32 Mr. Christie on the Magnetism of Iron fJuLY, 



was this — that the iron being evidently slightly polarized in par- 

 ticular points, the effect might be supposed to arise from an 

 impulse given to the needle by the motion of these points in a 

 particular direction, and that the directive power of the needle 

 not immediately overcoming the slight friction on the pivot, a 

 deviation might thus arise from the rotation of the plate. Had 

 this, however, been the cause of the deviations, I should have 

 expected that, when the centre of the plate was in the meridian, 

 the greatest effect would be produced with the plate parallel to 

 the norizon, and its centre vertical to that of the needle ; but I 

 had seen that the greatest deviation took place when the centre 

 of the plate was in the equator, its plane being perpendicular to 

 it ; and the deviation arising from the rotation, when the plate 

 was parallel to the horizon, was not a fifth of the deviation when 

 the plate was perpendicular to that plane. Besides it was 

 manifest that if this were the cause, any other impulse would 

 have a similar effect. I therefore made the needle revolve first 

 in one direction, and then in that opposite, by means of a small 

 bar-magnet, and invariably found that it settled at the same 

 point, in whichever direction the impulse was first given, and the 

 results obtained by the rotation of the plate were in these cases 

 of the same nature as before. It was also evident, that if the 

 deviations I have mentioned arose from this circumstance, the 

 needle being agitated after any particular point of the plate was 

 brought to the limb of the instrument, it ought to settle in the 

 same direction, whether that point were brought into this posi- 

 tion by revolving from east to west or from west to east ; but this, 

 except in the cases I have mentioned, where the rotation pro- 

 duced no deviation, was not found to take place. In order 

 wholly to obviate this objection, in all my future experiments, 

 after any point had been brought to the limb of the instrument, 

 I agitated the needle, and let it settle before I noted the devia- 

 tion. 



" Description of' particular Experiments. 



" As I had found in my first experiments that I could obtain 

 the nature of the deviation caused by the rotation by noting the 

 greatest and least deviations when the plate was made to revolve 

 in contrary directions, but that the quantity of that deviation 

 could not by this means be determined with any degree of pre- 

 cision, I resolved to make my future observations differently. 

 The method I adopted, when the change in the deviation from 

 one point of the plate to another was considerable, was this : the 

 plate being placed in any required position, I made it revolve 

 once, for example, the upper edge from east to west, without 

 noting the deviations, bringing the point marked 0^ to coincide 

 with tne line indicating the position for observation ; from hence 

 I continued the revolution of the plate until the point marked 



