532 JAMES CLERK MAXWELL. 



every direction perpendicular to the lines of force, on account 

 of the tendency of the vortices to expand equatorially. 



Now suppose that we have a north magnetic pole and 

 a south magnetic pole placed near to one another. Lines of 

 force will proceed from the North pole, generally in curved 

 lines, to the South pole. The space in the neighbourhood 

 of the poles will be filled with molecular vortices, which will 

 be most energetic along the line joining the poles, and the 

 velocities of the vortices will diminish as we pass into weaker 

 portions of the field. The tension along the lines of force, 

 tending to draw the North and South poles together, affords 

 sufficient explanation of the apparent attraction between the 

 poles ; the kinetic energy of the molecular vortices accounts 

 for the potential energy of the separated poles, which we 

 thus suppose to be really kinetic energy, though possessed by 

 the medium between the apparently attracting bodies and not 

 by the bodies themselves. (Perhaps all examples of so-called 

 potential energy we shall some day find to be really kinetic 

 energy possessed by a medium with the properties of which 

 we have been hitherto unacquainted.) When the poles 

 approach one another, the field which is occupied by the 

 vortices is diminished in extent, and though the velocity of 

 the vortices is increased, the whole energy of the field is 

 diminished, and the difference is expended in work done upon 

 the approaching magnets. If the poles are of equal strength, 

 and can come absolutely to coincide, the field is destroyed, 

 all the vortices come to rest, and the energy possessed by 

 them is all expended in work done on the magnets. 



If two like poles, north poles for example, be placed near 

 to one another, the lines of force proceeding from the one, 

 instead of going to the other, will be turned aside, and if the 

 poles be of equal strength, a plane bisecting, at right angles, 

 the line joining the poles, will separate the lines of force due 

 to the one from those due to the other, so that no line will 

 cut the plane (Fig. 1 0). The lines of force thus passing nearly 

 parallel to one another, the pressure exerted by the molecular 

 vortices in every direction at right angles to the lines of force 

 will cause an apparent repulsion between the poles. 



