390 POPULAR LECTURES AND ADDRESSES. 



count the turns of this escapement-wheel (a worm, 

 for instance, working upon a wheel with thirty 

 teeth, carrying a hand round, which will correspond 

 to the seconds' hand of the clock) completes the 

 instrument ; but if desired, minute and hour-hands, 

 though a superfluity in an astronomical clock, can 

 easily be added. 



In various trials which I have made since the 

 year 1865, when this plan of escapement first oc- 

 curred to me, I have used several different forms, 

 all answering to the preceding description, although 

 differing widely in their geometrical and mechanical 

 characters. In all of them the cscapcmcnt-whccl 

 is reduced to a single tooth or arm, to diminish as 

 much as possible the moment of inertia of the mass 

 stopped by the pendulum. This arm revolves in 

 the period of the pendulum (two seconds for a one 

 second's pendulum), or some multiple of it. Thus 

 the pendulum may execute one or more complete 

 periods of vibration without being touched by the 

 escapement. 



I look forward to carrying the principle of the 

 governed motion for the escapement-shaft much 



