SirW. Thomson on a new Astronomical Clock. 393 



June 10. — Lieut. -General Sabine, President, in the Chair. 



The following communications were read : — 



" On a new Astronomical Clock, and a Pendulum-governor for 

 Uniform Motion." By Sir William Thomson, LL.l)., F.R.S. 



It seems strange that the dead-beat escapement should still hold 

 its place in the astronomical clock, when its geometrical transforma- 

 tion, the cylinder escapement of the same inventor, Graham, only 

 survives in Geneva watches of the cheaper class. For better portable 

 time-keepers, it has been altered (through the rack-and-pinion move- 

 ment) into the detached lever, which has proved much more accurate. 

 If it is possible to make astronomical clocks go better than at present 

 by merely giving them a better escapement, it is quite certain that 

 one on the same principle as the detached lever, or as the ship-chro- 

 nometer escapement, would improve their time-keeping. 



But the inaccuracies hitherto tolerated in astronomical clocks may 

 be due more to the faultiness of the mercury compensation pendulum, 

 and of the mode in which it is hung, and of the instability of the sup- 

 porting clock-case or framework, than to imperfection of the escape- 

 ment and the greatness of the arc of vibration which it requires ; 

 therefore it would be wrong to expect confidently much improvement 

 in the time-keeping merely from improvement of the escapement. I 

 have therefore endeavoured to improve both the compensation for 

 change of temperature in the pendulum, and the mode of its support, 

 in a clock which I have recently made with an escapement on a new 

 principle, in which the simplicity of the dead-beat escapement of 

 Graham is retained, while its great defect, the stopping of the whole 

 train of wheels by pressure of a tooth upon a surface moving with 

 the pendulum, is remedied. 



Imagine the escapement-wheel of a common dead-beat clock to be 

 mounted on a collar fitting easily upon a shaft, instead of being rigidly 

 attached to it. Let friction be properly applied between the shaft 

 and the collar, so that the wheel shall be carried round by the shaft un- 

 less resisted by a force exceeding some small definite amount, and let a 

 governor giving uniform motion be applied to the train of wheel-work 

 connected with this shaft, and so adjusted that, when the escapement- 

 wheel is unresisted, it will move faster by a small percentage than it 

 ought to move when the clock is keeping time properly. Now let 

 the escapement- wheel, thus mounted and carried round, act upon 

 the escapement, just as it does in the ordinary clock. It will keep 

 the pendulum vibrating, and will, just as in the ordinary clock, be 

 held back every time it touches the escapement during the interval 

 required to set it right again from having gone too fast during the 

 preceding interval of motion. But in the ordinary clock the interval 

 of rest is considerable, generally greater than the interval of motion. 

 In the new clock it is equal to a small fraction of the interval of mo- 

 tion : -y-Jy in the clock as now working, but to be reduced probably 

 to something much smaller yet. The simplest appliance to count 

 the turns of this escapement-wheel (a worm, for instance, working 

 upon a wheel with thirty teeth, carrying a hand round, which will 



Phil. Mag. S.4. Vol. 38. No. 256. Nov. 1869. 2 D 



