Management of delicate Balances. 353 



The index needle is 2 inches long, and is fixed into a hole 



drilled in the projecting end of the beam, or to which the 



cross brass piece is soldered, which bears the scale-pan points. 



The weight of the beam with the 3000 grains, hob, and the 



scale pan is altogether 5,070 grains. 



The case is 18^ inches long, 15| high, and 12 inches wide, 

 with a drawer at the bottom to receive the beam, &c. when 

 not in use. Frames with single plates of glass are fitted 

 into the back and front, that at the back being fixed, while 

 the front one turns upon the pivots at the upper edge, by 

 which it is raised to admit the hand under the lower edge for 

 the management of the instrument, and it can be supported 

 in the raised position by a brass prop on the left side, which 

 catches into a set of pins at different heights as required. 



The centre bearing for the knife-edge is a brass pillar, 

 about 9 inches high, which is screwed to the bottom of the 

 case. The summit of this is divided to let in the beam, and 

 is surmounted by two planes of hardened steel, one inch long 

 and | an inch wide, which are worked down perfectly level to 

 the same plane by grinding, and four little pieces of brass 

 screwed on to the sides to prevent the knife-edge from 

 slipping off. 



The beam is lifted off the centre support when not requir- 

 ed by means of two pillar supports, which are fixed to the 

 bottom of the case, and which are hollow and have in them 

 two sliding rods with forked tops which receive the beam, 

 and these are raised by means of a circular nut on the top of 

 the pillar, which works into a double threaded screw cut into 

 the sliding rod. 



The vibration scale for the index is fixed by screws to the 

 left side of the case with the zero point as nearly as possible 

 the height of the centre-bearing planes. It is divided into 

 degrees to a 7 inch radius, and includes an arc of 16°. 



The scale-pan is borne upon the points of suspension by a 

 brass stirrup, in the flat part of which two holes are cut to 



