Application of the Pendulum to Time-Keepers. 251 



at the lower extremity with a weight in the form of a lens, so 

 placed as meet with as little resistance as possible from the air. 

 The axis also or point of suspension is fitted to have very little 

 friction. Connected with the pendulum, is a train of wheels and 

 pinions, the teeth and leaves of which are so adapted to each 

 other, that the motions correspond to the several divisions of 

 time, and their axes carry indexes that show by the arcs they 

 describe, the hours, minutes, and seconds. Around the axis at 

 one extremity of this train of wheels, is wound a cord bearing a 

 weight, that would put the whole system in rapid motion, but for 

 the appendage to the pendulum CFD at the other extremity of the 

 train of wheels, which, while the pendulum is at rest, effectually 

 prevents all motion. But if the pendulum be made to vibrate, 

 it will suffer one tooth to pass or escape at each vibration, while 

 at the same time the impulse of the teeth upon the arms FC, FD, 

 is so adjusted, by increasing or diminishing the weight, as just to 

 overcome the friction and the resistance of the air, and thus to 

 keep up the motion, while the action of the weight continues. 

 The contrivance by which the train of wheels is connected with 

 the pendulum, is called the escapement. 



373. On account of the constancy of gravity the oscillations 

 of the pendulum, other things being the same, must be equal or 

 of the same duration. There are, however, several causes that 

 tend to disturb this isochronism. (1 .) The air is subject to changes 

 of density, on account of which the arcs of vibration will some- 

 times be longer and sometimes shorter, while the maintaining 

 power remains the same.* But if these changes are noted, or if 

 the arcs of vibration are noted, the deviation from perfect regu- 

 larity can be calculated, and allowance made accordingly.! It 



*As the air becomes more dense the pendulum is more resisted 

 and would seem to be retarded, but the arc of vibration being dimin- 

 ished, the clock goes faster, so that one of these causes tends to coun- 343. 

 teract the other. In like manner, when the motion of the axis and 

 wheels is obstructed by dust or want of oil, the impulse of the weight 

 communicated to the pendulum is diminished, the arcs of vibration 

 are reduced, and hence there is a tendency to increase its rate of 

 going. 



f If the pendulum could be made to move in the arc of a cycloid, 

 it will be perceived from what has been said of this curve, that all 353< 



