Description of the Solar Index. 303 
quadrant ; and its position is secured by a screw at the back, 
like that of the common quadrant. 
In the piece N is a convex lens, which brings the sun’s rays to 
a focus upon the piece O, through which a hole is made of about 
>; of an inch diameter. 'The sun’s focal diameter upon this 
piece being a little larger, causes a fine, bright ring of light to 
appear about the border of this hole, when the instrument is 
properly adjusted. The lens is fixed at a proper distance from 
the piece O, and this piece at a proper height above the edge of 
the bar K, by means of screws; the particular mode of doing 
which will be understood at once from fig. 2. 
_ ‘The quadrant, or more properly, the septant, is divided into 
degrees and halves, from 0 to 70°; O being upon the equatorial 
axis. At 35° is placed another 0, and 10 and 20 at the corres- 
ponding number of degrees distant upon each side of it. The 
purpose of this last series of numbers is, the greater convenience 
in making the adjustment for the sun’s declination. The vernier 
is divided each way from the centre into 15 equal parts, which 
embrace an extent equal to 14 of the half degrees upon the limb 
of the quadrant ; by this means the vernier is capable of measur- 
ing an arc of 2’ on the limb. When the central line of division 
or 0 of the vernier, is upon the 0 of declination, or at 35° from 
the 0 on the equatorial axis, the bar K is at right angles to this 
axis. 
The quadrant is attached to the equatorial axis by three screws 
at the back, and the spirit level to the arm LM by two. The holes 
through this arm and the equatorial axis are about ;'; of an inch 
larger in diameter than the screws, and the heads of the screws 
set down square over them. I ought, however, to except that 
screw of the level nearest the bar K, the head of which is sunk, 
to allow a free motion over the radii GI. By this device it is 
easy to give the quadrant and level a proper adjustment. 
The compass being placed firmly upon its tripod, with the base 
A fixed in its place and turned towards the south in a clear day, 
adjust the 0 on the vernier of the index to a distance upon the 
quadrant from 0 on the equatorial axis, equal to the latitude of 
the place, and put the equatorial axis into the tube E; level the 
compass either by means of the levels upon it, or in defect of 
them, as nearly as may be by the eye, and bring the plane of the 
quadrant into a position perpendicular to the horizon ; loosen the 
screw F’, and give to the tube a position which will bring the bubble 
