Astronomical Circles and other Instruments, 421 
of silver or gold, on which is a well defined circular dot, is 
soldered to the lower surface of efy beneath, and projecting 
from e : the lower surfaces of this tongue, of the piece ef, and 
of Uy a, j, Uy are all in the same plane. It is evident from this 
construction, that the tongue with the circular dot is moveable 
on the surface of the limb of the instrument, both to and from 
its centre, and in a direction at right angles to 'the radius. 
The screws d, and^, are to be turned by a key with a handle 
of wood or ivory, in order to avoid any change of temperature 
which might arise from the near approach of the hand; the 
sliding pieces should move easily, and yet be perfectly free 
from shake. 
There is another mode of constructing the adjustable pieces, 
which I conceive may be found far preferable to that I have 
just described, though in the use of them greater care and 
nicety will be required. It is to draw two fine lines at right 
angles to each other on the silver tongue of the piece fig. 1. * 
which lines may be used instead of the dot, by causing them 
to bisect the angles formed by the cross wires of the micro- 
scopes, and thus ensuring, as far as vision is concerned, as great 
a degree of accuracy in dividing as can afterwards be attained 
in reading ofi. 
The line in the direction of the piece ef, must be drawn 
when the adjustable piece is fixed on a circle, and when the 
piece cCy is placed about half way between the limits of its 
adjustment; in which }x)sition this line will be directed towards 
the centre of the instrument, and, unless the circle to be 
divided is very small, the difference from this direction within 
the limits of its adjustments will scarcely become sensible. 
This will be better understood when the manner of using the 
MDCCcxiv. 3 I 
