308 Description of the Selar Index. 
axis and its appendages in one department, the base and parts 
connected with itin another. When applied to other instruments 
than the compass, this index may be made of any size thought 
proper; but I would advise to have the same relative proportion 
of parts maintained. 
Of the Adjustments. 
Fig. 3 is an adjuster; which is simply a piece of smooth sheet 
copper or brass, 8 inches long and 4 wide at the top.* Over the 
centre of the hole 8, are drawn the very fine lines TU, VW, at 
right angles. X, isa small weight or plumb, suspended by a 
fine black hair fatonéd in a cut in the lower edge of the hole 8. 
At the foot of the line TU, is another cut. 
X. being suspended by its hair on the side of the plate shown 
in the figure, let the bar K with its appendages be taken from 
the quadrant, and placing its pivot in the hole 8, made just large 
enough to receive it, move the arm LM till the 0 on the vernier 
shall be on the line VW, and here make it fast by the screw at 
the back; then fix the whole in such a position that the hair 
shall hang freely by the side of the plate and directly over the 
line TU; adjust the level upon the arm LM, so that the air bub- 
ble shall rest truly in the middle of the opening, and fix it per- 
manently in this position by means of its screws. 
Y and Z, are pieces of block tin or brass, } of an inch thick, 
fitting on to the equatorial axis ; the piece Y near the quadrant, and 
Z near the extremity R. A fixe line is drawn upon the edges of 
these pieces over the central section of the holes. 
Let these pieces be placed upon the equatorial axis, with their 
marked edges in a plane with the face of the quadrant ; take the 
plate fig. 3 and fix the hair in the cuts at S and U, on the side of 
the plate as seen in the figure, and placing the hole S to the 
centre of the quadrant, put the pivot of the bar K in its place 
with the arm LM opposite the quadrant, and bring the slit y under 
the screw head d and make the hair truly parallel with the lines 
upon the pieces Y and Z, and secure this position by tightening 
the screw ; then adjust the quadrant so that the 0 of the equato- 
rial axis shall appear exactly under this hair, and make it fast in 
this position. A good eye will enable one to get this parallelism 
* But drawn here, for the sake of convenience, of a size relatively too small. 
