511 
and magnetic rotations. 
needles were attached ; 7. By the electricity of the bridge ; 
8. By the bending of the wax needle. 
The first of these causes, namely, the action of currents 
of air in the room in which these experiments were made, was 
obviated by the form of the apparatus in which the needle 
was afterwards inclosed, and care was taken to render all 
the parts of it as close as possible. 
The second source of this retrograde motion might be 
supposed to arise from the movement impressed on the air 
within the lower cylinder of millboard being communicated 
through the screen, not by actually passing through its 
apertures, but by the vibrations it might impress on it. The 
fact of the direction of the motion in the lower cylinder being 
in the contrary direction to that of the needle in the upper 
one, was exceedingly unfavourable to this hypothesis. 
Before I relate the various experiments which I made, with 
the view of successively placing each of these causes out of 
action, I think it may be necessary to detail a few additional 
experiments, in confirmation of the fact. 
Experiment 13. 
Using the same apparatus, I substituted a disc of copper, 
5 inches in diameter, and inch thick, instead of that of 
pewter ; the sealing-wax needle was suspended half an inch 
above the leno screen ; the lamp was placed at one inch from 
the edge of the copper plate, towards its centre: — the flame 
was large and smoky. 
