ox TIMEKEEPERS. IQS 



action through an equal space in the middle of the vibration. Yet a good 

 clock on this construction may keep time without an error of the ten 

 thousandth part of the whole, and a watch within a two thousandth. In the 

 common watch scapement, there is little friction, for the force acts almost 

 perpendicularly on the pallet; it appears to have been the oldest scapement, 

 and was employed before the application of springs to balances: it requires a 

 considerable extent of^ motion in the balance, and cannot therefore well be 

 applied to clocks with such pendulums as vibrate in small arcs. The crutch 

 scapement, on the contrary, cannot be applied immediately to a vibration in 

 a very large arc; but by the interposition of a lever with a roller, or of a part 

 of a wheel with a pinion, it may be adapted to the balance of a watch; and 

 some watches thus constructed by Emery, Letherland, and others, appear to 

 have succeeded very well. , 



To avoid the inconveniences of the recoiling scapements, Mr. Graham in- 

 vented or introduced the dead beat for the clock, and the cylinder for the 

 watch. In both of these, the tooth of the scape wheel rests, during the 

 greater part of the vibration, on a cylindrical surface, and acts on the in- 

 clined plane for a short time only, in the middle of each vibration; so that a 

 change of the sustaining power scarcely produces a sensible derangement of 

 the isochronism ; for which ever way we turn the key of a horizontal watch, 

 as long as it continues to go, the frequency of its vibrations is scarcely 

 affected. A good horizontal watch will keep time within about a ten 

 thousandth part, especially if a little oil be frequently applied to it, or if the 

 cylinder be made of a ruby: and the timekeeper in the observatory at Green- 

 wich, with a dead beat scapement, made by Graham, varies from true time 

 only two parts in a million. (Plate XVI. Fig. 203, 204.) 



Still, however, the friction of the teeth of the scape wheel on the cylinder 

 or pallet, and the tenacity of the oil, where it is employed, may interfere in 

 a slight degree with the time of vibration, especially by the irregularities to 

 which they are liable. If the friction were perfectly uniform, it would 

 scarcely disturb the isochronism, but friction is always increased by an in- 

 crease of pressure ; hence, therefore, the effect of any addition to the sus- 

 taining force must tend in some degree to retard the vibrations ; and to ob- 

 viate this, the surfaces, on which the teeth rest, have sometimes been so 



