96 On the Magnetism dcvclo])cd hi) Rotation. 



The author, liaving satisfied himself of the reality and con- 

 stancy of tliis effect, in different plates, and of the necessity of 

 referring it to a peculiar agency of the earth's magnetic power 

 on the molecules of the plate, proceeded to ascertain the laws, 

 and measure the quantities of the deviation due to rotation (so 

 he terms it) in various positions; and detailed a great num- 

 ber of experiments, with their numerical results, arranged in 

 the form of tables. 



From these he deduced the following general law; viz. that 

 the deviation due to rotation in a dipping needle " will always 

 be such, that the sides of the equator of such dipping needle 

 will deviate in a direction contrary to the directions in which 

 the edge of the plate moves, that edge of the plate nearest to 

 either edge of the equator producing the greatest effect." 



The results of this law, it may be here observed, are in 

 many cases coincident with those of the following : conceive the 

 dipping needle orthographically projected on the plate. Then 

 will the deviation due to rotation of the projected needle take 

 place in a direction opposite to that of the rotation itself. 



The author then proceeded to a theoretical investigation of 

 the effect of a plate of soft iron, having within it two poles de- 

 veloped in given positions, and acting (in addition to the usual 

 magnetic action of soft iron) on a needle of infinitely small di- 

 mensions, in the plane of the plate. He referred the whole 

 ordinary action of the iron to its centre, and supposed that 

 this is attractive on both poles of the needle ; but the extra- 

 ordinaxy action on that of the newly-developed poles he sup- 

 posed to reside in them, and to be attractive or repulsive, ac- 

 cording as they act on the poles of the needle of the same or op- 

 posite names with themselves. On this hypothesis, assuming 

 symbols for the co-ordinates of the plate's centre, the distance 

 separating the newly-developed poles in the plate, and the angle 

 which the line joining them makes with the direction of the needle, 

 &c., he deduced (from the known laws of magnetism) formulae, 

 expressing the horizontal deviations of the needle: — first, on 

 the supposition of a rotation in one direction; secondly, on 

 that of a rotation in the opposite; and thirdly, in that of no 

 rotation at all. From these, by comparing them with a few 

 of the observations, he deduced numerical values for the con- 

 stants of the formulee, and then employed them to compute 

 the deviations due to rotation in all the rest. Fie regarded 

 the discrepancy between the calculated and observed results, 

 as in few cases, larger than what he considered may be fairly 

 attributed to eiTor of observation ; and that the theory above 

 stated is at least a general representation of what passes in 

 fact : admitting, however, that it does not give the exact posi- 

 tion 



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