COMMON THINGS CLOCKS AND WATCHES. 



spiral, will make the fusee revolve in the direction indicated by 

 the arrows. This tension at the commencement acts upon the- 

 highest and smallest groove of the fusee. As the chain is 

 gradually discharged from the fusee to the barrel, the tension is 

 gradually decreased by reason of the relaxation of the spring, and 

 at the same time the cSain acts upon a larger and larger groove of 

 the fusee. In this way the tension of the chain is continually 

 decreased, and the radius of the groove on which it acts is con- 

 tinually increased, until the entire action has passed from the 

 fusee to the barrel, and the clock goes down. 



Now the power of the chain to impart a motion of revolution 

 to the fusee depends on two conditions ; first the force of its 

 tension, and secondly the leverage by which this tension acts 

 upon the fusee. This leverage is in fact the semi-diameter of the 

 groove, upon which the chain is coiled at the point where it 

 passes from the fusee to the barrel. Without much mechanical 

 knowledge it will be easy to perceive that it requires less force to 

 turn a wheel or barrel if the force be applied at a great distance 

 from the axle than if it be applied at a small distance from it. 

 Upon this principle generalised, it follows that the power of the 

 tension of the chain to impart revolution to the fusee is augmented 

 in exactly the same proportion as the magnitude of the groove on 

 which it acts is increased. 



The form given to the fusee is such that as the chain is gra- 

 dually discharged from it, the diameter of the groove on which 

 it acts increases in exactly the same proportion as that in which 

 the tension of the chain decreases. It follows, therefore, that the 

 power of the chain upon the fusee gains exactly as much by the 

 increase of its leverage as it loses by the decrease of its tension, 

 and consequently it remains invariable. 



Complete compensation is therefore obtained by this beautiful 

 and simple expedient, and a variable force is thus made to produce 

 an invariable effect. It may be useful to state that this is only a 

 particular application of a mechanical principle of great generality. 

 In all cases whatever, the varying energy of a moving power may 

 be equalised by interposing between it and the object to be moved 

 some mechanism, by which the leverage, whether simple or com- 

 plex, through which its force is transmitted, shall vary in the 

 exact inverse proportion of the variation of the power, increasing 

 as the intensity of the power is decreased, and decreasing as the 

 intensity of the power is increased. 



33. Whatever be the moving power,- whether it be a weight or 



mainspring, it would, if not controlled and regulated, impart to the 



hands a motion more or less accelerated, and therefore unsuitable 



to the measurement of time, which requires a motion rigorously 



26 



