505 
and magnetic rotations . 
The three first oscillations of the needle were : 
ist. 127.8 
2nd. 22 6 
3rd. 127.8 
The disc was then set in motion, and kept revolving at 
about 78 turns in a minute. After six minutes no result was 
apparent in the needle different from those which had been 
observed before the motion commenced. This seems to 
show that the action of air, if it is driven through the screen, 
is insensible. 
Experiment 10. 
Having placed a screen of coarse gauze above the pewter 
plate at ^ inch distant, an excited needle of sealing-wax was 
substituted for that of brass. I observed the extent of the 
oscillations, and then setting the plate of pewter in motion, I 
was surprised to find that the returning arc (contrary to the 
direction of the plate’s motion) was almost invariably in- 
creased, generally at the first, and almost constantly at the 
second oscillation, and that the arc in the same direction was 
diminished. 
So unexpected a result naturally induced me at first to 
attribute it to some error in the observations ; but on repeat- 
ing it many times, and having measured the greatest error 
which the means I employed were susceptible of when used 
even a little carelessly, I found that half a degree was the 
greatest error I could make in reading off the oscillations, and 
that this retrograde motion was in many cases equal to three, 
