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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 Mélanges de l’école française de Rome, xxv. (1905) 157 seq.  (T. As.) 

CIRCLE (from the Lat. circulus, the diminutive of circus, a ring; the cognate Gr. word is κιρκος, generally used in the form κρίκος), a plane curve definable as the locus of a point which moves so that its distance from a fixed point is constant.

EB1911 Circle Figs 1-4.jpg

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.”

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 the same book demands the possibility of describing a circle for every “centre” and “distance.” Having employed the circle for the construction and demonstration of several propositions in Books I. and II. Euclid devotes his third book entirely to theorems and problems relating to the circle, and certain lines and angles, which he defines in introducing the propositions. The fourth book deals with the circle in its relations to inscribed and circumscribed triangles, quadrilaterals and regular polygons. Reference should be made to the article Geometry: Euclidean, for a detailed summary of the Euclidean treatment, and the elementary properties of the circle.

Analytical Geometry of the Circle.

In the article Geometry: Analytical, it is shown that the general equation to a circle in rectangular Cartesian co-ordinates is x2+y2+2gx+2fy+c=0, i.e. in the general equation of the second degree the co-efficients of x2 and y2 are Cartesian co-ordinates. equal, and of xy zero. The co-ordinates of its centre are –g/c, –f/c; and its radius is (g2+f2c)½. The equations to the chord, tangent and normal are readily derived by the ordinary methods.

Consider the two circles:—

x2+y2+2gx+2fy+c =0, x2+y2+2gx+2fy+c′=0.

Obviously these equations show that the curves intersect in four points, two of which lie on the intersection of the line, 2(gg′)x + 2(ff′)y + cc′ = 0, the radical axis, with the circles, and the other two where the lines x² + y² = (x + iy) (xiy) = 0 (where i = √–1) intersect the circles. The first pair of intersections may be either real or imaginary; we proceed to discuss the second pair.

The equation x² + y² = 0 denotes a pair of perpendicular imaginary lines; it follows, therefore, that circles always intersect in two imaginary points at infinity along these lines, and since the terms x² + y² occur in the equation of every circle, it is seen that all circles pass through two fixed points at infinity. The introduction of these lines and points constitutes a striking achievement in geometry, and from their association with circles they have been named the “circular lines” and “circular points.” Other names for the circular lines are “circulars” or “isotropic lines.” Since the equation to a circle of zero radius is x² + y² = 0, i.e. identical with the circular lines, it follows that this circle consists of a real point and the two imaginary lines; conversely, the circular lines are both a pair of lines and a circle. A further deduction from the principle of continuity follows by considering the intersections of concentric circles. The equations to such circles may be expressed in the form x² + y² = α², x² + y² = β². These equations show that the circles touch where they intersect the lines x² + y² = 0, i.e. concentric circles have double contact at the circular points, the chord of contact being the line at infinity.

In various systems of triangular co-ordinates the equations to circles specially related to the triangle of reference assume comparatively simple forms; consequently they provide elegant algebraical demonstrations of properties concerning a triangle and the circles intimately associated with its geometry. In this article the equations to the more important circles—the circumscribed, inscribed, escribed, self-conjugate—will be given; reference should be made to the article Triangle for the consideration of other circles (nine-point, Brocard, Lemoine, &c.); while in the article Geometry: Analytical, the principles of the different systems are discussed.

The equation to the circumcircle assumes the simple form aβγ + bγα + cαβ = 0, the centre being cos A, cos B, cos C. The inscribed circle is cos ½A √α cos ½B √β + cos ½C √γ = 0, with centre α = β = γ; while the escribed circle opposite the angle A Trilinear co-ordinates. is cos ½A √–α + sin ½B √β + sin ½C √γ = 0, with centre -α = β = γ. The self-conjugate circle is α² sin 2A + β² sin 2B + γ² sin 2C = 0, or the equivalent form a cosA α² + b cos B β² + c cos C γ² = 0, the centre being sec A, sec B, sec C.

The general equation to the circle in trilinear co-ordinates is readily deduced from the fact that the circle is the only curve which intersects the line infinity in the circular points. Consider the equation

aβγ + bγα + Cαβ + (lα + mβ + nγ) (aα + bβ + cγ) = 0  (1).

This obviously represents a conic intersecting the circle aβγ + bγα + cαβ = 0 in points on the common chords lα + mβ + nγ = 0, aα + bβ + cγ = 0. The line lα + mβ + nγ is the radical axis, and since aα + bβ + cγ = 0 is the line infinity, it is obvious that equation (1) represents a conic passing through the circular points, i.e. a circle. If we compare (1) with the general equation of the second degree uα² + vβ² + wγ² + 2uβγ + 2vγα + 2wαβ = 0, it is readily seen that for this equation to represent a circle we must have

kabc = vc² + wb² – 2ubc = wa² + uc² – 2vca = ub² + va² – 2wab.

The corresponding equations in areal co-ordinates are readily derived by substituting x/a, y/b, z/c for α, β, γ respectively in the trilinear equations. The circumcircle is thus seen to be a²yz + b²zx + c²xy = 0, with centre sin 2A, sin 2B, Areal co-ordinates.sin 2C; the inscribed circle is √(x cot ½A) + √(y cot ½B) + √(z cot ½C) = 0, with centre sin A, sin B, sin C; the escribed circle opposite the angle A is √(–x cot ½A) + √(y tan ½B) + √(z tan ½C)=0, with centre – sin A, sin B, sin C; and the self-conjugate circle is x² cot A + y² cot B + z² cot C = 0, with centre tan A, tan B, tan C. Since in areal co-ordinates the line infinity is represented by the equation x + y + z = 0 it is seen that every circle is of the form a²yz + b²zx + c²xy + (lx + my + nz)(x + y + z) = 0. Comparing this equation with ux² + vy² + wz² + 2uyz + 2vzx + 2wxy = 0, we obtain as the condition for the general equation of the second degree to represent a circle:—

(v + w – 2u′)/a² = (w + u – 2v′)/b² = (u + v – 2w′)/c².

In tangential (p, q, r) co-ordinates the inscribed circle has for its equation (sa)qr + (sb)rp + (sc)pq = 0, s being equal to ½(a + b + c); an alternative form is qr cot ½A + rp cot ½B + pq cot ½C = 0; the centre is ap + bq + cr = 0, or p sin A + q sin B + r sin C = 0. Tangential co-ordinates.The escribed circle opposite the angle A is –sqr + (sc)rp + (sb)pq = 0 or –qr cot ½A + rp tan ½B + pq tan ½C = 0, with centre –ap + bq + cr = 0. The circumcircle is a √(p) + b √(q) + c √(r) = 0, the centre being p sin 2A + q sin 2B + r sin 2C = 0. The general equation to a circle in this system of co-ordinates is deduced as follows: If ρ be the radius and lp + mq + nr = 0 the centre, we have ρ=(lp1mq1+nr1)/(l+m+n), in which p1, q1, r1 is a line distant ρ from the point lp+mq+nr=0. Making this equation homogeneous