ON BALANCES. 59 



the attainment of this important object, the supporting 

 frame is furnished at each of its extremities with two steel 

 pins terminating in cones, and made to fit exactly into corre- 

 sponding conical holes in the plane bearings over the knife- 

 edges at the ends of the beam. These two conical holes are 

 in a line with, and on each side of the knife-edge. The lines 

 joining the points of the two pins and of the conical holes 

 are thus in a line normal to the axis of the beam, and all the 

 points are in the same horizontal plane when the balance is 

 at rest. As the movement of the supporting frame in a well- 

 constructed balance of precision is always in the same vertical 

 line, being guided by vertical rods fitted to cylindrical drilled 

 holes in the column of the balance, the knife-edges and their 

 bearings are thus always brought into contact in the same 

 relative positions. 



The arrangements of the knife-edges and of the supporting 

 frame may be seen in the kilogram balance constructed by 

 Oertling, now exhibited and illustrated by a large drawing. 

 (See Fig. 1.) To show the great importance of avoiding every 

 risk of a displacement of the knife-edges on their bearings 

 by the most exact agreement being maintained between the 

 position of the supporting frame and of the beam itself, it 

 should be pointed out that in this balance the conical pins 

 are a fixed part of the supporting frame, and must descend 

 and ascend in the same vertical line, whilst the points of 

 the conical holes, being movable with the beam, must move 

 in the line of the circumference of a circle, the radius of which 

 is the length of the axial line from the centre of motion of 

 the balance to these points. There is consequently a risk of 

 displacement from any such difference in the line of motion 

 of the pins and their conical holes, although the risk is 

 diminished in proportion to the length of the beam. Such 

 risk did not escape the observation of Captain Kater, and 

 in some balances made under his direction by Mr. Bate an 

 arrangement was made by means of two side rods hinged to 

 the central vertical rod, and of the proper radial length, to 

 give the same circular motion to the supporting pins as the 

 conical holes. You may see a similar contrivance in the 

 small Mendeleef balance now before you, which is exhibited 

 by me, No. 344a in the catalogue. (See Fig. 2.) 



16. Attention should here be called to the difference of 

 construction of these two balances of precision, each of them 



