ON TIMEKEEPERS. 151 



scarcely liable to any variation ; the detent being unlocked by the motion 

 of the balance. Mr. Haley* has refined still further on this construction, 

 by causing the subsidiary spring to unlock the wheel in its return, so that 

 the balance is relieved from this action, which may sometimes produce a 

 slight irregularity. These constructions are, however, much too delicate 

 for common pocket watches. In a clock, Mr. Gumming has employed a 

 detached scapement, in which a lever is raised to a certain height by each 

 tooth of the scape wheel, and acts immediately on the pendulum in its 

 descent in the middle of the vibration. The scape wheel is unlocked by 

 the pendulum during its ascent, and a variation of the pressure may, there- 

 fore, produce a very slight inequality in the motion of the pendulum. Mr. 

 Nicholson has attempted to remove this cause of error, by a construction 

 in which the scape wheel only assists the pendulum in raising the lever ; 

 but it depends on the degree of force applied, to determine what part of 

 the weight the scape wheel shall sustain ; this scapement cannot, therefore, 

 by any means be considered as detached. It is, however, easy to remove 

 the defect of Mr. Gumming' s scapement, if it can be called a defect, by a 

 method similar to that which Mr. Haley has applied to watches ; each 

 tooth of the wheel being unlocked by the descent of the lever on the op- 

 posite side, at the moment that it ceases to act on the pendulum, and 

 remaining inactive until the pendulum meets it. (Plate XVI. Fig. 206, 207.) 



The detents of the scapements of Mudge and Gumming are parts of the 

 pallet, but in the timekeepers now commonly made by Arnold, Earnshaw, 

 and others, the tooth is detained by a pallet or pin projecting from a lever, 

 the point of which is forced back by the balance, at the moment that the 

 pallet presents itself to another of the teeth. Mr. Arnold employs an 

 epicycloidal tooth, acting on a single point of the pallet ;t Mr. Earnshaw 

 makes a flat surface of the tooth first act on the point of the pallet, and 

 then the point of the tooth on a flat surface of the pallet.^ In other 

 respects there is little difference in these scapements ; and both the artists 

 have been judged worthy of a public reward for their success. (Plate 

 XVI. Fig. 208, 209.) 



The last of the three principal objects, which require the attention of the 

 watchmaker, is to employ a pendulum or balance of which the vibrations 

 are in their nature perfectly isochronous. For this purpose the weight of 

 the pendulum ought to move in a cycloidal arc, but the difficulty of pro- 

 ducing such a motion in practice is much greater than the advantage derived 

 from it, and a circular vibration, confined to a small arc, is sufficiently 

 isochronous for all practical purposes. The error of such a vibration is 

 nearly proportional to the square of the arc described by the pendulum, 

 and amounts to a second and a half in a day of 24 hours, for a single degree 

 on each side the point of rest ; so that a pendulum keeping true time in 

 an arc of three degrees, would gain 13i seconds if the arc were very much 



* Haley's Patent Timekeeper, Repertory of Arts, vi. 145. 



f Explanation of Mr. Arnold's Timekeeper. Questions proposed by the Board 

 of Long, relative to the same, 4to, 1804-5. 



J Explanation of Mr. Eamshaw's Timekeeper. Questions proposed by the 

 Board of Long, relative to the same, 4 to, 1804-5. 



