Management of delicate Balances. 351 



is neatly effected by heating alternately each portion in the 

 flame of a spirit lamp. On the upper part of the centre a 

 piece of brass is soldered, a thin side of which embraces the 

 frame on each side, a portion of which is left for that pur- 

 pose, and through this a triangular hole is worked for the 

 central knife-edge bearing piece. The knife-edge is a trian- 

 gular bar of steel \ inch in length and \ inch in the side, 

 which is hardened by being heated to a bright red heat in a 

 crucible filled with charcoal powder, and then removed by a 

 pair of bamboo forceps, which do not injure the edge, and 

 immersing it slowly end-ways in cold water. 



It is worked nearly true by the file and polishing on a flat 

 surface of brass with powdered corundrum previous to har- 

 dening ; after which the charcoal powder having prevented 

 the oxidization, it is tempered in boiling water, and a little fur- 

 ther polishing removes any slight bending which may have 

 taken place. 



The knife-edge goes loosely into the brass piece, by which 



it is easily removed when not in use, and can be rolled up 



in oiled rag to prevent its rusting. When in use, it is fixed in 



its place by two short projecting arms attached to the central 



brass piece at the upper edge of the triangular hole. Through 



these a small screw is worked about t th inch in diameter, the 



10 J 



flat points of which press upon the upper surface of the knife- 

 edge, and bind it down tight in the socket. The screw holes 

 in these arms serve also to receive two pointed screws, which 

 upon occasions, such as adjusting weights, can be used in- 

 stead of the knife-edge, by which all chance of changing the 

 position of the bearings is obviated. 



The points of suspension for the scale pan are the points of 

 two screws 1th inch diameter, about \ an inch apart, which 

 are carried by a cross piece of brass soldered on to a square 

 projecting piece at the end of the beam. The screws have 

 capstern heads projecting below the brass piece, by which they 

 are turned when adjustment is required ; they are hardened 



