1831 .] 
Accurate Balances . 
221 
tlie real knife-edge is brought to bear. This improvement is of the greatest im- 
portance to the preservation of the good qualities of a balance for large weights ; 
anil there is nothing difficult in its adaptation to the ordinary commercial pur- 
poses in a more simple form. One amendment may still be suggested to the makers ; 
namely, that the false supports or pins should be situated in the prolongation of 
the true edges, to prevent the rubbing which must take place, when the relief is 
not quite vertically directed : nay, it would be better to increase the length of the 
prisms themselves, and relieve upon the outer extremities of them, using the re- 
served part of the edge only for final adjustment of the weight. 
I do not think it necessary to enter into a minute detail of the construction of 
the several parts of these very effective balances, because it may be found in the 
Cabinet Cyclopedia, vol. v. p. 286, or in the Phil. Trans, for 1 826. And the general 
form will be understood from the accompanying plate XI. figs. 7, 8, 9. The 
beams are made of cast bronze, with feather edges. 
The plan of making the relief of the fulcrum upon a portion of the edge itself, 
had suggested itself in the construction of an accurate assay balance, of which I 
have taken the liberty to trouble you with a representation in Plate XII. As 
there are a number of practical conveniences introduced in this instrument, a 
short description may not be without interest to those who have constant use for 
a balance of exceeding delicacy : that of which I am speaking will measure one 
ten thousandth of a grain, with 100 grains in the scales, which is equal to the one 
millionth part of the whole weight. 
1 . — Of the Balance itself. 
I he beam, (figure 13,) is made of a double cone of silver with one-fourtli copper, 
beaten hard, and very thin : it is twelve inches long, and weighs less than a steel 
beam of only half the section ; the centre block is perforated to admit a turned 
shoulder-piece, (fig. 12,) containing a prismatic knife-edge of agate, which is 
protected by a thin silver elastic jacket, fitting closely over its upper surface. 
I he agate prism is two and half inches long : it was ground and polished until 
the line of light on the edge was no longer visible under the microscope ; from 
being worked by the hand on the hones, it acquired a very small degree of curva- 
ture towards the centre, (fig. 13,) of which advantage was afterwards taken 1 , 
the length of the prism is of great service in preventing any lateral motion on the 
bearings, but its principal object was to allow room for three different supports, so 
that the same beam should have three degrees of sensibility, according to the pur- 
pose for which it might be required. The mode of effecting this object is repre- 
sented in fig. i 3 . w h ere anc j C c , are the three pairs of supports, all made 
°f polished agate. 
^he first, or common supports, cc', were made with concave surfaces, as most 
t0 <iU‘nient for ordinary use, in preventing the displacement of the beam; they 
' Vere Cached to the upper stage, and bore upon the part of the knife-edge nearest 
' the beam. The next pair bb' were fitted upon a moveable crutch, (fig. 11 .) cap- 
c °f rising through the opening in the brass stage, just sufficiently to lift the 
11 1 rum off of the fixed -supports cc. The crutch was worked by a wfire passing 
‘rough one of the brass pillars of the stage, and connected by a lever and rod with 
^ 1 of the flower ornaments in the corner pedestal of the case, as may be traced in 
e sect ‘on, fig. 14 . the surface of the bearings bb' was only slightly curved, and from 
lr a Pplication to the part of the prism outside of the first, the position of the ful- 
tlif ent ^ s *he prism were bevilled off to a point, so as to allow no contact with 
u . ° Il ^ ll ' n 8 > walls of the support; and this point ought not to be neglected by 
ers, as the want of it is a source of much trouble in many excellent balances. 
