to rotation of the projected needle take place in a direction opposite to 

 that of the rotation itself, 



The author then proceeds to a theoretical investigation of the effect 

 of a plate of soft iron, having within it two poles developed 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 refers the whole ordinary action of the iron to its centre, 

 and supposes that this is attractive on both poles of the needle ; but 

 the extraordinary action, or that of the newly developed poles, he 

 supposes to reside in them, and to be attractive or repulsive accord- 

 ing as they act on the poles of the needle of the same or opposite 

 names with themselves. On this hypothesis, assuming symbols for 

 the coordinates of the plate's centre, the distances separating the 

 newly developed poles in the plate, and the angle which the line 

 joining them makes with the direction of the needle, &c. &c. deduce 

 (from the known laws of magnetism) formulae expressing the hori- 

 zontal deviations of the needle : 1st, on the supposition of a rotation 

 in one direction ; 2ndly, on that of a rotation in the opposite ; and 

 3rdly, in that of no rotation at all. From these, by comparing them 

 with a few of the observations, he deduces numerical values for the 

 constants of the formulae, and then employs them to compute the 

 deviations due to the rotation in all the rest. He regards the dis- 

 crepancy between the calculated and observed results as in few cases 

 larger than what he considers may be fairly attributed to error of 

 observation, and that the theory above stated is at the least a gene- 

 ral representation of what passes in fact ; admitting, however, that 

 it does not give the exact position of the point where the deviation 

 due to the rotation vanishes, and suggesting partial magnetism in 

 the iron plate used, as one mode of accounting for the difference. 

 At all events, by an examination of the case on the ordinary sup- 

 position of induced magnetism in the iron, he shows that a greater 

 coincidence between theory and fact would not result from that hy- 

 pothesis than from the one here employed. 



He then proceeds to inquire into the degree of permanence of the 

 polarity thus produced in iron by rotation ; from which inquiry it 

 appears that (at least during twelve hours after the plate was brought 

 to rest,) the influence of a single rotation had scarcely suffered any 

 diminution. It appears also that the effect is so far from depending 

 on the rapidity of the motion, that the plate can scarcely be made 

 to revolve so slowly that the whole effect shall not be produced. 



Lastly, by a slight change in the formulae, the results of compu- 

 tation can be made to agree with observation, to a degree of ex- 

 actness as near as can be wished. This change consists in the 

 omission of certain terms introduced by the theory, and the author 

 regards it as very possible so to modify the theory as to get rid of 

 them. 



The author closes this communication with an Appendix, com- 

 paring the magnetic effects produced by slow and rapid rotation. 

 The result of the comparison is, that the forces exerted on the needle 



