167 



evolve the largest quantity of electricity; but the second, electricity 

 of the highest tension, by the same amount of motion : the accord- 

 ance of these results with the principles advanced is pointed out. 

 The author then refers to the use of wire rings of one or many con- 

 volutions, and indicates cases in which they may supply valuable 

 means of experimental inquiry. 



The relative amount and disposition of the forces of a magnet 

 when it is alone, or associated with other magnets, forms the next 

 point in the present paper ; and a distinction is first taken between 

 ordinary magnets, which are influenced much by other magnets, so 

 that the amount of their external force varies greatly, and those 

 which are very hard, where this influence is reduced to little or 

 nothing. The power of a given magnet was measured according to 

 the method described in the last series, by a loop once passed over 

 its pole. A given hard magnet placed in an invariable position, 

 being thus estimated, was found to have a force equivalent to 16^'3 

 of deflection. Another magnet, having a power of 25^*74, was then 

 placed close to the first in different positions, with like or unlike 

 poles near together, so as to tend sometimes to exalt its power and 

 at other times to depress it ; and the results observed. In the ex- 

 tremest favourable case, namely, when the two were conjoined as a 

 horseshoe magnet, the force of the first magnet was only raised 

 2°"45, which fell directly the dominant magnet was removed ; in the 

 corresponding adverse case the depression was only 1°. A very 

 hard magnet, made by Dr. Scoresby, of 6°"88 power, when under the 

 influence of another of double its power, was not sensibly affected 

 either way. When under the influence of one of six times its force, 

 it could be affected to the extent of nearly 1°. Ordinary magnets 

 can be afi^ected to the extent of one half of their power or more ; and 

 indeed in extreme cases can be altogether overruled and inverted. 



From these results the author concludes, that, with perfect un- 

 changeable magnets, the lines of force (as before defined) of chfferent 

 magnets in favourable positions, coalesce : that there is no increase 

 of the total force by this coalescence ; the sections between the 

 associated poles giving the same sum of power as the sections of 

 the lines of either magnet alone : that as the external amount of 

 force of the magnet is not varied, neither is the internal amount at 

 all changed : that the increase of power upon a magnetic needle, 

 or a piece of soft iron, placed between two opposite favourable poles, 

 is caused by concentration of the lines which before were diflused, 

 and not by the addition of the power represented by the lines of 

 force of one pole to that of the hues of force of the other. There is 

 no more power represented by all the lines of force than before, and 

 a line of force is not in itself more powerful because it coalesces with 

 a line of force of another magnet. In this and in other respects, the 

 analogy of the magnet with the voltaic pile is perfect. 



The paper concludes with some practical remarks upon the deli- 

 neation of the forms of the lines of force by iron filings, and by a 

 description of the inflection of the lines by hemispheres of hot and 

 cold nickel ; which the author considers as the corresponding case 

 to the action of warm and cold oxygen in the atmosphere, as applied 



12* 



