MECHANIC POWERS. 33 



sion be in the same right line, the balance will 

 have no tendency to one position more than an- 

 other, but will rest in any position it may be 

 placed in, whether the scales be on or offj empty 

 or loaded. 



If the centre of gravity of the beam, when level, 

 be immediately above the fulcrum, the beam will 

 overset by the smallest action ; that is, the end 

 which is lowest will descend j and it will do this 

 with more swiftness, the higher the centre of gravity 

 be, and the less the points of suspension be loaded. 



But if the centre of gravity of the beam be im- 

 mediately below the fulcrum, the beam will not 

 rest in any position but when level ; and if dis- 

 turbed from that position, and then left at liberty, 

 it will vibrate, and at last come to rest on the level. 

 In a balance, therefore, the fulcrum ought always 

 to be placed a little above the centre of gravity. 

 Its vibrations will be quicker, and its horizontal 

 tendency stronger, the lower the centre of gra- 

 vity, and the less the weight upon the points of 

 suspension. 



4. The friction of the beam upon the axis ought 

 to be as little as possible ; because, should the 

 friction be great, it will require a considerable 

 force to overcome it; upon which account, though 

 one weight should a little exceed the other, it will 

 not preponderate, the excess not being sufficient to 

 overcome the friction, and bear down the beam. 

 The axis of motion should be formed with an edg-e 

 like a knife, and made very hard : these edges are 

 at first made sharp, and then rounded with a fine 

 hone, or piece of buff leather, which causes a suffi- 

 cient bluntness, or rolling edge. On the regular 

 form and excellence of this axis depends chiefly 

 the perfection of this instrument. 



vol. i. d 



