on the Plane of no Deviation in those places. 319 



on this point, Captain Wilson provided himself with an appa- 

 ratus like that which I employed at Woolwich ; and the fol- 

 lowing is his own account of these experiments, from which it 

 appears, that, abstracting from the errors of his dipping nee- 

 dle, the accuracy of the deduction is ascertained ; the discre- 

 pance between the observed and the inferred position of the 

 plane differing very little more than the known error of the 

 needle in London, although the experiments were made in 

 places having dips of contrary denominations, viz. north and 

 south. Besides these results, it will be seen that Captain 

 Wilson detected in his needle in China a very curious anomaly, 

 depending upon the unequal distribution of magnetism in its 

 two branches ; and these having been since examined on the 

 same apparatus in England by Captain Beaufort, R. N. F.R. S. 

 by Captain Wilson, and by myself, they have been found to 

 possess a greater importance in the theory of magnetism than 

 Captain Wilson supposed, and a paper is at present before 

 the Royal Society on these experiments. It would therefore 

 be improper to enter upon the subject in this communication, 

 but you will probably (should they appear in the Transac- 

 tions) make them the subject of a future article. The follow^ 

 ing is Captain Wilson's account of his experiments on the 

 plane of no deviation. 



Having provided myself with a stout deal table, the surface 

 of which was planed perfectly true, and also with a dipping 

 needle made by Nairne and Blount, with several light hori- 

 zontal needles, and with some powerful magnets, on my ar- 

 rival in Calcutta I procured a 13 inch mortar shell from the 

 arsenal, with the intention of ascertaining the plane of no at- 

 traction, which, according to Mr Barlow, should be at right 

 angles to the dip in all latitudes. 



Some defects, however, in the apparatus, and other circum- 

 stances, prevented my obtaining any satisfactory results before 

 the ship's departure. 



On our arrival in China, my attempts to repeat Mr Bar- 

 low's experiments were again unsuccessful, till at length it oc- 

 curred to me, that, if the poles of the needle were not equally 

 distant from the pivot on which it turned, all the inconsisten- 

 cies would be accounted for which had defeated my former 



