an improved Reflecting Circle . 369 
sentations, in which the same parts are marked with the same 
letters, are sufficient to give an accurate idea of the arrange- 
ment of the whole, and make it unnecessary for me to enter 
now into a minute detail of the mechanism of 'the apparatus. 
I therefore shall content myself with adding here only what 
concerns the general use, and the peculiar properties, of this 
instrument. 
M is the divided limb of the circle, and N the Flying Nonius, 
(Plates XXX. XXXI. and XXXII.) to each of which the 
horizon index may be occasionally attached, by means of the 
clamps D, C ; as well as the centre index, by means- of the 
other clamps A, B. The peculiar property of the instrument 
being that of giving double the distance, I have thought proper 
to divide the circle into 360 degrees, and not into 720 accord- 
ing to the nature of the Sextant. Thus, after a crossed obser- 
vation, the reading of the Nonius will, without reduction, exhibit 
the measure of the simple distance. I have likewise extended 
the Nonius round the circumference* so that, by the coincidence 
of two divisions, the number of degrees will appear on the 
limb, and that of the minutes and seconds on the Flying Nonius. 
The manner of making the observations with this instrument 
is as follows. 
recommended to him this improvement, which he has adapted to his Reflecting 
Circle. 
I shall observe here, that Mr. Troughton’s Circles ar v e not of the kind which I 
have endeavoured to improve. The scheme of his construction may be said to consist 
in completing the limb of a Sextant to the whole circumference, and making it capable 
thereby of performing Bor da’s crossed observations, with as many Noniuses as may 
be attached to the centre glass. But, Mr. Troughton’s instrument is deprived of 
the principle invented by Mayer, for obtaining at one reading a multiple of the 
distance required ; which is the great property of Circles, and, in my opinion, the best 
means of diminishing their errors. 
MDCCCI. 3 B 
