122^ NEW BALANCE. 



often visible, so that the balance has nothing permanent in 

 its nature. New adjustment is necessary much oftener than 

 the instrument requires cleaning : but that adjustment is of 

 no duration ; for, as the pores are more torn than at first, 

 the balance becomes worse and worse, and at last quite 

 useless for what it is intended. 

 The new ba- I make use of the direct expansion of metals; for the 



lance has no fcars of my balance are independent of each other. They 

 welded sur- ^^^ connected only at the extremities, and the excess or 

 face. difference of the expansion of the two metals is communi- 



cated to the short ends of the two spring levers. Its dura- 

 bility can therefore no more be doubted than that of the 

 gridiron pendulum, where the direct expansion of metals 

 produces the desired eiTect. 

 Its weights The two globular weights were described in my last letter 



move nearly in as moving constantly in the same plane, which passes 

 through their centres and the axis of the balance; and I 

 should have added that, as to sense, they also move in the 

 same right-line which passes through the centres of the 

 globes, and cuts the axis of the balance at right angles ; for 

 the versed sine of a very small arch, or the difference be- 

 tween the radius and co-sine, is in this case a quantity so 

 small that it cannot be perceived ; and however we increase 

 or diminish the expansion of the balance, or whatever may 

 be the degree of temperature, it still retains this admirable 

 property, namely, that the two spherical weights move not 

 only in the same plane in a strict mathematical sense, but 

 also in the same right Jine in a physical one. This quality, 

 united with the direct motion of the brass bars, renders the 

 motion of the globes simple and uniform, and therefore the 

 effect (depending on such simple and direct causes) is 

 regular and- certain. 

 An6xiou8 vi- The common balance, when in motion, causes the weights 

 bration of thre ^^ ^ ^^ ^^ recede from the axis of the balance, and this 

 weights in the •' , ,. . . i . , i , n . 



common ba- flying off- will increase and dumnish with the arch of vibra- 

 lance. ^ioa in the balance : for, as there is nothing to brace the 



rim at the extremity of which the weight is suspended ; 

 as the arch of vibration increases, the weight and rim are 

 thrown outward as much as the centrifugal force of the 

 weight exceeds that of the elasticity of the rim. An4 

 ^s the arc of vibmtioa diminishes, and consequently the 



centrifugal 



