248 Messrs, Bahhage and Herschel on the [Oct. 



maximum of polarity being immediately under the magnet. Now 

 let the magnet be moved horizontally along the surface, pre- 

 serving the same distance from it. The pomts over which in 

 succession it becomes vertical, not instantlj/ receiving all the 

 magnetism of which they are susceptible, will not have reached 

 their maximum of polarity at the precise moment of nearest 

 appulse, but will continue to receive fresh accessions during the 

 whole of that certain small portion of time when the distance 

 (being at or near its minimum) undergoes no change, or only a 

 certain very minute one. In hke manner, the points which have 

 attained their maximum of j;^olarity, being left behind by the 

 magnet, will by degrees lose their magnetism ; but the loss not 

 being sudden, they will continue near their maximum for a 

 certain finite time, during the whole of which the magnet conti- 

 nues receding from them, and leaving them further and further 

 behind. Thus from both causes, there will be always in arrear 

 of the magnet a space both more extensive and more strongly 

 impregnated with the opposite polarity, than in advance of it ; 

 and as the magnet moves forward, the point of actual maximum 

 (or the pole) of the plate, instead of keeping pace with it, and 

 being always precisely under it, will lag behind. There will 

 thus arise an oblique action between the pole of the magnet and 

 the opposite pole of the plate so lagging behind it ; and were 

 the plate free to move in its own plane, the resolved portion of 

 this action parallel to its surface, would continually urge it in 

 the direction of the magnet's motion. 



But besides the attracting pole of the opposite name (south) 

 produced by the (north) pole of the magnet at the spot imme- 

 diately under it, there will also be developed a corresponding 

 repulsion, or north polarity in the plate. This, however, will 

 not, like the attractive, be concentrated nearly in one spot 

 immediately below the magnet, but must of necessity be diffused 

 round it in a much less intense and more uniform state through- 

 out the more distant parts of the mass, and may be conceived 

 as arranged in spherical or other concave strata about the point 

 ■vertically under the magnet as a centre. Now when the magnet 

 by its motion is carried out of the axis of these strata, it is 

 obvious that the resultant force of each of them will be less and 

 less oblique to the surface as its radius is greater. The general 

 resultant, therefore, of all the repulsive forces exerted through- 

 out the whole extent of the plate is necessarily less oblique to 

 the surface than that of the attractive ones, whose influence, 

 from this cause alone, must, therefore, preponderate, and must 

 necessarily produce a dragging or oblique action, such as above 

 described. This force, however minute, acting constantly, must 

 at length produce a finite and sensible velocity, provided the 

 whole mass of the plate to be set in motion be finite, and the 

 force of the magnet sufficient to overcome friction, resistance, &c. 



