h great probability to a temple of
Venus or Circe) is of remarkable beauty; the whole mountain is covered
with fragrant shrubs. From any point in the Pomptine Marshes or on the
coast-line of Latium the Circeian promontory dominates the landscape in
the most remarkable way.

See T. Ashby, "Monte Circeo," in _Melanges de l'ecole francaise de
Rome_, XXV. (1905) 157 seq. (T. As.)

CIRCLE (from the Lat. _circulus_, the diminutive of _circus_, a ring;
the cognate Gr. word is [Greek: kirkos], generally used in the form
[Greek: krikos]), a plane curve definable as the locus of a point which
moves so that its distance from a fixed point is constant.

The form of a circle is familiar to all; and we proceed to define
certain lines, points, &c., which constantly occur in studying its
geometry. The fixed point in the preceding definition is termed the
"centre" (C in fig. 1); the constant distance, e.g. CG, the "radius."
The curve itself is sometimes termed the "circumference." Any line
through the centre and terminated at both extremities by the curve, e.g.
AB, is a "diameter"; any other line similarly terminated, e.g. EF, a
"chord." Any line drawn from an external point to cut the circle in two
points, e.g. DEF, is termed a "secant"; if it touches the circle, e.g.
DG, it is a "tangent." Any portion of the circumference terminated by
two points, e.g. AD (fig. 2), is termed an "arc"; and the plane figure
enclosed by a chord and arc, e.g. ABD, is termed a "segment"; if the
chord be a diameter, the segment is termed a "semicircle." The figure
included by two radii and an arc is a "sector," e.g. ECF (fig. 2).
"Concentric circles" are, as the name obviously shows, circles having
the same centre; the figure enclosed by the circumferences of two
concentric circles is an "annulus" (fig. 3), and of two non-concentric
circles a "lune," the shaded portions in fig. 4; the clear figure is
sometimes termed a "lens."

[Illustration: FIG. 1]

[Illustration: FIG. 2]

[Illustration: FIG. 3]

[Illustration: FIG. 4]

The circle was undoubtedly known to the early civilizations, its
simplicity specially recommending it as an object for study. Euclid
defines it (Book I. def. 15) as a "plane figure enclosed by one line,
all the straight lines drawn to which from one point within the figure
are equal to one another." In the succeeding three definitions the
centre, diameter and the semicircle are defined, while the third
postulate of