Bodies are repelled from the Poles of a Magnet. 283 



ting from N is repre- 

 sented by the arrows ; 

 and if this force be 

 purely repulsive, the ac- 

 tion upon every single 

 particle of the diamag- 

 netic mass furnishes 

 a moment which, in 

 the position here as- 

 sumed, tends to turn 

 the rectangular box in 

 the direction marked 

 by the arrow. It is 

 perfectly impossible 

 that such a system of 

 forces could cause the box to turn in a direction opposed to the 

 arrow ; yet this is the precise direction in which the box turns 

 when the magnetic force is developed. 



Here, then, we have a mechanical eflfect which is perfectly 

 inexjjlicable on the supposition that the diamagnetic force is 

 purely repulsive. But if the conclusions arrived at in the fore- 

 going memoir be correct, if the diamagnetic force be a polar 

 force, then we must assume that attraction and repulsion are 

 developed simultaneously, as in the case of ordinary magnetic 

 phsenomena. Let us examine how this assumption will affect 

 the analysis of the experiment before us. 



The marked end of a magnetic needle is pulled towards the 

 north magnet'c pole of the earth ; and yet, if the needle be 

 caused to float upon a liquid, there is no motion of its mass 

 towards the terrestrial pole referred to. The reason of this is 

 known to be, that the south end of the needle is repelled by a 

 force equal to that by which the north, or marked end, is at- 

 tracted. These two equal and opposite forces destroy each other 

 as regards a motion of translation, but they are effective in pro- 

 ducing a motion of rotation. The magnetic needle, indeed, when 

 in a position oblique to the plane of the magnetic meridian, is 

 solicited towards that plane by a 

 mechanical couple, and if free to 

 move, will turn and find its position 

 of equilibrium there. 



Let such a needle,///, be attached, 

 as in fig. 3, to the end of a light 

 wooden beam, viv; let the beam 

 and needle be suspended horizon- 

 tally from the point a, round which 

 the whole system is free to turn, 

 the weiglit of the needle being ba- 

 U2 



