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great diminution of magnetic force, when a ring of the same weight 

 was substituted for a disc ; and pursuing this point of inquiry, he 

 found that in all cases of solution of continuity, not only by cuts in 

 the direction of radii from the centre, but in concentric annuli or 

 otherwise, there is always a great loss of force, the magnetism of the 

 whole being always much greater than the sum of that of the parts. 

 He describes in detail these experiments, and the apparatus used for 

 them. His method of estimating the intensity of the force developed, 

 was by suspending the body, set in rotation by a revolving magnet, 

 by a wire, and preserving a constant velocity of rotation in the mag- 

 net, to note the time when the velocity acquired by the disc, was 

 just destroyed by the torsion of the wire, and the disc just began to 

 revolve in a contrary direction. Applying analysis to the dynamical 

 problem arising, he thence deduces the intensity of the force urging 

 the suspended body. Applying the resulting formula to the experi- 

 ments, he finds, in almost all cases, a small diminution in the inten- 

 sity of the force as the arc of rotation increases, which he attributes 

 to a very minute degree of magnetism accumulated in the disc, and 

 retained by it till the revolving magnet comes round again. 



In reasoning on the experiments detailed, Mr. Christie concludes 

 that the greatest developement of magnetism in a disc, subjected to 

 the action of revolving magnets, takes place when the axes of the mag- 

 nets are vertically under points bisecting the radii, and that the mag- 

 netism decreases very rapidly as they approach the edge; thus indi- 

 cating that for a full development of magnetism, a continuity of 

 substance, in all directions from the point acted on, is principally re- 

 quisite. This result is corroborated in a striking manner by the effect 

 produced by concentric circular cuts in the disc, leaving the interior 

 attached to the exterior in several places. On successively destroying 

 these points of connexion, a very great diminution of force is perceived. 



The effect of removing, by a circular concentric cut, the interior of 

 a disc, appears to be, to destroy or prevent the developement of a 

 quantity of force directly proportional to the mass removed, the mag- 

 nets acting at a constant difference from the centre ; and reasoning 

 from this and other phenomena, Mr. Christie concludes that the re- 

 duction of the disc, by concentric and radiating cuts, into very small 

 portions, though not actually to powder, would render its magnetism 

 quite insensible. 



The author next proceeds to investigate, by experiments of the 

 same kind, the law of variation of the magnetic force regarded as de- 

 pending on the distance of the revolving magnets from the suspended 

 body, which in this case was a cylindrical annulus of copper, about 1 

 inch in breadth, 10 inches in internal diameter, and ^inch thick, weigh- 

 ing 32'375 ounces troy ; and the axes of the magnets were made to 

 revolve during the whole series of experiments, with an uniform velo- 

 city of five turns per second, exactly under the middle of the breadth 

 of the annulus, being fixed vertically with their south poles upwards. 

 He assumes, first, that the action of each magnet may be referred to 

 a single point near its extremity ; secondly, that the action may 



