304 Description of Mr Wither spooiCs 



the balance spring, is accurately proportional to the distance of 

 evagation ; but in clocks, being obtained by the circular motion 

 of the pendulum, it is only approximately so. The oscillations 

 of the balance of a watch, however unequal in extent, are thus 

 performed in equal times ; while those of the clock pendulum 

 deviate a little from perfect isochronism. 



The balance or pendulum is the real time-measurer, the train 

 of wheels being attached only for the purposes of counting the 

 vibrations, and of supplying the slight loss of momentum which 

 attends the motion of every piece of mechanism. For these pur- 

 poses, it it absolutely necessary that the motion of the train be con- 

 nected with that of the pendulum ; the apparatus for making this 

 connection is called the escapement, and on the construction of 

 this escapement the accuracy of the time-keeper mainly depends. 



The earliest contrivances of this nature were called recoil es- 

 capements, because that, during part of the oscillation, the whole 

 train, and along with it the maintaining force, is driven back- 

 wards by the momentum of the vibrating body. During the 

 entire oscillation, the train is in connection with the pendulum, 

 the impulse teeth rubbing upon the backs of the pallets. Now, 

 to obtain accuracy in going, the momentum of the pendulum 

 ought to be gradually generated and extinguished by the sole 

 action of gravity ; whereas, with this escapement it is affected 

 by the friction and by the resistance of the maintaining force ; 

 the changes, then, to which, from the gradual thickening of the 

 oil, these disturbing forces are liable must occasion considerable 

 errors in the movement. 



The first amelioration of the common recoil escapement was 

 made by forming the rubbing parts of the pallets cylindric, so 

 that the train might merely be detained by them without being 

 subjected to a recoil. But this still left the motion of the pen- 

 dulum exposed to the effects of changes in the viscidity of the 

 oil, or in the smoothness of the rubbing surfaces. 



The great imperfection of the dead-beat escapement is, that, 

 while the train is communicating no impulse to the pendulum, 

 it continues to retard its motion by pressing upon the back of 

 the pallet, thus creating additional work for itself. This evil 

 has been nearly removed by the contrivance of detached escape- 

 ments. In these the train is prevented, by means of a detent, from 



