296 French Royal Academy of Sciences. 
structed by Ramsden, which they had made use of in all their 
preceding operations, and we, on: our part, brought, one of our 
repeating circles. These circles, in themselves, are instruments 
of small dimensions, and of a price compar atively moderate 3 but 
their precision is founded on a principle independent of their 
size, and which consists in the angles of observation being re- 
peated, and replacing each other on the. same circular limb, so 
that the errors which might embarrass them combat each other, 
(if we may be allowed. the phrase,) and mutually destroy each 
other.. A sector, on the contrary, is an expensive instrument, 
and of considerable size ; it is, properly speaking, a long tele- 
scope, placed vertically, and which is capable of deviating some 
degrees to the north as well as to the south of the vertical point, 
in following a circular limb whose divisions measure its devia- 
tion.. Such stars as pass to the meridian near the zenith, may 
thus be observed, and also the quantity which are both north and 
south of the vertical point may be ascertained, This observation 
being repeated on the same stars and- on a great number, at the 
two extremities of an arc of the ¢errestrial meridian, gives the 
breadth of the celestial arc, included between the verticals, pro- 
duced to these two extremities ; ; or, what is the same thing, it 
gives the mutual inclination of these two vertical dines ; which. 
may he ther compared to the length of the terrestrial are, which’ 
they confine. We know what art is necessary in the’ construction 
to establish thus the exact verticality of a telescope twelve feet 
long, and to maintain it invariably either in this position, or in 
the vertical plane which it ought to describe in following its limb. 
But. what we cannot’ conceive without seeing it, is the infinite 
multitude of precautions, cares, and we may say atttentions, 
which the English artist has employed to render the observations 
more exact, anda circumstance of more importance than one 
might imaeing more easy. In the old sectors,—in that, for ex- 
ample, which Clairault, “Lemonier, and Maupertuis employed i in 
the operation of Laponie—the observer was obliged to be hori- 
zontally aboye the telescope, his face turned towards the heavens, 
and in this constrained posture had to wait the transit of the star. 
In the sector of Ramsden, a little mirror, placed at the lower end 
of the telescope, reflects the image of the star in a horizontal di- 
rection ; and the astronomer, conveniently seated, observes it 
with much ease. Moreover, the steadiness of the instrument is 
estimated by the aid of a plumb-line suspended from the top, the’ 
higher extremity of which should always correspond to a mark 
finely traced by the artist at the centre of the rotation of the 
glass.—The proof of this coincidence, which must often be made, - 
was very inconvenient in the ancient sectors, the observer being 
obliged, in order to effect this purpose, to mount on a seaffold 
to 
