NA VIGA TION. 33 



another. The common hinge or joint is open in 

 its centre ; the actual central point from which the 

 three edges of the three rulers radiate is marked 

 by a pointer attached to one of the three limbs. 



27. THE CHRONOMETER. For the second mode 

 of navigation, the chronometer is the only other 

 instrument I have to mention. The object of the 

 chronometer is to show Greenwich time all over the 

 world. It is merely a watch adapted to go with 

 the greatest possible accuracy. The main feature 

 of the chronometer, besides very fine finish in all its 

 parts, and an escapement movement of peculiar 

 excellence, is that the vibrating balance-wheel is 

 " compensated " for variation of temperature. An 

 ordinary balance-wheel, with continuous rim of 

 one metal, vibrates more slowly at high than at 

 low temperatures, because the hair-spring has less 

 of elastic stiffness, and because the balance-wheel 

 is larger, at higher temperatures ; but a small part 

 only of the whole difference in time-keeping de- 

 pends on the last-mentioned cause. About twelve- 

 thirteenths of it is due to the diminished elastic 

 stiffness of the hair-spring. In the compensated 



VOL. III. D 



