32 



PROFESSOR FORBES'S EXPERIMENTS ON 



TABLE TIL 



64. It is easy to see that of these results, those with the needle A. 2. are 

 most worthy of confidence, and, after this clearly appeared, the observations 

 were made with that one almost exclusively. In the reductions presently to be 

 given, I shall in all cases adopt the results given by that needle. 



65. The principal reason of the superiority of the needle A. 2. is probably 

 the greater accuracy of adjustment of its centre of gravity. It is well known the 

 varying force of magnetization in opposite directions necessarily produces an error 

 besides the accidental ones, owing to errors likely to occur in a needle evidently 

 less carefully adjusted than the other. The repeated observations at Bonn on 

 different days with both needles, were made solely with a view of determining 

 the degree of accuracy which the instrument was capable of attaining, and they 

 must be owned to be very satisfactory. Needle A. 2. gave : 



23d May . . . 67 50.0 



67 51.1 



7th June ... 67 51.8 

 16th June ... 67 52.2 



Mean, 67 51.3 

 Greatest deviation from mean, 1.3 



