the devebpement of' Magnetism by Rotation. 215 



forms the subject of his paper. Now the plain fact is this : — 

 In 1819, I proposed my correcting plate, and from that time 

 it became necessary for me to ascertain the attractive power 

 of circular iron plates in different positions, and at different 

 distances from the needle, and to determine their strong and 

 weak points of local magnetism ; which was done by making 

 them revolve on their axis in certain situations near the com- 

 pass. 



In 1821 Mr Christie, with some particular views of his 

 own, undertook a series of experiments on the attraction of 

 circular iron plates, on which it was likewise necessary for him 

 to determine their strong and weak points of local magnetism, 

 by making them revolve on their axes ; and while thus engag- 

 ed, he fell upon the property in question. 



His object, of course, was different from mine, arid the results 

 he obtained were entirely his own ; but one may see clearly, 

 that it was the similarity in the two processes that led to the 

 passage in the report complained of. Strictly speaking, how- 

 ever, these experiments were not a repetition of mine, and it 

 was certainly not my wish they should have been so repre- 

 sented. At the same time, as mine were probably the first 

 experiments in which iron was put in rotation for magnetical 

 observation, I think they were very properly introduced into 

 the historical notice. 



I must beg, also, to bear testimony with Mr Christie, that 

 his paper in the Camb. Phil. Trans, was undertaken with a 

 different view to mine ; but still, as the experiments were not 

 made till after mine were completed, and for the purpose of 

 verifying an hypothesis suggested by a comparison of my re- 

 sults,* on the apparatus which had been constructed for me, 

 and with the same designations of latitude and longitude of 

 position, I can hardly consider the inquiries as " quite un- 

 connected." 



There was indeed this difference, as Mr Christie has stated 

 in his letter, that, both in this paper and that more particu- 

 larly in question, he referred the centre of the ball to the 

 sphere circumscribing the compass, while I referred the com- 

 pass to the sphere circumscribing the ball, and pages 21 and 



* See p. 110, 1st Edition. 



