784 



HOROLOGY 



it is wound up ; a ratchet preventing its unwinding 

 without turning the wheel with it. The hands 

 have a separate train of wheels, called the dial 

 or motion train, between the front plate and the 

 dial. The arbor of the centre wheel, b, is pro- 

 duced to the dial, and on it is put the minute- 

 wheel, revolving once an hour, with a long socket 

 on which the minute-hand is fixed. Over this is 

 placed a larger wheel, the hour- wheel, I, revolving 

 in twelve hours, which is set in motion by the 

 pinion of a duplicate minute-wheel, m (and also 

 seen at h, fig. 6 ). The attachment of the minute- 

 wheel to the centre- wheel arbor is, by means of a 

 spring, enough to ensure the hands being carried 

 round with the clock, but not enough to prevent 

 the hands being turned, when necessary, by hand, 

 without disturbing the interior works. 



Striking-clocks have an additional train of 

 wheels with separate weight (or spring) for the 

 striking ; it will be described further on. 



Spring-clocks i.e. clocks having a coiled spring 

 as a motive power instead of a falling weight 

 have an arrangement of barrel and fusee chain 

 similar to that of the watch, to be afterwards 

 described. The spring is used when it is wished to 

 save space, as the necessary fall of a weight re- 

 quires a case deep enough to hold it, something 

 about 4 feet for an eight-day clock. Their size also 

 necessitates a short pendulum, which, of course, 

 does not indicate seconds. 



Previous to the invention of the pendulum, the 

 regulating apparatus was generally as shown in 

 fig. 2, which represents 

 part of De Vick's clock 

 already mentioned. The 

 teeth of the escape- 

 ment-wheel, I, acting 

 on the two pallets, h, i, 

 attached to the upright 

 spindle or arbor, KM, 

 to which is fixed the 

 balance, LL, gave to 

 the latter an alternate 

 or vibrating motion, 

 which Avas regulated 

 Fig. 2. by two small weights, 



m, TO. The further 



these weights were moved from the centre, the 

 more they retarded the movement ; and, by means 

 of numerous notches, their position could be 

 shifted till the proper speed was secured. 



The great epoch in the history of horology was 

 the introduction of the Pendulum (q.v.)'as a 

 regulating power. This has generally been attrib- 

 uted to Huygens, a Dutch philosopher, who was 

 undoubtedly the first to bring it into practical use 

 {1657). The fact of the actual invention, however, 

 is obscure, and Sir E. Beckett says : ' The first 

 pendulum clock was made for St Paul's church, in 

 Covent Garden, by Harris, a London clock-maker, 

 in 1621, though the credit of the invention was 

 claimed also by Huygens himself, and by Galileo's 

 son, and Avicenna, and the celebrated Dr Hooke.' 

 In adapting the pendulum to the clocks previously 

 existing Huygens had only to add a new wheel 

 and pinion to the movement, to enable him to 

 place the crown-wheel and spindle in a horizontal 

 instead of a perpendicular position, so that the 

 balance, instead of being horizontal as in De Vick's 

 clock, should be perpendicular and extended down- 

 wards, forming a pendulum at one end. 



The principle of construction adopted by Huygens, 

 from the peculiar action of the levers and spindle, 

 required a light pendulum and great arcs of oscilla- 

 tion ; and it was consequently said that ' Huy- 

 gens's clock governed the pendulum, whereas the 

 pendulum ought to govern the clock.' About ten 

 /ears afterwards the celebrated Dr Hooke invented 



an escapement which enabled a less maintaining 

 power to impel a heavier pendulum. The pendu- 

 lum, too, making smaller arcs of vibration, was 

 less resisted by the air, and therefore performed its 

 motion with greater regularity. This device is 

 called the anchor escapement. It was brought 

 by Hooke before the notice of the Royal Society 

 in 1666 ; and was practically introduced into 

 the art of clock-making by Clement, a London 

 clock-maker, in 1680, It is the escapement 

 still most usually employed in ordinary English 

 clocks. Fig. 3 represents the more modern 

 form of the 

 anchor or recoil 

 escapement : A, 

 its axis ; BC, the 

 pallets ; and D, 

 the escapement- 

 wheel revolving 

 in the direction 

 of the arrow. The 

 connection be- 

 tween the pendu- 

 lum and escape- 

 ment may be seen 

 in fig. 1. When Fig. 3. 



the pendulum 



swings to the right AC rises, and a tooth escapes 

 from C, while another falls on the outside of 

 B, and, owing to the form of the pallet B, the 

 train goes back during the remainder of the 

 swing. The same thing occurs on the pendulum's 

 return; the arm AB rises, a tooth escapes from 

 B, and another falls on the inside of C and backs 

 the wheelwork as before. As each of the thirty 

 teeth of the wheel thus acts twice on the pallets, 

 at B and again at C, it follows that a hand fixed 

 on its arbor will move forward ^th of a circle 

 with each vibration of the pendulum and mark 

 seconds on the dial. At each contact the onward 

 pressure of the wheel gives an impulse to the 

 pendulum, communicated through the crutch, 

 sufficient to counteract the retarding effects of 

 the resistance of the air and friction, which would 

 otherwise bring it to a standstill. The length of a 

 pendulum oscillating seconds is, for the latitude 

 of London, about 39 '14 inches. 



The defect of Hooke 's escapement is the re- 

 coil, and various modifications have been devised 

 to obviate this. The first and most successful was 

 made by George Graham, an English watch-maker, 

 in the beginning of 

 the 18th century, and 

 his improved form is 

 called the dead-beat 

 escapement ( fig. 4 ). 

 There the outer sur- 

 face of B and inner 

 surface of C are arcs of 

 circles whose centre is 

 A, and a little consid- 

 eration will show that 

 there can be no re- 

 coil. This escapement 

 is adopted in time- 

 keepers when great 

 accuracy is required. 



Many other escapements for clocks have been 

 devised ; but no one seems to have met with 

 general favour except a certain form of remontoire 

 or gravity escapement. The form of it shown in 

 fig. 5 is called the double three-legged escapement, 

 and was invented for the great clock at West- 

 minster, in 1854, by E. B. Denison (afterwards Sir 

 E. Beckett, Q.C.). In this clock the pendulum is 

 13 feet \ inch long, to vibrate in two seconds, 

 and its bob weighs 6 cwt. The escapement con- 

 sists of two gravity impulse pallets, AB and AC, 



Fig. 4. 



