l 5 8 



THE POPULAR SCIENCE MONTHLY. 



pulse to the pendulum ; the spring at the end of the detent immedi- 

 ately locks the wheel again, and the pendulum swings on freely to the 

 left. When the pendulum swings to the right, the light spring at the 



end of the detent lets it pass 

 without unlocking the wheel. 

 The right-hand pallet is only in- 

 tended to catch the wheel in 

 case of accident and forms no 

 essential part of the escapement. 

 Thus, it will be seen, the pendu- 

 lum is quite free except during 

 a part of every alternate second, 

 when it releases the escapement 

 and receives an impulse ; the 

 seconds-hand, attached to the 

 escape - w T heel, moves only once 

 every two seconds. 



The most important source of 

 error in the running of a fine 

 clock is the change in the length 

 of the pendulum due to change 

 of temperature. Two methods 

 suggest themselves of eliminat- 

 ing this error : 1. To put the 

 clock where it will not be sub- 

 ject to changes of temperature. 

 2. To counteract the effect of 

 changes of temperature. To this 

 end various kinds of pendulums 

 have been devised, notably the 

 mercurial and gridiron forms, 

 which are known under the gen- 

 eral name of " compensating pen- 

 dulums." At Greenwich the two 

 methods are combined to insure 

 complete success. The clock is 

 placed in the magnetic basement 

 of the observatory, where the 

 temperature is as nearly uniform 

 as possible, and apparatus is pro- 

 vided to annul the effect of any 

 change of temperature which 

 might occur. 



Tests made with a mercurial 

 pendulum disclosed the fact that the steel rod responded more quickly 

 than the mercury to a change of temperature, and that consequently 



