266 Proceedings of the Royal Society, 



horizon, 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 mo- 

 lecules 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 number 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 qA^q 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 dip- 

 ping 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 

 dimensions, 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 extaordinary 

 action on that of the newly-developed poles he supposed to reside 

 in them, and to be attractive or repulsive according as they act 

 on the poles of the needle of the same or opposite 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 join- 

 ing 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 



