HOROLOGY 



787 



in 1 is fixed, und of course the spring in also fully 

 \viMiinl. At this point t he spring is st rongest ; ami, 

 polling upon the narrow end >i' tin- fusee, Im-^ tin- 

 Ira-t leverage. As it gradually unwinds, and at 

 the sunn' tinir heroines weaker, the leverage, owing 

 to the shapr of the funee, l>econieH in exactly the 

 saim- ratio greater, and thus the power on tin- 

 machinery u equalised till the whole chain in 

 unwound. Tin- .spring is wound up by the squared 

 arhur, m, of the fusee through an opening in the 

 inside case ; the arbor of the spring-barrel being of 

 course fixed. An ingenious stop arrangement pre- 

 vents the iMissibility of damage by over-winding. 

 Tin- maiii-.|pi iii^ is a thin ribbon of finely tempered 

 steel (fig. 8). The inner 

 end is hooked on to a 

 catch on the arbor of 

 the barrel round which 

 it is coiled, and the outer 

 end to a catch on the 

 inside of the rim of the 

 barrel. In the Ameri- 

 can watches, now so common, the fusee is 

 dispensed with, and the great wheel is on the 

 barrel and directly give,s the motion. In recent 

 years this form is also used in almost all keyless 

 watches. The verge escapement shown in fig. 

 7 is exactly the same as that shown in De 

 Vick's clock (fig. 2). Two pallets, hj t, moved 

 alternately in opposite directions by the teeth 

 of the escapement-wheel, cause a vibrating motion 

 in the balance k, which is steadied and regu- 

 lated by the balance-spring I. The balance and 

 spring are shown in plan in fig. 9 : a is the 

 balance and b the spring, which 

 is arranged spirally. The inner 

 end is fixed to the staff of the 

 balance, the outer to a stud c, 

 fixed to the watch-plate. Its 

 beautifully delicate motion may 

 be observed in any watch, as 

 all watches have the spiral 

 spring except chronometers, 

 which have a cylindrically 

 coiled spring instead. The 

 length of the balance-spring 

 in proportion to the weight of the balance 

 is an important factor in regularity of motion, 

 and for minute adjustment an instrument, d, e, 

 called a regulator is attached to it. Two curb- 

 pins at '/ enclose the outer coil of the spring, and, 

 in the case of the watch going fast, a movement 

 to the left lengthens the spring and retards the 

 speed in proportion. For too slow a motion a 

 movement to the right will shorten the spring and 

 quicken it. 



The principle involved in the clock-pendulum 

 and watch-balance alike is that by their regularity 

 of movement they shall keep the mechanism from 

 going either too fast or too slow, and that in return 

 the mechanism shall give repeated impulses suffi- 

 cient to keep them perpetually in motion. 



As the vertical 

 or verge escapement, 

 owing to the recoil 

 of the escape-wheel 

 and other causes, is 

 not to be depended on 

 for very great accur- 

 acy, attempts were 

 immediately made 

 Fig. 10. after the invention 



of the balance-spring 



to devise some form of escapement which would 

 give better results. Hooke, Huygens, Hautefeuille, 

 and Tompion introduced improvements, but the 

 first to succeed was made by George Graham, the 

 inventor of the dead-beat escapement in clocks. 



Fig. 9. 



Tliis is called tin- h<,rir.i,nt(d or cylinder eacapement 

 (lig. 10). It wan introduced in tin; beginning of 

 tin- istli century, and it is still the eacapeini-nt 

 u-i-'l in many foreign wutcluw. The impulse in 

 xiveu to a hollow cut in the cylindrical axw of tin- 

 balance by teeth of a peculiar form projecting 

 from a horizontal crown-wheel. 



Other forms of escapement in high estimation 

 are the lever, the duplex, and the chronometer 

 ' spring-detent ' escapement. The lever eucape- 

 inent (invented about 1770 by Thomas Mudge) 

 is the dead-beat escapement (see fig. 4) adapted 

 to the altered conditions of a watch. rig. 

 11 shows the form used in most modern Kn^'li-ii 

 watches. The pallets, P, P, are fixed to a 

 lever, A (pivoted at F), and there is an impulse 



Sin, B (usually a piece of ruby), set in a small 

 isc, C (called the roller), on the axis of the balance. 



Fig. 11. 



The ruby pin works into a notch at the end of the 

 lever, and the pin and notch are so adjusted that 

 when a tooth of the escape-wheel D leaves the 

 pallet the pin slips out of the notch, and the 

 balance is detached from the lever during the 

 remainder of its swing ; whence the name detached 

 lever escapement, originally applied to this arrange- 

 ment. On the balance returning, the pin again 

 enters the notch, moving the lever just enough to 

 allow the tooth next in order to escape from the 

 dead face of the pallet on to the impulse face ; then 

 the escape-wheel acts upon the lever and balance ; 

 the tooth escapes, and another drops upon the dead 

 face of the pallet, the pin at the same time passing 

 out of the notch in the other direction, leaving the 

 balance again free. This arrangement is found to 

 give great accuracy and steadiness of performance. 

 A safety pin, E, on the lever, prevents the wheel 

 being unlocked, except when the impulse-pin is in 

 the notch of the lever. Two banking-pins, GG, 

 keep the motion of the lever within the desired 

 limits. 



In the duplex escapement (invented about 1780) 

 the escape-wheel has two sets of teeth, hence the 

 name. One set, something like the lever-wheel 

 (fig. 11), lock the wheel bv pressing on the balance 

 staff, and the other, standing up from the side of 

 the rim of the wheel, give impulse to the balance. 

 It is rarely used now. 



The chronometer spring-detent escapement was 

 invented in principle by Le Roy about 1765, and 

 perfected by Earnshaw (who also invented the 

 cylindrical balance-spring) and Arnold about 1780. 

 It is shown in fig. 12 ; a is the escape-wheel, 

 which has fifteen teeth ; b, the impulse-roller, 

 fixed on the same staff as the balance; c, the 

 impulse-pallet ; d, discharge-pallet ; e, locking- 

 pallet all the pallets are of ruby or sapphire ; /. 

 the blade of the detent fixed at X- by its spring q;- 

 and /<, the gold-spring. In the fig. a tooth of MM 

 escape-wheel is caught on the locking-pallet ; the 

 discharge-pallet (carried round by the roller in the 

 direction of the arrow), by pressing on the end of 

 the gold-spring, which in turn presses on the horn 

 of i In- detent /. bends the detent enough to 

 allow the tooth to escape from the pallet. The 



