REFLECTING CIRCLE. 
objects of the angle to be measured by dis- 
tinct vision, and the other by reflection of 
plane mirrors. The first instrument of this 
kind was invented by Tobias Mayers, in 
1 770, a celebrated astronomer of Gottingen, 
who calculated the lunar and solar tables for 
determining the longitude at sea, for which a 
reward of 3,0001. was given by the board of 
longitude. In making use of these tables he 
found that the Hadley’s quadrants, though 
made by the first artists of that time, were 
not divided with sufficient accuracy for his 
purpose; he therefore contrived the reflect- 
ing and repeating circle ; to comprehend 
which, the reader must turn to our article 
Quadrant, by Hadley, from which this in- 
strument differs principally, in being a whole 
circle of divisions instead of an octant ; 
and is so contrived, that when an observa- 
tion has been made, it is repeated upon a 
fresh portion of the divisions, then a third 
time, a fourth, and so on as many times as is 
necessary ; the observation is then read off, 
and the product is divided by tlie number 
of observations made so as to take a mean 
of the errors there may be in any part of the 
divisions on the circle. This contrivance, 
though useful, was found so tedious, in 
taking so many observations, that it was 
laid aside in favour of the Hadley’s quadrant, 
to which in point of accuracy it was really 
superior. 
For the particular description of this in- 
strument we must refer our readers to a 
work, entitled, “ Tabula Motum Solis et 
Luuae,” by Tobias Mayer, London, 1770. 
This instrument received an improvement 
from the Chevalier de Borda, at Paris, 
which rendered its operation much more 
simple ; but it was not until the year 1796 
that the instrument became much used in 
the British navy, when it was new modelled 
by Mr. Edward Troughton, and the objec- 
tions to the former instruments done away. 
We have obtained permission from this gen- 
tleman to make a drawing of this instrument 
(see Plate Reflecting Circle) where (fig. 1) is 
a plan of the divided side of the instrument, 
(and fig. 2) a perspective view of its upper 
side. A A, in both figures, is a circle of 
brass, with a narrow ring of silyer let into a 
circular groove in it, as is seen in fig. 1 on 
which silver the divisions are made. B B B 
are three arms carrying verniers at their 
ends, they are all cast in one piece, and 
screwed to a truly turned steel axis, fitting 
into a tube, which is screwed to the centre 
of the circle (this tube cannot be seen in 
the pl&n but is denoted by a in fig. 2) the in- 
dex glass, b, which is a plane silvered 
mirror, is fastened to the other end of this 
axis by three screws in such a position that 
the centre line of the steel axis it is fixed to, 
if produced, would exactly coincide with 
the plane of the silvered surface of the mir- 
ror, and consequently that the plane of the 
mirror produced passes through the centre 
of the circle A A, perpendicular to its 
plane. 
To the upper end of the tube, a (fig. 2) a 
crooked plate of brass, d d, is fastened, and 
connects it with two other tubes, e and f, 
whose lower ends are fixed to the cross bar 
frame of the circle, one of these,/) has the 
mirror, h, called the object glass placed on 
it ; the other has a telescope, k, fixed to it, 
directed to the object glass, h. The instru- 
ment is held, when in use, by handles 
adapted to different occasions, of which 
there are four ; two perpendicular and two 
parallel to the plane of the circle ; of the 
latter, e, is one on the upper side, supported 
by a small pillar coming from the intercep- 
tion of two of the bars of the frame, and 
steadied by entering the tube, a, the other 
handle is at the divided side of the circle, 
and is fastened to the circle at the upper 
side, in the. same manner as e ; by a crooked 
hollow tube, G, going round the circle ; of 
the other two handles, one m, is, above the 
circle, screwed into a cock fixed to it, so 
that it is perpendicular to the centre of the 
circle, it comes over the index glass, but 
does not touch it. The other handle, is 
screwed into the handle of the crooked 
tube, G, so as to be in the same line with 
the upper handle m ; o are three dark 
glasses, between the index and horizon 
glasses, turning on a joint, so as to be put 
out of the way when necessary, or any one 
or two of them can be turned in the line of 
the telescope to darken the light, more or 
less, in observations of the sun ; p, are 
three other glasses supported by a small pil- 
lar behind the horizon glass, which can also 
be turned back as is necessary. The tele- 
scope is screwed into a brass ring, r, this is 
supported by a square piece of brass, tapped 
at the corners, so as to form a screw ; and 
by turning, s, a nut upon a screw, the te- 
lescope can be raised or lowered parallel to 
itself ; there is also an adjustment to bring 
the line ofcoHimation of the telescope to be 
parallel to the plan of the circle. 
The circle is divided on the silver ring 
shown in the plan, into 720 parts, each of 
which answers to a degree, as this instru- 
ment measures double the angle shown upon 
