TRANSACTIONS OF THE SECTIONS. 51 



Imagine a pendulum with single tootli escapement mounted on a collar loose on 

 the escapement-shaft just as described above, the shaft being vertical in this case 

 also. 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 escapement-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. Below the screw and long nut-collar 

 three-quarters of the length of the escapement-shaft is surrounded by a tube which 

 by wheel-work is carried round about 5 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 escapement-tooth ia 

 stopped by either of the pallets attached to the peudidum. A stiff cross-piece 

 (like the head of a T), projecting each way from the top of the tubular shaft, carries, 

 tanging 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 -svith the main shaft holds the governing masses in when their centrifugal 

 forces exceed the forces of the springs, and resists the motion by forces of friction 

 increasing approximately in simple proportion to the excess of the speed above that 

 which just balances the forces of the springs. As long as the escapement-tooth 

 is imresisted the nut-collar is carried round with the quicker motion of the outer 

 tubular shaft, and so it screws iqnvards, diminishing the force of the springs. Once 

 every semiperiod of the pendulum it is held back by either pallet, and the nut-col- 

 lar screws down as much as it rose during the preceding interval of freedom, when 

 the action is regular; and the central or main escapement-shaft turns in the same 

 period as the tooth, being the period of the pendulum. If through increase or 

 diminution of the driving-power, or diminution or increase of the coefficient of 

 friction between the governing masses and the ring on which they press, the shaft 

 tends to turn faster or slower, the nut-collar works its way down or up the screw, 

 until the governor is again regulated, and gives the same speed in the altered cir- 

 cumstances. It is easy to arrange that a large amount of regulating power shall 

 be implied in a single turn of the nut-collar relatively to the central shaft, and yet 

 that the periodic application and removal of about yV of this amount in the half- 

 period of the pendulum shall cause but a very small periodic variation in the speed. 

 The latter important condition is secured by the great moment of inertia of the 

 governing masses themselves round the main shaft. My communication to the 

 Royal Society ended as follows : — 



"I hope after a few months trial to be able to present a satisfactory report of the 

 performance of the clock now completed according to the principles explained 

 above. As many of the details of execution may become modified after practical 

 trial, it is unnecessary that I should describe them minutely at present. Its general 

 appearance, and the arrangement of its characteristic parts, may be understood 

 from the photograph now laid before the Society." 



I am sorry to say that the hope here expressed has not hitherto been realized. 

 Year after year passed producing only more or less of radical reform in various me- 

 chanical details of the governor and of the fine movement, until about six months 

 ago, when, for the first time, I had all except the pendulums in approximately 

 satisfactory condition. By that time I had discovered that my choice of zinc and 

 platinum for the temperature compensation and lead for the weight of the pendu- 

 lums was a rnistake. I had fallen into it about ten years ago through being in- 

 formed that in Kussia the gridiron pendulum had been reverted to because of the 

 difficulty of getting equality of temperature throughout the length of the pendu- 

 lum; and without stopping to perceive that the right way to deal with this diffi- 

 culty was to face it and take means of securing practical equality of temperature 

 throughout the length of the penduliun (which it is obvious may be done by 

 simple enough appliances), I devised a pendulum in which the compensation is 

 produced by a stiff tube of zinc and a platinum wire placed nearly parallel each to the 

 other throughout the length of the pendulum ; and the two pendulums of the clock 

 shown to the British Association were constructed on this plan. Now it is clear 



