feet, and the wheel A turns from A toward B. Meanwhile the pendulum M 

 oscillates to the other side, and the pallet K falls under a tooth of the wheel 

 A, and checks for a moment its further motion. On the returning vibration, 

 the pallet K becomes again disengaged, and allows the tooth of the wheel to 

 escape, and by the influence of the weight W another tooth passes be- 

 fore the motion of the wheel A is again checked by the interposition of the 

 pallet I. 



From this explanation it will appear that, in two vibrations of the pendulum, 

 one tooth of the wheel A passes the pallet I, and therefore, if the wheel A be 

 furnished with 30 teeth, it will be allowed to make one revolution during 60 

 vibrations of the pendulum. If, therefore, the pendulum be regulated so as 

 to vibrate seconds, this wheel will revolve once in a minute. From the ac- 

 tion of the pallets in checking the motion of the wheel A, and allowing its 

 teeth alternately to escape, this has been called the escapement wheel ; 

 and the wheel and pallets together are generally called the escapement, or 

 J scapement. 



We have already explained that, by reason of the friction on the points 

 of support, and other causes, the swing of the pendulum would gradually 

 diminish, and its vibration at length cease. This, however, is prevented 

 by the action of the teeth of the 'scapement wheel upon the pallets, which 

 is just sufficient to communicate that quantity of force to the pendulum which 

 is necessary to counteract the retarding effects, and to maintain its mo- 

 tion. It thus appears that, although the effect of the gravity of the weight 

 W in giving motion to the machine is at intervals suspended, yet this part of 

 the force is not lost, being, during these intervals, employed in giving to the 

 pendulum all that motion which it would lose by the resistances to which it is 

 inevitably exposed. 



In stationary clocks, and in other cases in which the bulk of the machine 

 is not an objection, a descending weight is used as the moving power. But in 

 watches and portable chronometers, this would be attended with evident incon- 

 venience. In such cases, a spiral spring, called the main-spring, is the mov- 



