and magnetic rotations. 523 
alluded are common to all the experiments in which the 
phenomena occurred, I shall now offer that explanation of 
them which appears to me to be warranted by the observa- 
tions that have been recorded in the preceding part of this 
paper. They naturally divide themselves into two classes. 
1st. Those in which the motion communicated is in the same 
direction as that of the revolving plate. 2ndly. Those in 
which the motion of the needle is in a contrary direction to 
that of the plate. 
The instances in which the needles employed followed the 
directions of the revolving plate are so numerous, and the 
extent of their motion so great, that no doubt can remain of 
the fact, and some of them may be repeated with a very 
simple apparatus. The circumstance of one of the bodies 
employed being generally an electric, such as wax, resin, 
glass, and of the increase of the observed motions when these 
were previously excited by friction, or by a change of tem- 
perature during the experiment, naturally points out electri- 
city as the cause. It is indeed the only one common to all 
the cases detailed. 
If it be admitted that induced electricity is neither acquired 
nor given up in an instant of time, then it necessarily follows, 
from the reasoning I have explained at the commencement of 
this paper, that such motions must arise. 
2nd. The case of a retrograde motion is much more diffi- 
cult of explanation. Its existence rests on measures of much 
smaller magnitude, and it is by no means so easily re-pro- 
duced, requiring some delicacy both in the adjustment of the 
apparatus, and in the observations of the needle. I have 
myself observed it so frequently, as to have not the smallest 
