OJV BALANCES. 51 



I should explain to you that in all scientific weighings, or 

 weighings in which special accuracy is required for deter- 

 mining the exact amount of difference between two bodies 

 weighed, the weights placed in each pan should be so ad- 

 justed that the beam only oscillates very slightly, and the 

 pointer does not range beyond the limits of the index. The 

 adjustment is effected by adding to the lighter pan very 

 small balance weights, the value of which has been accurately 

 determined, until the requisite approach to equilibrium is at- 

 tained. The difference of the two bodies weighed can then 

 be read off on the index in divisions, the value of which is 

 known. 



We now arrest the balance, and place one of two equal 1 

 Ib. weights in each pan, and again set the balance in motion. 

 These are gilt bronze standard weights very accurately veri- 

 fied. You will see that the beam of the balance continues 

 horizontal, as shown by the position of the pointer. 



7. I shall next show you two specimens of balances with 

 unequal arms. The first of these is part of a model kit, con- 

 taining all the instruments necessary for an inspector of 

 weights and measures, these instruments being made to pack 

 in a case. It is a septimal balance, in which, as you will see, 



1 Ib. placed in the pan suspended from one of the ex- 

 tremities of the beam balances 7 Ibs. placed in the pan sus- 

 pended from a knife-edge placed one-seventh of the distance 

 from the middle of the beam to the other extremity. It is 

 of consequence to make the inspector's kit, which he has to 

 carry with him, as light as possible. It contains a 4 Ib. a 



2 Ib. and a 1 Ib, weight, and a nest of small weights, weighing 

 altogether 1 Ib. more. By placing all these weights, amount- 

 ing together to 8 Ibs., in the pan suspended from the end of 

 the beam, the inspector is enabled, as you may see, to test 

 a 56 Ib. weight placed in the other pan. This septimal 

 balance has also the advantage of being a serviceable equal- 

 armed balance, by having a second knife-edge at the extre- 

 mity of the beam, from which a pan of equal weight to that 

 at the other end can be suspended. 



The other balance with unequal arms is a French balance, 

 and is part of a similar kit supplied to the verifiers of 

 weights and measures in France, where the decimal metric 

 system is established. You see there is a knife-edge placed 

 one-tenth of the distance from the middle of the beam to 



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