BALANCE-WHEEL. 



uniform. It is on that account that the moving power must be 

 controlled and governed by some expedient, by which it shall be 

 rendered uniform. 



How the combination of a pendulum and escapement-wheel 

 accomplishes this has been already explained. But this expedient 

 requires that the timepiece to which it is applied shall be sta- 

 tionary ; the slightest disturbance of its position would derange 

 the mutual action of the pendulum and the escapement- wheel, and 

 would either stop the movement, or permanently derange the 

 mechanism. It is evident that a pendulum is not only inap- 

 plicable to all forms of pocket timepiece, but that it cannot even 

 be used for marine purposes, the disturbances incidental to which 

 would be quite incompatible with the regularity of its action. 



The expedient which has been substituted for it with complete 

 success in all such cases is the balance-wheel. 



This is a wheel, like a small fly-wheel, having a heavy rim con- 

 nected with the centre by three or more light arms, as shown at 

 ABC, in fig. 16. Under, and parallel to it, is placed a spring 

 resembling in form the mainspring, but much finer and lighter, 

 and having much less force. 



This spring is formed of ex- Fi s- 16 - 



tremely fine and highly tem- 

 pered steel wire, so fine that 

 it is sometimes called a hair- 

 spring. One extremity of this 

 spring is attached to the 

 axis of the balance wheel, and 

 the other to any convenient 

 fixed point in the watch. The spring is so constructed that when 

 at rest it has a certain spiral form, to which it has a tendency to 

 return when drawn from it on the one side or the other. If we 

 suppose it, therefore, to be drawn aside from this position of rest 

 and disengaged, it will return to it, but on arriving at it, having 

 acquired by the elasticity a certain velocity, it will swing past it 

 to the other side, to a distance nearly as far from its position 

 of rest as that to which it had been originally drawn on 

 the other side. It will then swing back, and will thus oscillate 

 on the one side and the other of the position of rest, in 

 the same manner exactly as that in which a pendulum swings 

 on the one side or the other of the vertical line which is its 

 position of rest. 



34. The balance-wheel thus connected with a spiral spring, 

 like the pendulum, is isochronous, that is, it performs all vibra- 

 tions long and short in the same time. It will be recollected 

 that this property of the pendulum depends on the fact that the 



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