M. Poisson on the Theory of 'Magnetism in Motion. 329 



where they are enclosed, are constant in magnitude and direc- 

 tion. In this case, the two magnetic fluids spread over the 

 surfaces which envelope these small spaces, and are distribut- 

 ed over these surfaces in a manner determined by the magni- 

 tude and direction of the external forces. 



The second effects take place when the external forces, vary- 

 ing continually either in magnitude or direction, there are con- 

 tinually new portions of the neutral magnetic fluid decompos- 

 ed, so that the austral and the boreal fluid resulting from that 

 decomposition act in the time even that they take to pass from 

 the interior of the small spaces, where their decomposition 

 takes place, over the surfaces which envelope these spaces. 



Admitting, as M. Poisson has done, that a species of fric- 

 tion hinders this transport from being instantaneous, the ac- 

 tion which is thus produced upon an external point of the bo- 

 dy rendered magnetic, may, according to the nature of that bo- 

 dy, predominate greatly over that which the same fluids exert 

 in the first case upon the same external point. 



With regard to the friction above mentioned, which, with- 

 out opposing itself to the transport of the molecules of the 

 fluid, only diminishes the velocity with which the transport is 

 effected, we must carefully distinguish it from the coercive force, 

 the effect of which is absolutely to prevent the displacement of 

 the magnetic fluids, till it is overcome by an external force more 

 considerable. There is no coercive force in most bodies sus- 

 ceptible of being magnetised ; and it is chiefly in steel that this 

 force shows itself, both by the property which this body has 

 of being magnetised, and by that which it possesses of pre- 

 serving its magnetic properties when once acquired. 



M. Poisson admits, on the contrary, in all bodies, the ex- 

 istence of the force which he compares to friction, and those 

 even in which we cannot discover any coercive force arc not 

 exempt from it. He remarks, that the action produced by 

 the magnetic fluids in motion is nothing in the two extreme 

 cases, viz. the case where we would suppose that the magnetic 

 fluids would be transported instantaneously into the position 

 where they should remain in equilibrium, and the case in which 

 wc would admit a coercive force sufficient to oppose itself to 

 any displacement of these fluids. 



