349 
iron arising from its rotation. 
ordinate. The requisite computations being necessarily 
tedious, when I wished to pursue the subject further, I found 
them, from their number, so laborious, that I resolved, if 
possible, to supersede the necessity of them by the construc- 
tion of an instrument, by which I could adjust the iron and 
the needle in their proper relative positions, without any 
previous computation. In this I succeeded ; but as the iron 
was to be supported on an arm of brass, it became necessary 
to make use of a plate of iron instead of the heavy shell of 
nearly 500 lbs. weight ; and in consequence of this, when I 
expected that I had overcome the principal difficulties, I 
found they had only commenced. It is well known that 
almost every mass of iron, but especially sheet iron, pos- 
sesses polarity in a slight degree, and of a very variable 
nature in some parts of it, whatever care may have been 
taken in its manufacture ; and I soon found, to my no small 
vexation, that the effects apparently produced by it in that 
which I made use of were so various, that they would for a 
long time baffle me in my investigations, if they did not ulti- 
mately frustrate all my attempts at drawing any conclusions 
from the experiments. 
The instrument which I have mentioned is represented in 
Plate XXV. fig. 1. The principal part consists of two 
strong limbs of brass : one, SO N, a semicircle, 18 inches in 
diameter, 2.15 inches broad and .3 inch thick: the other 
consists of two semicircles joined together ; S iT N, 1.2 inches 
broad and .22 thick, and its outer diameter 18 inches \ s cen 
.9 inch broad, .22 thick, and its inner diameter 9.2 inches^ 
S iT N ^ ^ and S Q N are attached to each other by strong 
brass pins passing from S to .v and N to ; so that s N.n will 
