Measurement of Weight 



51 



to its ends in which the bodies to be weighed may be 

 placed. In a simple chemical balance, as shown in 

 Fig. 20, the beam is made of brass and turns upon 

 one edge of a polished triangular prism of agate a 

 hard stone which is supported upon a flat surface 

 of the same material when the balance is in use. Near 

 the ends of the beam are two similar " knife-edges " 

 of agate, with their edges directed upwards, parallel 

 to, and equidistant from, the middle knife-edge. Upon 

 these are hung the pans of the balance by means of 



E L 



Fig. 20. 



stirrups, the upper inside surfaces of which are also 

 made of agate. Depending from the middle of the 

 beam in front of the stout central support is a long 

 pointer, the lower end of which moves over a graduated 

 scale attached to the wooden base of the balance. In 

 front of the base is a milled brass head or a small 

 brass handle which may be turned from left to right 

 and vice versa. This works the " arrestment " of the 

 balance, a mechanical arrangement whereby the knife- 

 edges on the beam are kept out of contact with the 

 centre agate plane and the inner portions of the stirrups, 



42 



