194- LECTURE XVII. 



dent 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 improvements 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 pen- 

 dulum 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 ac- 

 tion of the teeth, to retire from the centre of the wheel : and great care i& 

 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. £01.) 



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 circumference 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 ap- 

 plied principally at the extremities of the vibrations, disturbs their isochronism, 

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

 creasing 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 subjectetl to its 



9 



