CIR 
given against him in the Common Pleas, 
this judgment shall bind him, for the land is 
not exempted out of the county, and the 
tenant may wave the benefit of his privi- 
lege. These five ports are Dover, Has- 
tings, Romney, Hytlie, and Sandwich ; to 
which Winchelsea and Rye have been since 
added. 
CIPHER. See Cypher. 
CIRC^ A, in botany, a genus of the Dian- 
dria Monogynia class and order. Natural 
order of Aggregatae. Onagrae, Jussieu. 
Essential character: corolla two-petalled ; 
calyx two-leaved, superior; seed one, two- 
celled. There are two species ; viz. C. lu- 
tetiana, common enchanter’s nightshade, 
and C. alpina, mountain enchanter’s night- 
shade. 
CIRCLE, the name of various astrono- 
mical instruments for observing right ascen- 
sions, declinations, azimuths, altitudes, and 
likewise for the purposes of the most im- 
proved theodolite. 
Plate “ Circular Instrument” is a represen- 
tation of an instrument made by Mr.'Trough- 
ton, and of which he liberally permitted our 
draughtsman to take a drawing. It is an in- 
strument which measures both horizontal 
and vertical angles with great accuracy, and 
is equally, adapted for astronomical purposes 
and surveying. 
The instrument is supported on three 
screws, two of which, x, y, are shewn in the 
figure; the. three arms through which these 
pass meet in the centre, and hold a strong, 
vertical steel axis, truly turned, and very 
exactly fitted into two sockets, one at the 
top and the other at the bottom of a cone, 
A : upon this axis the upper part of the in- 
strument turns. B is the azimuth circle 
laying upon the tlnee arms of the tripod, 
and capable of turning round on the steel 
axis before mentioned : it is held by a screw, 
g, which moves the circle slowly round 
when turned ; this motion is to adjust the 
circle, so that the plane of the verticle circle, 
P, shall be in the meridian when the index 
is set to zero. The circle is divided into 
degrees and every five minutes, and the 
microscope subdivides them into seconds. 
Anotlier similar microscope is fixed diame- 
trically opposite, upon the circular plate H, 
and tiuns round upon the vertical axis with 
the rest of the instrument. (For the con- 
structions of these microscopes see that ar- 
ticle.) I, I, are two hollow conical pillars, 
screwed on the index plate to support the 
axis of the vertical circle, P, by means of 
two bars (one only of which can be seen, 7t), 
VOL. II. 
t CIR 
screwed at the top of the pillars, and hold- 
ing at their outer ends tubes, which contain 
angular bearings for the pivots of the axis ; 
these bearings, or Y’s, as they are called, 
from resembling that letter, can be elevated 
or depressed by screws, e, beneath them, to 
bring the axis parallel to the plane of the 
azimuth circle, m, m, are two crooked hol- 
low tubes, screwed to tire upright pillars, 
holding two microscopes, n, n ; reading divi- 
sions diametrically opposite to each other 
on the vertical circle P. 'Fhe vertical circle 
is composed of two circles, each cut from a 
solid plate, and attached to two flanches on 
a hollow conical axis E ; they are firmly 
braced togetlier by short pillars, as in the 
figure ; between the circles the telescope F is 
fixed, it is 30 inches long and 2 in diameter. 
O is a thin plate of metal, screwed to the 
further main pillar, I, by its lower end, and 
its upper end supporting a clamp for fixing 
the circle when set at any elevation, and a 
screw for moving it slowly a small quantity 
after clamping. A similar screw, for occa- 
sionally attaching the index plate, H, to the 
azimuth circle, B, is seen at p. a is a small 
roller, pushed upwards by a spring, I; it 
acts against a ring upon tlie conical axis 
E, and its use is to support part of the 
weight of the-circle and telescope, and take 
the bearing from the pivots at the end of 
the axis. R is a spirit level hung to the two 
horns m, m, and adjustable by a screw at its 
end. S is a telescope beneath the instru- 
ment, which is set to any distant object 
when the instrument is in use, and serves to 
shew that the instrument does not change 
its position. See Observatory and Sur- 
veying. 
Circle, in geometry, a plane figure 
comprehended by a single curve line, called 
its circumference, to which right lines, or 
radii, drawn from a point in the middle, 
called the centre, are equal to each Otlier. 
The' area of a circle is found by multi- 
plying the circumference by the fourth part 
of the diameter, or half the circumference 
by half the diameter: for every circle may 
be conceived to be a polygon of ain infinite 
number of sides, and the semidiameter 
must be equal to the perpendicular of such 
a polygon, and the circumference of the 
circle equal to the periphery of the poly- 
gon : tlierefore half the circumference mul- 
tiplied by half the diameter, gives the area 
of the circle. 
Circles, and similar figures inscribed in 
them, ai-e always as the squares of the dia- 
meters; so that they are in a duplicate 
O 
