observation of distant stations in geodcetical operations. 327 
in its construction, and easy of management. Fig. 1, Plate 
XII. represents an instrument contrived with this view, 
which was employed last year in Ireland with much advan- 
tage. ab is a telescope of 12 inches focal length, and serves 
as the axis of the instrument ; the bars bd and be form a 
right angle ; and the bar gg, placed so that bf (fig. 2), shall be 
equal to fg, works between bd and be, carrying a small tele- 
scope, such as is usually attached to sextants, and provided 
with a rectangular eye-piece. The mirror mm , of which 
different sizes may be used according to circumstances, is con- 
nected with the instrument by three adjusting screws r. The 
bars be, b'c', being now made to coincide with ab, a moveable 
spirit level is placed across them in the position ll, fig. 2, 
and rendered horizontal by the foot screws ; by the same 
means the axis ab, to which a level is permanently attached, 
is also brought into a horizontal position. The moveable 
spirit level being now transferred to the surface of the mirror, 
the three adjusting screws r, are employed to render it 
horizontal. The mirror will then be parallel to ab and ll, 
and will have the required position on the instrument. The 
telescope ab being now directed upon the object to which the 
reflection is to be thrown, and the small telescope gg turned 
towards the sun, its rays will then be reflected parallel to 
the axis of the instrument ab. The head of the screw R, fig. 
2, is graduated, so that by means of it and the spirit level at- 
tached to the axis, the required elevation or depression may 
be given to the instrument when the object towards which it 
is directed happens to be invisible, its direction only being 
known relatively to some nearer object ; and which, it may 
be remarked, has been the case in every instance in which it 
