40 Rev. J. Challis 07i the Resistance to the Motion 



fig. S-t. In this way the elementary vortices in fig. 23. and 

 24. will explain the compound distributions of force in each 

 individual case, as represented in the figures. 



In fig. 16. and 17, where the disc revolves on a centre, the 

 excitation arising from the motion being in the direction o m 

 on one side of the magnet, fig. 26, is counteracted by the op- 

 posite excitation on the other side of the line a m ; for as on 

 one side of the magnet the motion would be advancing, and 

 on the other side retiring, as in fig. 23. and 24. respectively, 

 the forces arising therefrom would nearly, perhaps completely, 

 destroy each other. It is possible, however, nay it is even pro- 

 bable, that all the systems of forces arising from the four recti- 

 linear motions are in play when the disc is revolving on its axis; 

 but the insignificancy of the two last contemplated forces, with 

 regard to those which are due to the motions indicated by fig. 

 21. and 22, must necessarily render them exceedingly ineffi- 

 cient. If the force be electric, it is likely that the remote parts 

 of the disc serve merely as conductors to that excited in the 

 parts vicinal to the magnet. 



The small curved arrows in fig. 19. and 20. indicate the dis- 

 tribution of the force in annular discs of copper or zinc, when 

 rotated on an axis in the manner described for complete discs. 

 The large exterior arrows indicate the direction of motion in 

 each figure. The distribution in these annular discs is pre- 

 cisely the same, so far as the metal permits, as that in complete 

 discs. 



Fi^. 27. is intended to show the position of the neutral line 

 on the rectangular plate, when moved in the direction of the 

 arrow between the magnetic poles. The arrow is a right line 

 crossing the magnetic pole, and two inches in front of it. The 

 small needles are placed an inch from each other, and their 

 positions, with regard to the arrow, show the inclination at each 

 station, or the position in which the excited forces in the plate 

 alone would place them. 



X. On the Resistance to the Motion of small Spherical Bodies 

 in clastic Mediums. By the Rev. J. Challis, Fellow of the 

 Cambridge Philosophical Society*. 



I^HE following observations have reference to the com- 

 munication I made to the Philosophical Magazine and 

 Annals for last March, and the mathematical reasoning therein 

 contained, vvhich being of a novel kinil, requires to be con- 



* ComniunicateJ bv the Autlior. 



