ON BALANCES. 57 



of hardened and polished steel ; the line of the knife-edge 

 is also in a horizontal plane at right angles to the axis of 

 the beam. The whole length of the knife-edge ought con- 

 sequently to be in contact with the plane of the fulcrum at 

 every point. This knife-edge is at the lower edge of a steel 

 prism, the section of which is an equilateral triangle ; and the 

 edge is ground with a small facet on each side so as to in- 

 crease the angle at the edge from 60 to 120. Captain Kater, 

 who was an eminent member of the Eoyal Society fifty years 

 ago, and took the most active part, as a member of the Stan- 

 dards Commission, in superintending the construction of the 

 imperial standard weights and measures which were legalised 

 in 1825, considered the angle of 120 to be practically the 

 best form for the knife-edge of a balance of precision, and the 

 balances of the Standards Department have since been ad- 

 justed to this angle. It requires great care in a skilled work- 

 man to adjust the knife-edges, as the excellence of the action 

 of balances depends very much upon them. The small 

 facets on each side of the knife-edge ought to be exactly 

 rectangular. 



Whilst the central knife-edge is at the lowest part of the 

 prism, the two knife-edges at the ends of the beam have, on 

 the contrary, their edges uppermost, and you will see that the 

 tendency of this arrangement is to bring the centre of gravity 

 of the balance a little below the centre of motion. The pans 

 are suspended from these two knife-edges by means of agate 

 or steel planes bearing upon them. 



In order to preserve the nice adjustment of the knife-edges, 

 they are never allowed to be in contact with their bearings, 

 except when weighings are actually being made. At all other 

 times the beam and pans are supported upon a frame attached 

 to the column of the balance, and movable in a vertical 

 direction upon it. "When the balance is required to be put 

 in action, the support is very gradually lowered by means of 

 a lever handle, and the knife-edges are brought upon their 

 bearings, so that the balance is left free to act. As soon as 

 the weighing is completed the supporting frame is again 

 raised, and the knife-edges are thus lifted from their bearings. 



15. The chief cause of discordances in the results of suc- 

 cessive series of weighings with a balance of precision, which 

 it is necessary to guard against, is the risk of the knife-edges 

 not being brought again to exactly the same place on their 



