394 Royal Society : — 



correspond to the seconds' hand of the clock) completes the instru- 

 ment ; for minute- and hour-hands are a superfluity in an astrono- 

 mical clock. 



In various trials which I have made since the year 1865, when 

 this plan of escapement first occurred 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 escapement- wheel 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 one second's pendulum), or some multiple 

 of it. Thus the pendulum may execute one or more complete pe- 

 riods of vibration without being touched by the escapement. 



I look forward to carrying the principle of the governed motion 

 for the escapement-shaft much further than hitherto, and adjusting 

 it to gain only y^j per cent, on the pendulum ; and then I shall 

 probably arrange that each pallet of the escapement be touched only 

 once a minute (and the counter may be dispensed with). The only 

 other point of detail which I need mention at present is that the pal- 

 lets have been, in all my trials, attached to the bottom of the pen- 

 dulum, projecting below it, in order that satisfactory action with a 

 very small arc of vibration (not more on each side than jj^ of the 

 radius, or 1 centimetre for the second's pendulum) may be secured. 



My trials were rendered practically abortive from 1865 until a 

 few months ago by the difficulty of obtaining a satisfactory governor 

 for the uniform motion of the escapement-shaft ; this difficulty is 

 quite overcome in the pendulum-governor, which I now proceed to 

 describe. 



Imagine a pendulum with single-tooth escapement mounted on a 

 collar loose on the escapement-shaft just as described above — the 

 shaft, however, being vertical in this case. A square- threaded screw is 

 cut on the upper quarter of the length of the shaft, this being the part 

 of it on which the collar works, and a pin fixed to the collar projects 

 inwards to the furrow of the screw, so that, if the collar is turned 

 relatively to the shaft, it will be carried along, as the nut of a screw, 

 but with less friction than an ordinary nut. The main escapement- 

 shaft just described is mounted vertically. The lower screw and 

 long nut collar, three-quarters of the length of the escapement-shaft, 

 are surrounded by a tube which, by wheelwork, is carried round 

 about five per cent, faster than the central shaft. This outer shaft, 

 by means of friction produced by the pressure of proper springs, 

 carries the nut collar round along with it, except when the escape- 

 ment-tooth is stopped by either of the pallets attached to the pen- 

 dulum. A stiff cross piece (like the head of a T), projecting each 

 way from the top of the tubular shaft, carries, hanging down from 

 it, the governing masses of a centrifugal friction governor. These 

 masses are drawn towards the axis by springs, the inner ends of 

 which are acted on by the nut collar, so that the higher or the lower 

 the latter is in its range, the springs pull the masses inwards with 

 less or more force. A fixed metal ring coaxial with the main shaft 



