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iron arising from its rotation, 
C c at right angles to the axis of the cylinder. Over the 
plate of iron is a wooden washer D d, which is pressed on it 
by the screw h working on the short cylinder. The cylinder 
with the plate is fixed in any position on the arm by the 
clamp Mm. In the part Aj of the arm are two openings 
0, o', on the chamfered edges of which are indexes in a line 
with the axis of the cylinder B b, so that when each points to 
the same arc on the semicircles S tT N, s ^n^ the axis of the 
cylinder B 6 is directed towards their centre, and every point 
in the edge of the plate is at the same distance from that 
centre. As the weight of the plate was a considerable strain 
on the instrument, a scale to contain a counter-weight, was 
suspended from the ceiling of the room, and the line from it 
passed through a moveable pulley, attached to the arm B 5 , 
so that the weight might easily be adjusted to relieve nearly 
altogether the strain of the plate on the arm in any position. 
The arm was also occasionally supported, and kept steady in 
its position, by a sliding rod resting on the table T The 
compass consists of a circular box, containing a circle 6 inches 
in diameter, very accurately divided into degrees, and again 
into thirds of a degree ; and a very light needle, having an 
agate in its centre, and its point of suspension only .07 inch 
above the surface of the needle. The extremities of the 
needle are brought to very fine points ; so that by a little 
practice, with the assistance of a convex lens, I could read off 
the deviations very correctly to two minutes, being the tenth 
of the divisions on the circle. To this compass I have ano- 
ther needle, which has a vernier at each end ; but this being 
much larger and heavier, and consequently not so sensible, I 
greatly prefer the other for all delicate experiments. • In the 
