79 
the particles of magnetic bodies, &c. 
exerted by the wire be proportional to the torsion, the torsion 
divided by the sine of the deviation should be constant. Now 
the values which I obtained for this quotient with the torsions 
713°*8, 1427 °*5, 2i4i°'i7, were 6600, 6595, 6634, the greatest 
difference among which is only the 165th part of the mean. 
The resistance of the air would have a tendency to increase 
the whole time during which the disc continued to revolve in 
the direction of the magnets, and consequently to diminish the 
values of «, as those of 6 increased, but not to a sufficient ex- 
tent ; a diminution of 6 or 7 seconds in the whole time, and in 
some instances more, being required to account for this decrease 
in the value of a : nor can the decrease arise from the small 
changes which take place in the relative velocity of the disc 
and the magnets. Upon the whole, I think it very probable, 
that the values of u really decrease in the successive revolu- 
tions of the disc ; and in this manner : according to the prin- 
ciple adopted in the very interesting Paper by Mr. Herschel 
and Mr. Babbage, on the magnetism manifested by various sub- 
stances during rotation, and so well supported by all the pheno- 
mena hitherto observed, the effect of the magnets upon the 
disc will depend upon the excess of the magnetism in those parts 
of the disc in the rear of the magnets above that in the parts in 
advance of them ; and if this excess were constant, so would 
also be the value of & ; but if on the magnets coming succes- 
sively under any point, that point has not parted with all the 
magnetism which it is capable of losing by the removal of the 
magnets to the opposite side of the disc, there may be a gra- 
dual, though small, accumulation of the magnetism left in 
every point of the disc ; and as only a certain portion of mag- 
netism can be developed in each point during the time that the 
