steel 
beam, (fig. 5,) perforated at the centre and the two ends ; through these holes are 
introduced the wires upon which it turns : now even supposing the three holes to 
be in the same line, the fulcrum must rest on the upper y while the supports turn 
on the lower surfaces ; which are the conditions of fig. 3. 
When the edges of the fulcrum and supports are circular, as must be the case, 
more or less, in all large beams, it should be remembered, that the condition of 
equal levers requires that the horizontal line should pass through the centres of 
these curves, (figs. A. and B.) provided that the fulcrum rests upon a plane surface : 
if this surface is curved also, it takes much from the accuracy of weighments, the 
descending lever being shortened by the oblique contact of the edge rolling thereon . 
thus, by shifting a loaded common beam with the finger to either side of its 
support, a preponderance may easily be produced, of very notable extent. 
1. In delicate balances, and of these alone we now treat, it is necessary that the 
whole materials should be as light as is consistent with proper stiffness. Steel, 
therefore, seems the fittest material for their construction every where but in Ben- 
gal, where other inconveniences occur from the use of this metal. Platina is unfit 
for the purpose, or certainly not so fit as an alloy ol silver and coppei, which pos- 
sesses less than half its weight. A hollow cone of this substance may be made 
lighter, and as strong -as a steel beam : brass has the same advantages. Some 
makers prefer an open truss beam, and this form is certainly preferable for \ery 
heavy loads, but for delicate work it has an air of needless power, and a want of 
compactness. Captain Kater thought lightness so essential a property, that he 
constructed his standard beam of a mahogany plank, 70 inches long, 22 wide, and 
2 thick, tapering from the middle to the extremities. Such an instrument 
might be better adapted to temporary purposes than to constant use. 
2. The knife edges or fulcra should be exactly parallel to each other, and at rig it 
angles to the axis : in olden balances the fulcrum was generally a square cross 
Piece rivetted in to the beam, and filed under its two extremities to an edge ; this 
defect is now avoided, by grinding the fulcrum pieces into triangular prisms, and 
setting them through the beam by means of a cylindrical shoulder-piece, turned on 
the lathe. An exemplification of this structure is given in fig. 15, of a Wollaston 
balance ; (also in fig. 7.) it presents the further advantage of an adjustment verti- 
Cl % for the centres of gravity and collimation, and laterally , by means 
c «Pstan screws, for the equalization of the arms. The construction of the end 
supports does not require so much nicety : the same figure points out a neat and 
sample construction adopted by Mr. Robinson : the prism is fitted into a divu e 
piece at the end of the beam, which acts as a strong spring compres&ei or 
°pened by two small screws ; by the oblique position of the cut, a minute motion 
Pwards is also obtained for the adjustment of collimation. 
Mr. Robinson has the merit of introducing another improvement : in a miner 
Mai >ces, the weight was sustained by short portions, at the extremities only, of 
