94 Mr Christie on the Developernent 



Art. XI. — Observations connected with the History of' the De- 

 velopernent of Magnetism by Rotation. By S. H. Christie, 

 Esq. M. A., F. R. S. of Trinity College, Cambridge, Fel- 

 low of the Cambridge Philosophical Society, and of the 

 Royal Military College. In a Letter to the Editor. 



Dear Sir, 



IVIr Barlow's " Illustration of some facts connected with 

 the Developernent of Magnetism by Rotation," published in 

 your last Number, requires some comment; and however much 

 I may regret that the pages of your valuable Journal should 

 be occupied with such matter, I must beg that you will in- 

 sert a few observations : it must, I assure you, be something 

 of more importance which can again call my attention to the 

 subject. 



If Mr Barlow, about the years 1819, 1820, &c. was engag- 

 ed in a series of observations, in order to determine accurately 

 the attractive power of circular iron plates, in different posi- 

 tions, and at different distances from a magnetized needle, 

 and likewise, with precision, their strong and weak points of 

 local magnetism, I can only say that I had no knowledge of 

 the circumstance. At the same time, I cannot avoid expressing 

 my surprise, that he should not have fallen upon the property 

 in question, as no sooner had I found it necessary to note ac- 

 curately the deviation of a needle due to each particular posi- 

 tion of an iron plate upon an axis perpendicular to its plane, 

 than I discovered the peculiar effect due to rotation. Refer- 

 ring to the experiments in which I was engaged when I first 

 noticed this effect, Mr Barlow says, " Strictly speaking, how- 

 ever, these experiments were not a repetition of mine." I 

 know not any latitude of speaking that could make them be 

 so considered, since I am not even now aware of any which 

 Mr Barlow has made at all similar to them, — mine having been 

 made with particular views of my own, with an instrument 

 expressly constructed in conformity with those views, and 

 which had no reference to any views which Mr Barlow had 

 taken of the subject. 



