of an Arc of the Meridian » 389 
In this general description, I am now to speak of one of the 
most ingenious contrivances attending the sector, which is, the 
means of readily adjusting the plumb-line in its several positions. 
I refer to the Plates and their descriptions, for a full account of it ; 
but, as it will enable the reader to understand that which repre- 
sents the instrument in its perspective view, (Plate IX.) I shall 
shortly describe this part. 
The telescope of the sector is nearly eight feet long, and has 
an object-glass of four inches in diameter. It is attached to an 
axis, similar in shape to that of a transit-instrument, having at 
one end a lens, and near to the tube an arrangement of brass 
work, carrying a thin and diaphanous slice of mother-of-pearl, 
having, as appears to the naked eye, a dot upon it. The centre 
of this dot is by construction the true centre of the conical axis, 
and consequently of the circle of which the divided limb is a 
part. It is unnecessary to say, in this place, how that diaphragm 
was so adjusted as to have the centre of its dot where it should 
be, or the means by which it has been permanently fixed ; it 
suffices that I say the point was placed most accurately, and the 
diaphragm fastened so firmly in the cone, that no readjustment 
of this part has been found necessary, since the sector came 
into my hands. 
As the axis is hollow, a light, as that of a candle, held at its 
open end, is transmitted through the mother-of-pearl, which,, 
stopping a part of its rays, exhibits a circle of red light to an 
eye looking through the lens at the opposite end of the axis ; 
a well defined and exceedingly small dot appearing in the 
middle of the illuminated circle. Through proper openings in 
the upper and under parts of the axis, and suspended from a 
point not connected with it, passes the plumb-line, having its 
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