12 Mr Christie on the Magnetism of Iron in Rotation. 



Barlow having made any experiments with an iron plate to 

 which your correspondent could refer mine as a repetition or 

 even extension; but as he very pointedly so refers them, I am 

 under the necessity of giving an account of the only connection 

 between any experiments of mine on the magnetism of soft 

 iron, and those in which Mr Barlow had been engaged. In the 

 spring of the year 1819,* Mr Barlow informed me that he had 

 found there was a plane passing through the centre of a sphere 

 of iron, in which, if the centre of a magnetised needle were 

 any where placed, the iron would in no case cause deviation 

 in the needle. He did not, however, at the time, state to me 

 from what experiments he drew this conclusion, but wished to 

 have my opinion on the cause of the existence of such a plane, 

 and likewise that I should witness some of his experiments. 

 Previously to my doing this, I pointed out the nature of the 

 deviations of the needle that ought to take place in different 

 positions of the iron sphere, according to a particular view 

 which I had taken of the subject, and the result in all cases 

 perfectly accorded with those which I had predicted. I was 

 immediately afterwards induced to make an extensive series of 

 experiments with an iron ball, in order to ascertain how far 

 the peculiar views which I had taken were correct, but quite 

 unconnected with Mr Barlow's inquiry, which, at the time, 

 was wholly practical. An account of these experiments is 

 given in a paper in the first volume of the Transactions of the 

 Cambridge Philosophical Society. In the first edition of his 

 " Essay on Magnetic Attractions,' 1 '' Mr Barlow has given a 

 brief statement of my views of the subject at that time, and, 

 by a reference to p. 113, you will see that, by adopting these 

 views, Mr Barlow was then enabled to correct some of the laws 

 which he had deduced. Your correspondent has thought pro- 

 per to state, that I was adopting Mr Barlow's views, by con- 

 ceiving an ideal magnetic sphere to circumscribe the needle. 

 By referring to p. 21 and p. 28, first edition, p. 23 and p. 30, 



• It was at this time that Mr Barlow's earliest experiments with an iron 

 ball were made, and some time afterwards he proposed correcting the local 

 attraction of a ship by means of an iron plate ; but the account of this me- 

 thod was not published until 1820, instead of 1818, as stated in your 

 Journal. 



