156 



NATURE 



[April 24, 1919 



This table gives the losing rate for variations of 

 arc : — 



Semi arc D.iily loss Difference 



It must be remembered that these figures only relate 

 to a -free pendulum, and with some escapements the 

 errors introduced mask this result completely. 



Many attempts, some of great ingenuity, were 

 made to get better results from the verge, especially 

 as regarded the reduction of the arc, but they were 

 all superseded by the anchor, or recoil, escapement, 

 invented (most probably) by the celebrated Dr. Hooke, 

 and first made by William Clement in 1675. This is 

 the escapement still used in all common clocks, but 

 it has disadvantages which render it unsuitable 

 for high-class work. The train exercises great 

 "dominion," as it used to be called, over the pen- 

 dulum, and is assisting gravity the whole time, 

 hindering the rise of the pendulum and accelerating 

 its fall, so that T may be considerably diminished 

 when the train has been recently oiled without any 

 corresponding variation of a. 



But in 1715 George Graham, pupil of Tompion (both 

 of whom were so esteemed as to be accorded burial 

 in Westminster Abbey), made a most important 

 modification of the anchor. He removed most of the 

 flukes, leaving only a small sloping part near the 

 tip, by sliding along which the extremity of the scape- 

 wheel teeth could give the necessary impulse to the 

 pendulum. The rest of the fluke he fashioned so 

 that it should be a portion of a circle having its 

 centre on the axis of the crutch-arbor, thus entirely 

 preventing recoil of the movement, and, to a great 

 extent, releasing the pendulum from the "dominion" 

 of the train. During the time when the circular part 

 of the fluke is passing along the tooth of the scape- 

 wheel the motion of the train is entirely held up, 

 and it is neither doing work on the pendulum nor 

 having work done on it. The device is consequently 

 known as the "dead-beat." 



Numbers of escapements were devised after 

 Graham's invention, which, though differing much 

 from it in design, were, nevertheless, broadly speak- 

 ing, mechanical equivalents of it. Such were 

 Thiout's, V^rit^'s. Perron's, Leonhard's, Vulliamy's, 

 Robert's, Befthoud's, Lepaut^'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 func- 

 tion 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 construc- 

 tion was proposed by Huygens, who did so much for 

 the science of accurate timekeeoin^. The principle of 

 these " remontoirs," as they are called, is verv much 

 NO. 2582, VOL. 103] 



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 be 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? Whv 

 not simply relift the pallets and let them fall by 

 gravity on the pendulum ? " This question was 

 answered about 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 escapement 

 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. i ' shows Mudge's escapement, 

 and will be easily understood. 



The tooth marked i has just lifted the gravity piece 

 A'B', and is resting on the dead face. The pen- 

 dulum, moving to the right, Ms 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 



Fii;. I. KiG. 2. Fig. 3. 



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 I 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' is out of contact with 

 it, and so the action of the train cannot disturb 

 the pendulum except by the friction of the dead 

 faces. 



There is, however, a source of error to which 

 Mudge's escapement is liable which was sufficient 

 to condemn it. The driving power had to be ample, 

 and there was danger either that the gravity pieces 

 might be thrown clean off the wheel, allowing the 

 latter to race and destroying all timekeeping, or that, if 

 this complete "tripping" did not occur, they might, 

 at all events, be thrown a little too high, so that 

 the teeth of the scape-wheel, instead of resting in the 

 exact corner, as tooth i is seen to be doing, would 

 rest on the dead face nearer its extremity, and prob- 

 ably hold up the gravity piece, by friction, higher than 

 it should. This fault was called by Lord Grimthorpe 

 "approximate tripping," and if it occurred the con- 

 stancy of the maintenance would be lost. This might 

 probably have been cured by the use of a dashpot, 



