104 On a Proposed JS T eiu Method of Weighing, 



beam. The range of the rider thus includes the whole extent 

 of one arm of the balance. 



In adapting this piece to the assay beam of the usual 

 pattern it will be found necessary to set the divided upper 

 edge of it a little out of the vertical plane, which equally 

 divides the beam from end to end, and to incline it towards 

 the back of the lantern, so as to accommodate its position to 

 the sweep of the lever by which the rider is lifted (this is 

 shown in the sketch attached to the present paper). The 

 same lever will then serve for placing the rider on any part 

 of the beam itself, or on this auxiliary piece. 



Besides the weights already enumerated, I propose an 

 additional series of twenty small weights, of gold or platinum, 

 weighing actually one-tenth of a grain, two-tenths of a grain, 

 and so by regular progression of one-tenth of a grain, up to 

 one and nine-tenths grains ; and at the outset we must 

 convince ourselves that these weights are as accurate as care 

 and skill can make them, and then I think it can be shown 

 that with them we shall be able to perform all the work of 

 the assay independently of the relative lengths of the arms 

 of the balance, and that fortified by the series of weights "99 

 to *8 (as above described) in the position of weights of 

 reference, they will enable us to eliminate one of the chief 

 sources of error in the assay, that, namely, which results from 

 the wearing of the weights. 



But the use of this series of weights which I now propose, 

 involves this principle, namely, that the load of the balance 

 is in all cases constant (the pound often grains), a condition 

 which is also highly favourable to uniform and accurate 

 results. We adopt the conjoint advantages of double 

 weighing and a constant load. 



We adjust our balance so as to effect an equipoise with 

 the pound in the right-hand pan, and removing this pound 

 we weigh in its place and adjust to equality with it each of 

 our samples to be assayed. The assay is conducted through 

 its several processes in the usual manner, and the resulting 

 gold cornets are each in succession placed in the pan of the 

 balance. What the trial piece has lost is base metal ; what 

 is retained of its ponderable substance, subject however to 

 the usual variable correction for surcharge,* is pure gold. 



* For the general reader, it is explained that the technical term " sur- 

 charge," used in its broadest sense, is understood to mean the correction 

 which it is necessary to make in order to reduce the weight of the cornet to 

 that of the pure gold which it represents ; there is loss of gold on the cupel 



