150 LECTURE XVII. 



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

 invented 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 

 inclined plane for a short time only in the middle of each vibration ; so 

 that a change of the sustaining power scarcely produces a sensible derange- 

 ment of the isochronism ; for which ever way we turn the key of a hori- 

 zontal 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 obser- 

 vatory at Greenwich 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 cylin- 

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

 terfere in a slight degree with the time of vibration, especially by the 

 irregularities to which they are liable. Since friction is always increased 

 by an increase of pressure, the effect of any addition to the sustaining 

 force must tend in some degree to retard the vibrations, even if the friction 

 be somewhat less increased than the force propelling the balance. In order 

 to obviate this retardation, the surfaces on which the teeth rest, have some- 

 times been so formed as to create a slight recoil ; but this construction does 

 not appear to have been very successful in practice. The friction may, 

 however, be considerably diminished by the duplex scapement, apparently 

 so called from the double series of teeth employed. The teeth of the more 

 prominent series are detained on a cylinder so small as to be unfit for re- 

 ceiving an impulse from them, the balance is therefore impelled by the other 

 series of teeth, acting on a pallet at a greater distance from its axis. The 

 French have sometimes employed a construction somewhat similar, which 

 they call the comma scapement, the teeth first resting on a small arch of 

 repose, and then impelling the curved surface of a pallet extending to a 

 considerable distance beyond it. In both these cases the single pallet, 

 which is impelled by a tooth of a simple form, requires less labour in the 

 execution than a number of larger teeth, each of which is to be finished 

 with great accuracy : but watches on these constructions, especially those 

 with the comma scapement, are too liable to be stopped by any sudden 

 motion, although the duplex scapement begins to be frequently employed 

 for pocket timekeepers. (Plate XVI. Fig. 205.) 



Mr. Harrison avoided all friction on the pallet, by connecting it with 

 the pendulum by means of a slender spring, so flexible as to follow the 

 motion of the scape wheel to a sufficient extent without sliding on its 

 teeth. But the construction which is most usually employed where the 

 greatest accuracy is required, is the detached scapement: in which the 

 teeth of the scape wheel always rest on a detent, excepting a short interval 

 when it is unlocked in order to impel the pallets. Mr. Mudget employed a 

 detached scapement actuated by a subsidiary spring, of which the force is 



* See Nich. Journal, 4to, ii. 49. 



f Mudge on a Scapement, 1763. On a Timekeeper, 4to, 1799. 



