and magnetic rotations. 5%7 
nation agrees well with the observed facts ; since the motion 
communicated to the needle is the difference of two forces, 
each of which is very small, it must itself be smaller than 
either; and in case of any approach to equality in those 
opposing forces, it will become exceedingly minute. 
With respect to the action of heat in encreasing these rota- 
tions, I am inclined to conjecture, that the distribution of 
electricity on the metallic plate is altered by the application 
of heat ; that the part under the needle becoming electrified, 
produces in it induced electricity ; and thus the plate being 
made to revolve, the rotation of the needle follows from the 
explanation previously given. It may, however, happen, 
that the wax needle becomes itself electric by the heat it re- 
ceives from the plate below it. 
It will naturally be enquired, whether a parallel to this 
retrograde motion cannot be produced with magnets. I have 
made a few trials with a plate of soft iron, an iron gauze 
screen, and neutralized magnets, but have not succeeded. I 
found the needles stationary over whatever point they were 
placed. 
I shall conclude this paper with mentioning some of the 
methods by which I observed the angles passed over by the 
needles ; this is the more necessary, because it was desirable 
to remove all the parts of the apparatus as far from them as 
could be conveniently done. 
If the angle was small, a fine thread of glass was made to 
project from the end of the needle, and a lamp was placed in 
such a position as to throw its shadow on the graduations 
below ; this of course would only do for the measure of a 
few degrees, and was inconvenient by day. 
