52 LECTURES TO SCIENCE TEA GHEES. 



one extremity, and when a weight of 10 kilograms is placed 

 in the pan suspended from this knife-edge, it is balanced by 

 a 1 kilogram weight in the pan suspended from the other 

 end of the beam. 



8. In both these kinds of balance the beams are levers of 

 the first order, the fulcrum upon which the beam turns being 

 placed between the power and the weight, that is to say, 

 between those points of the beam which carry the testing 

 power on one side and the tested weight on the other side. 

 On the principle of the lever, the power of any weight to 

 move a balance beam is proportionately greater according as 

 the part of the beam which is pulled down by that weight 

 is more distant from the fulcrum or the centre of motion 

 of the balance. Hence it follows that the power of a weight 

 to move a balance is in a ratio compounded of the weight 

 itself and of the distance of its point of suspension from the 

 centre of motion of the balance. 



A multiplying or proportionate balance can consequently 

 be constructed, as you have seen, for determining the weight 

 of a body placed in the pan suspended from the shorter arm 

 of 'the beam, by its being found equal to a multiple of a unit 

 weight placed in the pan suspended from the longer arm of 

 the beam, usually termed the weight-pan. Thus, if the beam 

 be divided into say three equal parts, and the centre of mo- 

 tion be placed at the first of these divisions, one-third of the 

 whole length of the beam, 1 Ib. placed in the weight-pan 

 will form an equipoise with 2 Ibs. placed in the other pan, 

 and so on. 



9. The two balances with unequal arms, which have been 

 exhibited to you, are specimens of this class of balance where 

 there is a fixed proportion between the two arms. In these 

 balances any weight placed in the weight-pan will equipose a 

 proportionally larger weight in the other pan. But there 

 are also balances of the same class where a similar result is 

 produced by a variable proportion between the two arms, and 

 by one fixed weight only, which is made to traverse along the 

 graduated long arm of the balance. Such balances are, as 

 you know, called by us steelyards. They appear to have been 

 first used by the Romans, and to have been the earliest form 

 of a well-constructed multiplying balance. In France they 

 are still called Roman balances. You may here see a speci- 

 men of an ancient Roman steelyard, or statera, which has 



