ON THE MOTION'S 01' CONNECTED BODIES. 91 



Where a uniformly accelerating force, like that of gravitation, is employed 

 in machines, it might often be of advantage to regulate its operation, so that 

 it might act nearly in the same manner as the forces that we have been con- 

 sidering ; at first with greater intensity, and afterwards with sufficient power 

 to'sustain the equilibrium, and overcome the friction only. This might be done, 

 by means of a spiral barrel, like the fusee of a watch ; and a similar modifica- 

 tion has sometimes been applied, by causing the ascending weight, when it 

 -arrives near the place of its destination, to act on a counterpoise, which resists 

 it with a force continually increasing, by the operation of a barrel of the same 

 kind, so as to prevent the effect of the shock which too rapid a motion would 

 occasion. 



On the whole, we may conclude, that on account of the limited velocity 

 which is usually admissible in the operation of machines, a very small por- 

 tion of the moving force is expended in producing momentum ; the velocity 

 of 3 miles an hour, would be generated in a heavy body, descending by its 

 own weight, in one seventh of a second, and a very short time is generally 

 sufficient for obtaining as rapid a motion as the machine or the nature of the 

 force will allow; and when this has been effected, the whole force is employed 

 in maintaining the equilibrium, and overcoming the resistance : so that the 

 common opinion, which has probably been formed without entering minutely 

 into the consideration of the subject, and which appears, when first we examine 

 its foundation with accuracy, to lead to material errors, is in great measure jus- 

 tified by a more profound investigation. 



To seek for a source of motion in the construction of a machine, betrays a 

 gross ignorance of the principles on which all machines operate. The only 

 interest that we can take in the projects which have been tried for procuring a 

 perpetual motion, must arise from the opportunity that they afford us to ob- 

 serve the weakness of human reason; to see a man spending whole years in the 

 pursuit of an object, which a week's application to sober philosophy might have 

 convinced him was unattainable. The most satisfactory confutation of the 

 notion of the possibility of a perpetual motion, is derived from the considera- 

 tion of the properties of the centre of gravity: we have only to examine whe- 

 ther it will begin to descend or to ascend, when the machine moves, or whe- 

 ther it will remain at rest. If it be so placed, that it must either remain at 



