HOROLOGY 



7*1 



: mi pivot* at A. The escape-wheel consist* 



Fig. 5. 



ot i wn t lin-r Ir^'i'il wheel)*, "'" ana / /, -| nan-il on 

 one arbor a little distance apart, witli three lifting 

 pins (or three- lea veil pinion) 

 fixed between them. The 

 three pin- are shown by the 

 three note in the centre of the 

 fig. The legs of the wheels 

 are generally arranged alter- 

 nately as in the fig. The 



n'lrts, with their arum 17 and 

 ie between the wheels ; at 

 C is a block to lock the legs 

 a fa-, and at B, on the other 

 side, and of course acting 

 in the reverse direction, is 

 another to lock the legs def. 

 The leg a is, in the fig., 

 locked on the block at C. 

 The pendulum, part of which 

 is removed to show the escape- 

 wheel, is supposed to be mov- 

 ing in the direction of the 

 arrow, and has received im- 

 pulse from the falling pallet 

 AB at t ; it is just beginning 

 to touch the other pallet at / , 

 which has been kept in position clear of the 

 pendulum by one of the centre pins bearing on 

 the arm g. The pendulum before turning again 

 moves the pallet AC just enough to allow the leg 

 a to escape from the locking-block at C ; the 

 wheel Hies round, impelled by the clock-weight, 

 till the leg / locks on the block at B ; by the same 

 movement the pin which is seen near the end of 

 the arm h pushes the pallet AB away from the 

 pendulum, which now gets impulse from the fall of 

 the pallet AC. This goes on at each side alter- 

 nately, the pallets being raised by the clock train, 

 the pendulum only unlocking them. To make 

 the motion go smoothly and prevent jar, a fly is 

 attached to the arbor of the escape-wheel by a 

 spring ; it is seen in the figure. As the height 

 to which the pallets are lifted is the same, how- 

 ever unequal the force communicated by the train 

 may be, the arc of vibration of the pendulum 

 remains constant, as the weight of the arm and 

 the distance it falls are always the same. 



The gradual perfection of the clock required 

 also improvements in the regulating power which 

 finally resulted in the compensation pendulum (see 

 PENDULUM ). 



The improvements in the escapement and the 

 pendulum bring the mechanical perfection of the 

 clock, as a time-keeping instrument, to the point 

 which it has attained at the present day. But 

 the art of horology would be incomplete unless 

 there were some standard, independent of indi- 

 vidual mechanical contrivances, by which the 

 errors of each may be corrected. This standard 

 is supplied by observatories, and the methods by 

 which time is determined l>elong to the details of 

 practical astronomy. There are in most parts of 

 the United Kingdom now sufficient opportunities 

 of setting clocks by a communication more or less 

 direct with these establishments. When these 

 are not to be had the sun-dial may still be used 

 with advantage as a means of approximation to 

 the correct time. The time which a clock ought 

 to mark is mean time, the definition of which will 

 be found in the articles DAY and TIME. The mean 

 tint < at any place depends on the longitude. Sup- 

 posing a clock to le set to ( In'onwirli moan time, a 

 clock keeping mean time of any place will be 4 

 minutes faster for every degree of* longitude east of 

 ^reenwich, and 4 minutes slower for every degree 

 v jst. Since the introduction of railways, clocks 

 are usually set within (Jreat Britain to Greenwich 

 258 



mean time. In the United State*, where the 

 extent of country make* it unadvixahle to use the 

 nifaii timr of one meridian, four standard nifridiiuiH 

 were ndopt'd in Iss.'t vix. 7-V, 90*. 105, 120" went 

 of (irtwnwirh. Clock* Hhowing ' Eastern,' 'Cen- 

 tral,' '.Mountain,' ami ' Pacific ' time are therefore 

 respectively five, BIX, seven, or eight bourn glower 

 than (ireenwich mean time. 



For the more ready transmission of correct 

 time to the public there is at Greenwich Obser- 

 vatory, as well as some others, a ball which U 

 dropped by means of electricity precisely at one 

 o'clock. Several attempts have been made to 

 keep the public clocks of a town in perfect agree- 

 ment with the mean-time clock in the observatory. 

 One means of effecting this was by an electric 

 connection and a modification of Bain's electric 

 pendulum (1840), by Mr R. L. Jones of Chester 

 (1857), on the suggestion of Mr Hartnup, the 

 astronomer of the Liverpool Observatory. For a 

 description, see article ELECTRIC CLOCKS in Vol. 

 IV., page 253. A clock in the castle of Edinburgh, 

 by whose mechanism a gun is fired precisely at one 

 o'clock every day, is controlled by the mean-time 

 clock in the observatory on the Calton Hill. 



It is not known when the alarum or when the 

 striking-mechanism of the clock was first applied. 

 The first striking-clock probably announced the 

 hour by a single blow, as they still do in churches 

 to avoid noise. During the 17th century there 

 existed a great taste for striking-clocks, and hence 

 a great variety of them. Several of Tompion's 

 (died 1713) clocks not only struck the quarters on 

 eight bells, but also the hour after each quarter. 



The striking part of a clock (see fig. 6, which 

 shows an English striking-clock by Ellicott, taken 



Fig. 6. 



from the engraving in Moinet's work) is a peculiar 

 and intricate piece of mechanism. The motive 

 power is a weight used in a similar manner to 

 that in the time-keeping train shown in fig. 1. In 

 fig. 6, a, b, c, d, e are the striking-train ; e is a fly 

 which acts as a drag to prevent tne striking being 

 too rapid. The striking-train is kept in a normal 

 condition of rest by the tumbler or gathering pallet 



