538 



Major J. Herschel. 



necessary ; as alterations may be necessary which one wonld otherwise 

 have no right to make. 



I append a separate account of the weighings of the major and 

 minor weights. Supposing the instrument ultimately brought inte- 

 nse, it would become necessary to know the total weight of the 

 suspended parts with some accuracy. This need should be kept in 

 view, if alterations are made. 



Account of the Weighing. 



I have weighed the suspended parts on three occasions, the 

 circumstances differing in each case somewhat. In the first two 

 weighings, the cross piece at the top of the shaft having been 

 removed, the whole of the swinging parts were suspended from one 

 arm of a balance — with exception of three mirrors absent for repair 

 at the time. This suspension involved certain additions and sub- 

 tractions by way of allowance for parts not present or redundant. 

 These were either determined or estimated for. I have since been 

 able to correct all the estimations, and the first weighing makes the 

 sum of the two parts A and B equal to 3,721 + 69+14-35-18-22 

 = 3,729 grs. 



In the second weighing — being uncertain of the accuracy of the 

 weights used — I prepared two lead blocks representing approxi- 

 mately the masses A and B ; and, by more directly counterpoising the 

 redundant parts, and reducing to a minimum all corrections, I ob- 

 tained a mass A equal to 3,109 grs. and a mass B equal to 615 grs., 

 the sum of which or 3,724 grs. balanced, or would have balanced 

 (for I could not eliminate two small pieces which had to be allowed 

 for) the whole of the suspended parts. The separation into two 

 parts provided for the removal of the lesser when the minor weight 

 was supported. I was quite aware that this was a doubtful partition, 

 owing to the impossibility of exactly supporting the lower weight 

 without imparting a thrust through the single wire. But there was 

 no way of obviating this without a separation of the two parts of the 

 mechanism. The total weight was free from suspicion. 



When later events led me to take the whole apparatus to pieces, I 

 took care before building it up again to weigh the two principal 

 parts separately — as well as to take some measurements which seemed 

 important. The result showed that this thrust had been much 

 stronger than I supposed, having transferred 9 or 10 grs. from the 

 lower to the upper estimate. I now found, directly and without any 

 allowances or reductions — the large mass being in one scale, and the 

 major weight as it would hang in the other, that the former required 

 paring down. The parings weighed 9 grs. Conversely the smaller 

 mass required an addition of 10 grs. to balance the minor weight. 

 Having made this transfer, I now have the two leaden masses, repre- 



