o7i> Prof. Challis s Theoretical Explanations of 



at its blackened surface than at the other, the two surfaces are 

 electrified in different degrees ; and, relatively to a neutral state, 

 one is positively electrified and the other negatively electri- 

 fied. Now by an experimental law of electrical action (which 

 is also accounted for by the hydrodynamical theory), a face of 

 the vane, whether positively or negatively electrified, as it 

 approaches the piece of pith will attract it ; and the oppo- 

 sitely electrified face of the same vane, after passing the pith, 

 will also attract it, supposing there is no contact between the 

 vane and the pith. Consequently the piece of pith is drawn 

 in opposite directions in quick succession if the rotation of the 

 vanes be very rapid ; and as impression of motion takes time, 

 it might veil happen that, under these circumstances, no per- 

 ceptible motion takes place. If, however, the rotation is slow, 

 the attraction of the vane in one direction may take effect 

 before that in the opposite direction commences, in which case 

 an oscillatory motion of the pith ball will be produced. Clearly 

 the oscillations will be most steady when by reason of synchro- 

 nism of the time of oscillation of the pith with the time of re- 

 volution of the vane, the attraction of a «;iven vane acts like 

 gravity on a pendulum. If the oscillation of the pith ball be 

 produced, as seems to be supposed, by the intervention of the 

 action of the vanes on the residuum of air in the globe, it is 

 wholly inexplicable that there should be no perceptible effect 

 when the rotation of the vane is very rapid, and might be ex- 

 pected to cause great disturbance of tLe air. The facts that 

 no oscillation is produced by a rapid rotation, and that oscilla- 

 tion commences after diminishing the rate of rotation, are well 

 accounted for by the present theory ; and these explanations 

 justify at the same time the assumption of the electric state of 

 the vanes. 



In the Tastitut of July 5, p. 213, an account is given 

 of an experiment by M. Ducretet, -according to which, by 

 throwing ether on the glass globe to produce depression of 

 temperature, the rotation of the vanes is first stopped, and 

 then, by continuing the cooling, is caused to take place in the 

 opposite direction. This result is quite in accordance with the 

 present theory. The cooling effect of the ether, according to 

 the law of heat-exchanges, causes the vanes to radiate less 

 heat, or even changes the heat into cold : and as the cooling, 

 for the same reason as the heating, is in excess at the blackened 

 surface, the rotation might thus be stopped, and the rotating 

 force might be made to act in ihe contrary direction. 



The movement of the vanes is affected by two causes — the 

 presence of a residuum of air, and the friction at the pivot 

 above spoken of. Before exhaustion of the globe, no motion 



