1919] on Clock Escapements 449 



it is neither doing work on the pendulum nor having work done 

 on it. The device is consequently known as the " dead-beat." 



Xumbers of escapements were devised after Graham's invention, 

 which, though differing much from it in design, were, neverthe- 

 less, broadly speaking, mechanical equivalents of it. Such were 

 Thiout's, Verite's, Perron's, Leonhard's, YuUiamy's, Robert's, 

 Berthoud's, Lepaute's, and Brocot's. 



The designer of a turret-clock, however, always has in mind 

 the serious variations in the force of the train caused by wind or 

 snow on the hands, as well as by the thickening and drying of 

 the oil on the bearings and the cutting and wearing of the pivots 

 and of the teeth of the wheels and pinions. It was, therefore, 

 long ago recognised that the proper function of the clock-train 

 was not to drive the pendulum, but to record the number of its 

 swings — that is, to tell the time— and to keep wound a smaller 

 clock which should be independent of these disturbances, and 

 could be made very simple, and even reduced to one wheel, if 

 often enough rewound. This construction was proposed by 

 Huygens, who did so much for the science of accurate time- 

 keeping. The principle of these " remontoirs," as they are called, 

 is very much the same in all. Some rewind a little weight, others 

 keep a spring wound, but in every case, directly or indirectly, 

 the pendulum has to unlock the rewinding mechanism by means 

 of some device which is itself an escapement, and this cannot l)e 

 effected without some friction. 



From the train-remontoir it is an easy step to the next great 

 improvement. The question naturally arises : " Why rewind the 

 train in the middle ? Why not simply relift the pallets and let 

 them fall by gravity on the pendulum ? " This question was 

 answered a.bout the year 1716, when Alexander Gumming produced 

 the first of the series of gravity escapements which have done so 

 much to make the accurate turret-clock a possibility. His escape- 

 ment is rather complicated and has several points where there is 

 friction, and very soon after it was greatly simplified and improved 

 by Thomas Mudge, a pupil of Graham's. Fig. 1 shows Mudge's 

 escapement, and will be easily understood. 



The tooth marked 1 has just lifted the gravity piece A'B', 

 and is resting on the dead face. The pendulum, moving to the right, 

 is just about to lift the gravity piece, causing the dead face to slide 

 along the tooth until it is clear of it. The wheel is then free to turn 

 further, and the tooth marked 2 lifts the other gravity piece AB in a 

 similar way. When the pendulum has attained its maximum 

 elongation to the right (carrying A'B' with it) and begins to return, 

 the pallet on A'B' falls midway between teeth 1 and 3, thus falling 

 rather farther than it rose, the balance of work done on the pendulum 

 serving to maintain the latter in motion against the resistances. 

 Each gravity piece is lifted by the wheel at a time when the pendulum 



