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Sir W. Thomson on a New Astronomical Clock. [June 10, 



hours there is a comparative absence of these phenomena. So much is 

 this the case that for the two years investigated I have not succeeded in 

 finding a single example of a peak or hollow, suitable for this research, 

 between the hours of 6 and 7 p.m., or between those of 9 and 10 p.m. 



I- forbear to make further remarks on this subject, but hope in a short 

 time to extend the investigation up to the present date, and to bring the 

 results before this Society. 



VI. " On a new Astronomical Clock, and a Pendulum Governor for 

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

 Received June 10, 1869. 



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

 in the astronomical clock, when its geometrical transformation, 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 movement) 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-chronometer 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 supporting 

 clock-case or framework, than to imperfection of the escapement 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 endea- 

 voured 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 unless 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 



