34 POPULAR LECTURES AND ADDRESSES, 



balance-wheel, the rim is composed of two metals, 

 the outer part brass, the inner part steel, and it 

 is cut into two halves, which are nearly semi- 

 circular, and are supported by one end attached 

 to one end of a stout diameter of the wheel, Fig. 9. 

 Weights are attached to the two semicircles in 



FIG. 9. Chronometer Balance Wheel. 



proper positions, to produce, as nearly as possible^ 

 the desired equality of period of vibration for 

 different temperatures, according to the following 

 principle : When the temperature is augmented, 

 the two halves of the rim, supported as they are on 

 two ends of one diameter, curve inwards from their 

 outer parts being brass (more expansible), and the 



