Balance, Steelyard, fa. 161 



Of the Balance, Steelyard, #c. 



256. A balance is a lever of the first kind in which the fulcrum Fig.142. 

 or axis is in the middle between the points of application of 

 the forces or weights. It is apparent that with such an instru- 

 ment, a state of equilibrium must indicate an equality in the 

 weights to be compared. 



There are several particulars to be attended to in the con- 

 struction of a good balance, (1.) The axis should be above and 

 not too far above the centre of gravity of the two arms or beam ; 

 for if it pass through this centre of gravity, the beam, when load- 

 ed with equal weights, or not loaded at all, will remain at rest 

 in any position whatever; and it is intended that a horizontal 

 position shall indicate equal weights. If the axis be below 

 the centre of gravity of the beam, the slightest deviation of this 

 centre from a vertical position over the axis would be followed 

 with a motion that would tend to reverse the position of the 

 beam. Moreover, if the axis be at a considerable distance 

 above the centre of gravity, the beam would have too great a 

 tendency to a horizontal position, independently of an equality 

 in the weights, and would accordingly want the requisite sensi- 

 bility. 



(2.) The axis and points of application of the weights, or points 

 of suspension, should be in the same straight line ; otherwise the 

 beam when loaded is liable to the defects just mentioned, by 

 having its centre of gravity shifted above the axis or too far be- 

 low it. It is accordingly of great importance that the arms 

 should be so constructed as not to bend or yield in any degree 

 in consequence of the weights attached to them, while at the 

 same time they should be as light as possible. To secure both 

 these objects, in the best balances instrument makers have given 

 to the arms the form of hollow cones. Care is taken also to 

 preserve the equality in point of length of the two arms, and to 

 diminish friction, by making the axis and points of suspension of 

 hardened steel, and in the shape of a very acute, wedge, or knife? 

 Meek. 21 



