120 



OUTLINES OF NATURAL PHILOSOPHY. 



A more refined artifice for producing a uniform mo- 

 tion in a machine impelled by gravity, is employed 

 in the pendulum clock. 



In that machine, a pendulum, by forcing the end of 

 a lever at each vibration, against a tooth of a 

 wheel, put in motion by a weight, prevents the ac- 

 celeration of that weight, and at the same time re- 

 ceives an impulse just sufficient to continue its mo- 

 tion. The exact adjustment of these forces, is one 

 of the nicest problems in practical Mechanics. 



In a watch, where the moving power is the elasticity 

 of a spring, the uniformity of the motion is, on si- 

 milar principles, produced by the vibrations of a ba- 

 lance. 



189. AVhen the momentum of the power appli- 

 ed to a machine, is greater than the momentum of 

 the resistance, the machine is put in motion, and 

 if the power be one which acts more forcibly on 

 bodies at rest than on bodies in motion, its action is 

 of course lessened, and the acceleration of the ma- 

 chine in the second instant is less than in the 

 first. Thus the effect of the accelerating force 

 continues to diminish till such time as it becomes 

 equal to the effect of the resistance, or till the ve- 

 locity generated every instant by the one force, 

 be just equal to the velocity destroyed every in- 

 stant by the other. The acceleration then ceases, 

 and the motion of the machine becomes uniform. 



