Management of delicate Balances. 347 



The rough adjustment is then complete ; all these opera- 

 tions are then to be again repeated, with the utmost care 

 and attention, and in adjusting the rapidity of the vibrations, 

 the ball on the top is to be screwed up to make them slower, 

 and down to increase their rapidity. It is sometimes custom- 

 ary to adjust a beam so as to ' set,' as it is called, by which 

 it is made less sensible when loaded with a small weight, but 

 the sensibility is increased by increasing the load, by which 

 means the effect of the increased friction of a heavier load is 

 counteracted. For this purpose, the beam alone is made to 

 vibrate quickly by making the centre of gravity low, and the 

 scale pans being hung on, the full load which the beam will 

 bear is placed in them, and the vibrations are made as slow 

 as possible by screwing up the points of suspension. When 

 the load is increased beyond the weights used in adjusting, 

 the beam will upset. 



Faraday (Chemical Manipulations) remarks, that in weigh- 

 ing, the beam should always be allowed to settle, but with a 

 fine instrument this is impossible, for a 7 inch beam will take 

 nearly half a minute to pass through one vibration, and will 

 not settle under half an hour, or even much more. It is best 

 therefore to estimate the place where it would settle by taking 

 half of the average arc of vibration, which should not be 

 greater than three or four degrees. 



The trouble required to perfect the adjustments of a very 

 fine balance is very great, and great skill, nicety, and patience 

 are required to make them satisfactory. Even when finished, 

 they will not remain perfect for three or four days togethei', 

 one or the other of the arms shewing a slight preponderance, 

 when the trial weights are again put in. 



Dr. Ure (Chemical Dictionary) remarks, that the results of 

 weighing, generally given by authors, cannot be depended 

 upon beyond four places of figures, or one 10- thousandth part 

 of the load, from which it is to be inferred that the balances 

 generally used by chemists in England are very indifferent, 



