ON TIMEKEEPERS. 149 



escape for a time from the action of the wheelwork, and in general, the 

 more independent its motion is rendered the better is the effect of the 

 machine. The simplest forms in common use are the crutch scapement 

 for a clock, and the pallets with a vertical wheel for a watch ; the dead 

 beat scapement, and the cylinder with a horizontal wheel, are improve- 

 ments on these ; and the detached scapement is a still further refinement. 



The crutch scapement, called by the French the anchor scapement, is 

 an arch in the plane of the scape wheel, and parallel to that in which the 

 pendulum vibrates, supporting at each extremity a pallet, of which the 

 face is a plane, and which is impelled in its turn by the teeth of the scape 

 wheel. The faces are so inclined that the pallets are alternately forced, by 

 the action of the teeth, to retire from the centre of the wheel : and great 

 care is taken in making the teeth exactly at equal distances, so that they 

 may fall regularly on the pallet, immediately after the disengagement of 

 the teeth on the other side from the opposite pallet. (Plate XVI. Fig. 201.) 



In the common watch, the axis of the balance is parallel to the plane of 

 the scape wheel, which is a contrate or crown wheel, and the flat pallets 

 are fixed on the axis of the balance at the opposite parts of the circum- 

 ference of the scape wheel. (Plate XVI. Fig. 202.) 



In both these cases the impulse given to one pallet carries the opposite 

 pallet with some force against the approaching tooth, and drives the wheel 

 a little backwards with a visible recoil. Here the sustaining power, being 

 applied principally at the extremities of the vibrations, disturbs their iso- 

 chronism or the equality of the times in which they are performed, by 

 partially increasing the force. We may recollect that, in order that all 

 vibrations, of whatever magnitude may be performed in equal times, the 

 force must be exactly proportional to the distance from a given point, 

 consequently if an additional force be applied near the extremities of the 

 vibration only, the longer vibrations will occupy less time than the shorter ; 

 and we may observe that, by adding to the force of the spring of a common 

 watch, with the key, we may accelerate its motion, at the same time that 

 the angular magnitude of the vibration is increased. The motion of the 

 balance also, being slowest at the extremities of its vibration where the 

 sustaining force is applied, is more affected by the inequalities of this force 

 than if it were subjected to its 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 applica- 

 tion of springs to balances : it requires a considerable extent of motion in 

 the balance, and cannot therefore well be applied to clocks with such pen- 

 dulums 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 suc- 

 ceeded very well. 



