Mr Christie on the Magnetism of Iron in Rotation. 15 



second edition, " Essay on Magnetic Attractions" you will 

 find that Mr Barlow's ideal sphere was in all cases described 

 about the centre of the iron ball, and that it was only when he 

 adopted the views which I had suggested respecting the de- 

 viations of the dipping-needle being referred to the horizontal 

 plane, and according to which views I required a magnetic 

 sphere to be described about the needle, that he so conceived 

 it to be described. 



Shortly after making the experiments to which I have re- 

 ferred, I had an instrument constructed, by means of which 

 I avoided the preliminary laborious calculations for fixing the 

 iron sphere and compass in required positions, but with this 

 it became necessary to substitute a circular plate of iron for 

 the heavy eighteen inch shell with which I had been previous- 

 ly experimenting. With this instrument, I commenced a se- 

 ries of experiments near the end of May 182], and in endea- 

 vouring so to adjust the iron plate that, when its centre was 

 on the magnetic meridian, the needle should not deviate from 

 that meridian, I almost immediately (I think on the 4th June) 

 discovered, that the simple rotation of iron had a considerable 

 influence on its magnetic properties. I think your corre- 

 spondent must allow that this discovery, so made, was quite 

 independent of any experiments by Mr Barlow. 



As I considered that the correction of the local attraction 

 of a ship, by means of an iron plate, might be sensibly affected 

 by the plate being turned in one direction or another, on apply- 

 ing it to the compass, previously to the sailing of the Leven 

 and Baracouta in February 1822, these vessels being fur- 

 nished with correcting plates, I communicated to Mr Barlow 

 the discovery which I had made respecting the effects pro- 

 duced on the needle by the rotation of an iron plate, and sug- 

 gested that the pin on which the plate was to be applied to 

 the compass should be so formed that the plate could only be 

 slid on. * At this time Mr Barlow witnessed several of my 



• The necessity of a precaution of this kind, under many circumstances, 

 if not under all, has been fully proved during the late voyage of discovery. 

 Lieutenant Foster very obligingly undertook to repeat my experiments on 

 the effects of rotation, with the correcting plate of the Hecla, in the high 

 magnetic latitudes which the expedition was likely to visit, and at Port Bow- 



