1919] on Clock Escapements 451 



A new principle ayrs introduced into the gravity escapement by 

 Capt. Kater about the year 1840, and is described in vol. cxxx. of 

 the Phil. Trans. Fig. 3 is taken from Kater's paper, and shows 

 clearly the design. The gravity pieces are lifted alternately as in 

 Mudg-e's and Bloxam's constructions, but they do not themselves un- 

 lock the escapement, merely serving to upset the equilibrium of a 

 heavy piece (seen in the figure above the wheel), which does the un- 

 locking, but, owing to its high moment of inertia, gets slowly under 

 way and so unlocks the wheel only when the gravity piece then in 

 contact with the pendulum is no louger touching it. 



Verite produced a gravity escapement in which pivot friction was 

 got rid of, but this escapement had four little balls hanging from 

 four silk threads, and was somewhat delicate and complicated. 



It occurred to me some time ago that Kater's principle might be 

 applied in such a way that the pendulum should be entirely freed 

 from all friction whatever, while the impulses given to the pendulum 

 were exactly uniform. A full description of this escapement will be 

 found in Patent Office Specification No. 113,501, but it may be said, 

 very briefly, to consist in two little weights which rest alternately on 

 the two ends of a rocking frame having considerable moment of 

 inertia, and on two little upright stems at the ends of arms fastened 

 to the pendulum near its point of support. When the rocker is 

 horizontal, and the pendulum at rest and vertical, things are so ad- 

 justed that the weights are resting indifferently on both the pendulum 

 arms and the ends of the rocker. If, then, the pendulum is pushed 

 to one side, say the right, it carries the right-hand little weight up- 

 wards, relieving the rocker of its weight, and deposits on the opposite 

 end of the rocker the other little weight. This upsets the equilibrium 

 of the rocker, which commences to turn over, and so releases the 

 scape-wheel, which turns the rocker back rather beyond the horizontal 

 in the sense opposite to that of its last motion, so that when on its 

 return the pendulum again exchanges weights with the rocker, it 

 deposits the right-hand weight at a lower level than that at which it 

 was picked up. The escapement is simple, and a clock fitted with it 

 has given results which are encouraging. 



Before concluding, I must refer to a remarkable series of papers 

 which commenced last year to appear in the Proceedings of the Royal 

 Society of Edinburgh by Prof. R. A. Sampson, the Astronomer 

 Royal for Scotland. Prof. Sampson is, as all astronomers must be, 

 much interested in accurate timekeeping, and has experimented with 

 three diffei-ent clocks, having escapements which I must very briefly 

 describe. One is by Mr. Cottingham, and is essentially the same as 

 an escapement which the late Sir David Gill, then Astronomer Royal 

 at the Cape, had imagined. The pendulum is driven by a gravity 

 piece which, so long as it is in contact with the pendulum, by that 

 very contact completes an electric circuit which holds up an armature 

 against the poles of an electromagnet. This armature is itself the 



