168 
MR. LLOYD ON THE DIFFERENCE OF LEVEL BETWEEN 
stantial Ys, is fixed the inner conical centre, three inches in diameter and 
eight in length, ground to move perfectly smooth in the hollow conical centre 
fixed to the stand. 
This limb projects over the upper part of the frame about one inch, and 
is bevelled at the edge, in order to slide in a groove attached in two pieces to 
a clamp, screwed underneath the upper part of the frame, and which, by 
means of a tangent screw, gives a slow motion in azimuth to the limb. This 
limb, having two solid projections, allows the Ys being at some distance over 
the periphery of the circle, by which means the supports to the telescope 
are nearer its extremities. One of these Ys has a short vertical motion, with 
a pushing screw at the side, which fixes it at any height required. The 
telescope itself is of thirty inches focal length, and magnifies about twenty- 
two times, having two thick gun-metal collars, by which the telescope rests 
on and is turned in the Ys. These were quickly worn by the continued 
friction from the supporting part of the Ys, and would have caused an error in 
using the instrument ; I therefore had small thick steel plates, highly polished 
and hardened, dovetailed into each supporting part of the Ys. The friction on 
the collars was now transferred to the pieces of steel ; and although they like- 
wise continually wear, it does not affect the correctness of the instrument in 
any other way, than in altering the adjustment of one of the Ys. (See page 20.) 
At the eye end of the telescope is an adjustment with a rack and pinion, 
for distinct vision, and another by the same means to regulate the distance 
of the eye from the wires : this I found to be indispensable, as the eye occa- 
sionally becomes fatigued, and requires a different focus to view the wires 
distinctly. 
The wires themselves are adjusted as usual, by a sliding piece for azimuth, 
and two pushing screws for vertical motion. 
In the centre of the telescope there are two orifices opposite one another ; 
the one to receive a small lamp, and the other admitting a spindle and specu- 
lum at the end, ground to an angle of 45°, which, by reflection, illuminates the 
wires for night observation. The speculum being made to turn, the quantity 
of reflected light may be regulated at pleasure. 
As the glass would not admit of distinct vision for objects at a less distance 
than one hundred feet, and it being necessary to use the instrument, at times, 
