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known that striking, twisting, or filing iron in different directions 

 with regard to the magnetic axis, materially influenced its polarity ; 

 but it does not appear to have been remarked, that the simple ro- 

 tation of iron in different directions has any such influence. This, 

 however, the author has ascertained to be the case ; and that the 

 laws which govern this peculiar action are so regular, that there can 

 remain no doubt of a corresponding regularity in their causes. 



The attention of the author was first drawn to these phenomena 

 by some apparent anomalies in the magnetic action of an iron plate 

 on the compass, observed in the course of a different investigation. 

 In order to avoid or allow for the disturbing influence of partial mag- 

 netism in the iron, it became necessary to attend minutely to the po- 

 sition of certain points in its circumference, which corresponded to 

 the maxima and minima of this magnetism. It was then found that 

 these points were not constant, but shifted their position as the plate 

 was made to revolve in its own plane ; or, in other words, that a 

 plate which, in a given position, produced a certain deviation in a 

 compass, no longer produced the same deviation after making an ex- 

 act revolution in its own plane, although brought to rest, and every 

 part of the apparatus restored precisely to its former place. It ap- 

 peared from this that the revolution of the plate in its own plane had 

 an influence on its power of deviating the needle independent of the 

 partial magnetism of particular points in it ; and the justice of this 

 idea was proved by giving it a rotation in an opposite direction, when 

 the effect on the directive power was also reversed. 



The change produced by rotation in the directive power of the 

 plate was found to be a maximum when its plane was parallel to the 

 line of dip, or the magnetic axis, and at the same time as little in- 

 clined to the horizon as this condition would allow ; but when the 

 plane of the plate was parallel to the horizon, the effect was di- 

 minished in the ratio of 5 to 1 ; and when perpendicular to the ho- 

 rizon, and coincident with the magnetic meridian, was altogether 

 destroyed. 



The author having satisfied himself of the reality and constancy of 

 this 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 quan- 

 tities of the deviation due to rotation (so he terms it) in various 

 positions, and details a great number of experiments, with their nu- 

 merical results arranged in the form of tables. From these he de- 

 duces 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 con- 

 trary to the direction in which the edges of the plate move ; that 

 edge, and the plate nearest to either edg 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 



