374 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK iv. 



ten degrees Fahrenhejt of rise in the temperature ; a watch or chrono- 

 meter with a plain uncompensated balance will lose one minute a day 

 for the same difference. 



The cause of this is, that in the case of a watch or chronometer, a 

 change of temperature, in addition to increasing or diminishing the 

 diameter of the balance-wheel, produces a very great alteration in the 

 elasticity of the balance-spring. The error arising from this latter 

 cause is at least five times as great as the error arising from alteration 

 in the diameter of the balance. 



.Fir, 257. Compensated balance. 



The method of compensation employed is the following. The 

 rims B B of the balance wheel (see Fig. 257) are composed of laminae 

 of brass and steel fastened together, the brass being upon the outside 

 of the rim, and the steel upon the inside. With any increase of 

 temperature, the brass endeavours to expand faster than the steel, but 

 as it is rigidly, throughout its entire length, fastened to the steel 

 (having in fact been melted on to it) it cannot well do this : the only 

 way it can manage matters is by bending in the rim a little ; it does 

 this by increasing the length of the outside of the rim, as compared 

 with the inside. 



