CIRCEIUS MONS (mod. Monte Circeo), an isolated promontory on the S.W. coast of Italy, about 8o m. S.E. of Rome. It is a ridge of limestone about 31 M. long by 1 m. wide at the base, running from E. to W. and surrounded by the sea on all sides except the N. The land to the N. of it is 53 ft. above sealevel, while the summit of the promontory is 1775 ft. The origin of the name is uncertain: it has naturally been connected with the legend of Circe, and Victor Berard (in Les Pheniciens et l'Odyssee, ii. 261 seq.) maintains in support of the identification that Main, the Greek name for the island of Circe, is a faithful transliteration of a Semitic name, meaning " island of the hawk," of which vijaos Kipro s is the translation. The difficulty has been raised, especially by geologists, that the promontory ceased to be an island at a period considerably before the time of Homer; but Procopius very truly remarked that the promontory has all the appearance of an island until one is actually upon it. Upon the E. end of the ridge of the promontory are the remains of an enceinte, forming roughly a rectangle of about 200 by roo yds. of very fine polygonal work, on the outside, the blocks being very carefully cut and jointed and right angles being intentionally avoided. The wall stands almost entirely free, as at Arpinum —polygonal walls in Italy are as a rule embanking walls—and increases considerably in thickness as it descends. The blocks of the inner face are much less carefully worked both here and at Arpinum. It seems to have been an acropolis, and contains no traces of buildings, except for a subterranean cistern, circular, with a beehive roof of converging blocks. The modern village of S. Felice Circeo seems to occupy the site of the ancient town, the citadel of which stood on the mountain top, for its medieval walls rest upon ancient walls of Cyclopean work of less careful construction than those of the citadel, and enclosing an area of 200 by 150 yds.
Circei was founded as a Roman colony at an early date—according to some authorities in the time of Tarquinius Superbus, but more probably about 390 B.C. The existence of a previous population, however, is very likely indicated by the revolt of Circei in the middle of the 4th century B.C., so that it is doubtful whether the walls described are to be attributed to the Romans or the earlier Volscian inhabitants. At the end of the republic, however, or at latest at the beginning of the imperial period, the city of Circei was no longer at the E. end of the promontory, but on the E. shores of the Lago di Paola (a lagoon—now a considerable fishery—separated from the sea by a Iine of sandhills and connected with it by a channel of Roman date: Strabo speaks of it as a small harbour) one mile N. of the W. end of the promontory. Here are the remains of a Roman town, belonging to the 1st and 2nd centuries, extending over an area of some 600 by 500 yards, and consisting of fine buildings along the lagoons, including a large open piscina or basin, surrounded by a double portico, while farther inland are several very large and wellpreserved waterreservoirs, supplied by an aqueduct of which traces may still be seen. An inscription speaks of an amphitheatre, of which no remains are visible. The transference of the city did not, however, mean the abandonment of the E. end of the promontory, on which stand the remains of several very large villas. An inscription, indeed, cut in the rock near S. Felice, speaks of this part of the promunturium Veneris (the only case of the use of this name) as belonging to the city of Circei. On the S. and N. sides of the promontory there are comparatively few buildings, while, at the W. end there is a sheer precipice to the sea. The town only acquired municipal rights after the Social War, and was a place of little importance, except as a seaside resort. For its villas Cicero compares it with Antium, and probably both Tiberius and Domitian possessed residences there. The beetroot and oysters of Circei had a certain reputation. The view from the highest summit of the promontory (which is occupied by ruins of a platform attributed with 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 coastline of Latium the Circeiali promontory dominates the landscape in the most remarkable way.
See T. Ashby, " Monte Circeo," in Melanges de l'ecole frangaise de Rome, xxv. (1905) 157 seq. (T. As.) CIRCLE (from the Lat. circulus, the diminutive of circus, a ring; the cognate Gr. word is KipKOS, generally used in the form Kp1KOS), 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. I); 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 ";
CIRCLE
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 noncon
centric 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 coordinates
is x2+y2+2gx+2fy+c=o, i.e. in the general equation
Cartesian of the second degree the coefficients of x2 and y2 are
coordi
nates. equal, and of xy zero. The coordinates of its centre
are —g/c, —f/c; and its radius is (g2+f2c)I. The equations to the chord, tangent and normal are readily derived by the ordinary methods.
Consider the two circles:
xi +y2 +2gx+zfy +c =0, x2+y2+2g'x+2f'y+c, =o.Obviously these equations show that the curves intersect in four points, two of which lie on the intersection of the line, 2(g—g')x+2(ff')y+c—c'=o, the radical axis, with the circles, and the other two where the lines x2hy2=(x+iy) (x—iy)=o (where i= —I) 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 =o 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 =o, 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 =a2, x2+y2 = 92. 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 coordinates 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, selfconjugate—will be given; reference should be made to the article TRIANGLE for the consideration of other circles (ninepoint, 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 ¢By+bya+ca/3=o, the centre being cos A, cos B, cos C. The inscribed circle is cos ;A v a+Cos IB sl /i+cos IC al y = o, with centre Trmnear a =13=7; while the escribed circle opposite the angle A coordiis cos IAA/ —a+sin iBVll3+sin zCdy=o, with centre mates. —a=/3=y. The selfconjugate circle is a2 sin 2A + /32 sin 2B +y2 sin 2C = o, or the equivalent form acosAa2 +bcosB$2+acosCy2 = o, the centre being sec A, sec B, sec C.
The general equation to the circle in trilinear coordinates 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
a$7+bya+cal+(la+'m#+ny) (aa+bQ+c?') =0 (1).
This obviously represents a conic intersecting the circle a$7+bya +a/3=o in points on the common chords la+m$+ny=o, aa+b$ +cy=o. The line la+m$3+ny is the radical axis, and since as+bf +cy=o is the line infinity, it is obvious that equation (I) represents a conic passing through the circular points, i.e. a circle. If we compare (I) with the general equation of the second degree ua2+v/i2+w72+2u'/3y+2v ya+2w'a$=o, it is readily seen that for this equation to represent a circle we must have
— kabc = vc2+wb2 2u'bc = wag+uc'  2v'ca = ub2 +vat 2w'ab. The corresponding equations in areal coordinates are readily derived by substituting x/a, y/b, z/c for a, /3, y respectively in
the trilinear equations. The circumcircle is thus seen Areal
to be a2yz+b2zx+c2xy=o, with centre sin 2A, sin 2B, coordl
sin 2C; the inscribed circle is 1/ (x cot ;A)+'/ (y cot 2B) mates. +'d (z cot 3C) =o, with centre sin A, sin B, sin C;. the
escribed circle opposite the angle A is al (—x cot %A)+ (y tan 2B) + (z tan IC) =o, with centre —sin A, sin B, sin C; and the selfconjugate circle is x2cot A+y2 cot B+z2 cot C = o, with centre tan A, tan B, tan C. Since in areal coordinates the line infinity is represented by the equation x+y+z=o it is seen that every circle is of the form a2yz.+b2zx{c2xy+ (lx+my+nz) (x+v+z) = o. Comparing this equation with ux2+vy2+wz2+2u'yz+2v'zx+2w'xy=o, 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+v2w')/c2.
• In tangential (p, q, r) coordinates the inscribed circle has for its equation (s — a) qr + (s — b) rp + (s — c)pq = o, s being equal to %(a+b +c) ; an alternative form is qr cot 1A+rp cot lB+pq cot5C =o; Tangential the centre is ap+bq+cr =o, or p sinA+q sin B +rsinC = o. coordi
The escribed circle opposite the angle A is —sqr+(s—c)rp nates. + (s—b)pq =oor—grcot IA+rptan iB+pgtan IC = o,with
centre —ap+bq+cr=o. The circumcircle is ail p+bslqq+cs/r=o, the centre being p sin 2A+q sin 2B+r sin 2C =o. The general equation to a circle in this system of coordinates is deduced as follows: If p be the radius and 1p+my+nr=o the centre, we have p(lpi+mqi+nri)/(l+m+n), in which r, is a line distant p from the point 1p+mq+nr=o. Making this equation homogeneous
bythe relation Ea2(p—q) (p —r) =4A2 (see GEOMETRY: Analytical, which is generally written la bq, cr}2we obtain lap, bq, cr}2p2=4p21(lp+mq+nr)/(l+m+n)}2, the accents being dropped, and p, q, r regarded as current coordinates. This equation, which may be more conveniently written lap, bq, cr}2 =(Xp+~2q+vr)2, obviously represents a circle, the centre being Xp+iq+vr=o, and radius 20/(X+µ+v). If we make )=,u =p=o, p is infinite, and we obtain lap, bq, cr}2=o as the equation. to the circular points.
Systems of Circles.
Centres and Circle of Similitude.—The " centres of similitude " of two circles may be defined as the intersections of the common tangents to the two circles, the direct common tangents giving rise to the " external centre," the transverse tangents to the " internal centre." It may be readily shown that the external ' and internal centres are the points where the line joining the centres of the two circles is divided externally and internally in the ratio of their radii.
The circle on the line joining the internal and external centres of similitude as diameter is named the " circle of similitude." It may be shown to be the locus of the vertex of the triangle which has for its base the distance between the centres of the circles and the ratio of the remaining sides equal to the ratio of the radii of the two circles.
With a system of three circles it is readily seen that there are six centres of similitude, viz. two for each pair of circles, and it may be shown that these lie three by three on four lines, named the " axes of similitude." The collinear centres are the three sets of. one external and two internal centres, and the three external centres.
Coaxal Circles —A system of circles is coaxal when the locus of points from which tangents to the circles are equal is a straighf line. Consider the case of two circles, and in the first place suppose them to intersect in two real points A and B. Then by Euclid iii. 36 it is seen that the line joining the points A and B is the locus of the intersection of equal tangents, for if P be any point on AB and PC and PD the tangents to the circles, then PA•PB = PC2 =PD2, and therefore PC = PD. Furthermore it is seen that AB is perpendicular to the line joining the centres, and divides it in the ratio of the squares of the radii. The line AB is termed the " radical axis." A system coaxal with the two given circles is readily constructed by describing circles through the common points on the radical axis and any third point; the minimum circle of the system is obviously that which has the common chord of intersection for diameter, the maximum
is the radical axis—considered as a circle of infinite radius. In the case of two nonintersecting circles it may be shown that the radical axis has the same metrical relations to the line of centres.
There are several methods of constructing the radical axis in this case. One of the simplest is: Let P and P' (fig. 5) be the points of contact of a common tangent; ,drop perpendiculars PL, P'L', from P and P' to 00', the line joining the centres,
then the radical axis bisects LL' (at X) and is perpendicular to 00'. To prove this let AB, AB' be the tangents from any point on the line AX. Then by Euc. i. 47, AB2=AO2—OB2=AX2+OX2—OP2; and OX2=0D2—DX2=0P2+PD2—DX2. Therefore AB2=AX2 —DX2+PD2. Similarly AB'2=AX2—DX2+DP'2. Since PD = PD', it follows that AB=AB'.
To construct circles coaxal with the two given circles, draw the tangent, say XR, from X, the point where the radical axis intersects the line of centres, to one of the given circles, and with centre X and radius XR describe a circle. Then circles having the intersections of tangents to this circle and the line of centres for centres, and the lengths of the tangents as radii, are members of the coaxal system.
In the case of nonintersecting circled, it is seen that the minimum circles of the coaxal system are a pair of points I and I', where the orthogonal circle to the system intersects the line of centres; these points are named the " limiting points." In the case of a coaxal system having real points of intersection the limiting points are imaginary. Analytically, the Cartesianequation to a coaxal system can be written in the form x2+y2+tax=k2=o; where a varies from member to member, while k is a constant. The radical axis is x=o, and it may be shown that the length of the tangent from a point (o, h) is h2 t k2, i.e. it is independent of a, and therefore of any particular member of the system. The circles intersect in real or imaginary points according to the lower or upper sign of k2, and the limiting points are real for the upper sign and imaginary for the lower sign.
The fundamental properties of coaxal systems may be summarized:
1. The centres of circles forming a coaxal system are collinear;
2. A coaxal system having real points of intersection has imaginary limiting points;
A coaxal system having imaginary points of intersection has real limiting points;
Every circle through the limiting points cuts all circles of the system orthogonally;
The limiting points are inverse points for every circle of the system.
The theory of centres of similitude and coaxal circles affords elegant demonstrations of the famous problem: To describe a circle to touch three given circles. This problem, also termed the " Apollonian problem," was demonstrated with the aid of conic sections by Apollonius in his book on Contacts or Tangencies; geometrical solutions involving the conic sections were also given by Adrianus Romanus, Vieta, Newton and others. The earliest analytical solution appears to have been given by the princess Elizabeth, a pupil of Descartes and daughter of Frederick V. John Casey, professor of mathematics at the Catholic university of Dublin, has given elementary demonstrations founded on the theory of similitude and coaxal circles which are reproduced in his Sequel to Euclid; an analytical solution by Gergonne is given in Salmon's Conic Sections. Here we may notice that there are eight circles which solve the problem.
Mensuration of the Circle.
All exact relations pertaining to the mensuration of the circle involve the ratio of the circumference to the diameter. This ratio, invariably denoted by 7r, is constant for all circles, but it does not admit of exact arithmetical expression, being of the nature of an incommensurable number. Very early in the history of geometry it was known that the circumference and area of a circle of radius r could be expressed in the forms 27rr and 7rr2. The exact geometrical evaluation of the second quantity, viz. 7r2, which, in reality, is equivalent to determining a square equal in area to a circle, engaged the attention of mathematicians for many centuries. The history of these attempts, together with modern contributions to our knowledge of the value and nature of the number in, is given below (Squaring of the Circle).
The following table gives the values of this constant and several exist essions involving it :
Number. Logarithm. Number. Logarithm.
rr 3'1415927 0'4971499 +r2 9'8696044 0'9942997
2a 6.2831853 0'7981799 1
4+r 12.5663706 1.0992099 0'0168869 2'2275490
3 r 1'5707963 0'1961199 6"2
to 1.0471976 0:020 286  11724539 0.2485750
in 0'7853982 3.8950889
1 r 0'5235988 37189986 1'4645919 0'1657166
an. 0.3926991 1'5940599
n 0'2617994 3.4179686 , 1 0.5641896 11514251
4.1887902 0.6220886 y ,r
180 0'0174533 2'2418774 y 11283792 0'0524551
1 0.3183099 3.5028501 2y 0'2820948 1'4503951
s 1'2732395 0'1049101 1'2407010 0'0936671
1 0'0795775 1'9007901 4n 0'6.1)3505 3 7926371
4,r
180 57'2957795 1'7581226 loge a 1 1447299 0'0687030
Useful fractional approximations are 22/7 and 355/113.
A synopsis of the leading formula connected with the circle will now be given.
1. Circle.—Data: radius=a. Circumference =27ra. Area = 7a'.
2. Arc and Sector.—Data: radius=a; e=circular measure of angle subtended at centre by arc ; c = chord of arc; c2 = chord of semiarc; c4 =chord of quarterarc.
,aodla
%IW
3. 4. 5.
Exact formulae are:—Arc = aO, where B may be given directly, or indirectly by the relation c =za sin a9. Area of sector = za26 =a radiusXarc.
Approximate formulae are :—Arc = 4(8c2 —c) (Huygen's formula) ; arc =4'z (c—4002+256c4).
•3. Segment.—Data: a, B, c, c2, as in (2); h=height of segment, i.e. distance of midpoint of arc from chord.
Exact formulae are: —Area =4a2(0—sin 0)=2a20—4c2 cot ZB = lag — ZC I/ (a2 If h be given, we can use c2+4h2 = 8ah, 2h =c tan 48 to determine B.
Approximate formulae are :—Area = 116 (6c+8c2)h; = a Al (c2+ bh2)•h; = 15 (7c+3a)h, a being the true length of the arc.
From these results the mensuration of any figure bounded by circular arcs and straight lines can be determined, e.g. the area of a lune or meniscus is expressible as the difference or sum of two segments, and the circumference as the sum of two arcs. (C. E.*)
Squaring of the Circle.
The problemof finding a square equal in area to a given circle, like all problems, may be increased in difficulty by the imposition of restrictions; consequently under the designation there may be embraced quite a variety of geometrical problems. It has to be noted, however, that, when the " squaring " of the circle is especially spoken of, it is almost always tacitly assumed that the restrictions are those of the Euclidean geometry.
Since the area of a circle equals that of the rectilineal triangle whose base has the same length as the circumference and whose altitude equals the radius (Archimedes, Khe)tov JdTp'70'es, prop.l), it follows that, if a straight line could be drawn equal in length to the circumference, the required square could be found by an ordinary Euclidean construction; also, it is evident that, conversely, if a square equal in area to the circle could be obtained it would be possible to draw a straight line equal to the circumference. Rectification and quadrature of the circle have thus been, since the time of Archimedes at least, practically identical problems. Again, since the circumferences of circles are proportional to their diameters—a proposition assumed to be true from the dawn almost of practical geometry—the rectification of the circle is seen to be transformable into finding the ratio of the circumference to the diameter. This correlative numerical problem and the two purely geometrical problems are inseparably connected historically.
Probably the earliest value for the ratio was 3. It was so among the Jews (1 Kings vii. 23, 26), the Babylonians (Oppert, Journ. asiatique, August 1872, October 1874), the Chinese (Biot, Journ. asiatique, June 1841), and probably also the Greeks. Among the ancient Egyptians, as would appear from a calculation in the Rhind papyrus, the number (A)4, i.e. 3.1605, was at one time in use .l The first attempts to solve the purely geometrical problem appear to have been made by the Greeks (Anaxagoras, &c.)2, one of whom, Hippocrates, doubtless raised hopes of a solution by his quadrature of the socalled meniscoi or lune.3
[The Greeks were in possession of several relations pertaining to the quadrature of the lune. The following are among the more interesting. In fig. 6, ABC is an isosceles triangle right D
C
angled at C, ADB is the semicircle described on AB as diameter, AEB the circular arc described with centre C and radius CA= CB. It is easily shown that the areas of the lune ADBEA and the triangle ABC are equal. In fig. 7, ABC is any triangle
r Eisenlohr, Ein math. Handbuck d. alien Agypter, cabers. u. erkldrt (Leipzig, 1877) ; Rodet, Bull. de la Soc. Math. de France, vi.
PP. 139149•
2 H. Hankel, Zur Gesch. d. Math. im Alterthum, &c., chap. v
(Leipzig, 1874) ; M. Cantor, Vorlesungen 'fiber Gesch. d. Math. i. (Leipzig, 1880) ; Tannery,Mem. de la Soc ., &'c., a Bordeaux; Allman, in Hermathena.
2 Tannery. Bull. des sc. math. [2], x. pp. 213226.right angled at C, semicircles are described on the three sides, thus forming two lunes AFCDA and CGBEC. The sum of the areas of these lunes equals the area of the triangle ABC.]
As for Euclid, it is sufficient to recall the facts that the original author of prop. 8 of book iv. had strict proof of the ratio being <4, and the author of prop. 15 of the ratio being >3, and to direct attention to the importance of book x. on incommensurables and props. 2 and 16 of book xii., viz. that " circles are to one another as the squares on their diameters " and that " in the greater of two concentric circles a regular zngon can be inscribed which shall not meet the circumference of the less," however nearly equal the circles may be.
With Archimedes (287—212 B.C.) a notable advance was made. Taking the circumference as intermediate between the perimeters of the inscribed and the circumscribed regular ngons, he showed that, the radius of the circle being given and the perimeter of some particular circumscribed regular polygon obtainable, the perimeter of the circumscribed regular polygon of double the number of sides could be calculated; that the like was true of the inscribed polygons; and that consequently a means was thus afforded of approximating to the
circumference of the circle. As a matter of fact, he started with a semiside AB of a circumscribed regular hexagon meeting the circle in B (see fig. 8), joined A and B with 0 the centre, bisected the angle AOB by
OD, so that BD became the semiside of a circumscribed regular
r 2gon; then as AB : BO: OA :: 1 : Al 3: 2 he sought an ap
proximation to A13 and found that AB: BO>153:265. Next
he applied his theorem4 BO+OA : AB: : OB : BD to calculate BD; from this in turn he calculated the semisides of the circumscribed regular 24gon, 48gon and 96gon, and so finally established for the circumscribed regular 96gon that perimeter : diameter <34r :1. In a quite analogous manner he proved for the inscribed regular 96gon that perimeter : diameter >3+l
The conclusion from these therefore was that the ratio of circumference to diameter is <3 7 and >3Pl. This is a most notable piece of work; the immature condition of arithmetic at the time was the only real obstacle preventing the evaluation of the ratio to any degree of accuracy whatever.5
No advance of any importance was made upon the achievement of Archimedes until after the revival of learning. His immediate successors may have used his method to attain a greater degree of accuracy, but there is very little evidence pointing in this direction. Ptolemy (fl. 127151), in the Great Syntaxis, gives 3.141552 as the ratios; and the Hindus (c. A.D. 500), who were very probably indebted to the Greeks, used 62832/20000, that is, the now familiar 3.1416.7
It was not until the 15th century that attention in Europe began to be once more directed to the subject, and after the resuscitation a considerable length of time elapsed before any progress was made. The first advance in accuracy was due to a certain Adrian, son of Anthony, a native of Metz (1527), and father of the betterknown Adrian Metius of Alkmaar. In refutation of Duchesne(Van der Eycke), he showed that the ratio was <3 °b and >31105g, and thence made the exceedingly lucky step of taking a mean between the two by the quite unjustifiable process of halving the sum of the two numerators for a new numerator and halving the sum of the two denominators for a new denominator, thus arriving at the now wellknown ap
proximation 3M or 1 , which, being equal to 3.1415929. • •, is correct to the sixth fractional place. 8
4 In modern trigonometrical notation, I+sec B : tame :: 1: tan +B.
5 Tannery, " Sur la mesure du cercle d'Archimede," in Mem.... Bordeaux [2], iv. pp. 313339 ; Menge, Des Archimedes Kreismessung (Coblenz, 1874).
6 De Morgan, in Penny Cyclop, xix. p. 186.
7 Kern, Aryabhattiyam (Leiden, 1874), trans. by Rodet (Paris, 1879).
8 De Morgan, art. " Quadrature of the Circle,"in English Cyclop. ; Glaisher, Mess. of Math. ii. pp. 119128, iii. pp. 2746; de Haan, Nicuw Archief v. Wisk. i. pp. 7086, 206211.
The next to advance the calculation was Francisco Vieta. By finding the perimeter of the inscribed and that of the circumscribed regular polygon of 393216 (i.e. 6X216) sides, he proved that the ratio was > 3.1415926535 and <3.14159z6537, so that its value became known (in 1579) correctly to so fractional places. The theorem for anglebisection which Vieta used was not that of Archimedes, but that which would now appear in the form 1 —COS 0 = 2 sine 20. With Vieta, by reason of the advance in arithmetic, the style of treatment becomes more strictly trigonometrical; indeed, the Universales Inspectiones, in which the calculation occurs, would now be called plane and spherical trigonometry, and the accompanying Canon mathematicus a table of sines, tangents and secants .l Further, in comparing the labours of Archimedes and Vieta, the effect of increased power of symbolical expression is very noticeable. Archimedes's process of unending cycles of arithmetical operations could at best have been expressed in his time by a "rule" in words; in the 16th century it could be condensed into a " formula." Accordingly, we find in Vieta a formula for the ratio of diameter to circumference, viz. the interminate product 2
From this point onwards, therefore, no knowledge whatever of geometry was necessary in any one who aspired to determine the ratio to any required degree of accuracy; the problem being reduced to an arithmetical computation. Thus in connexion with the subject a genus of workers became possible who may be styled " 7computers or circlesquarers "—a name which, if it connotes anything uncomplimentary, does so because of the almost entirely fruitless character of their labours. Passing over Adriaan van Roomen (Adrianus Romanus) of Louvain, who published the value of the ratio correct to 15 places in his Idea mathematica (1593),3 we come to the notable computer Ludolph van Ceulen (d. 161o), a native of Germany, long resident in Holland. His book, Van den Circkel (Delft, 1596), gave the ratio correct to 20 places, but he continued his calculations as long as he lived, and his best result was published on his tombstone in St Peter's church, Leiden. The inscription, which is not known to be now in existence,' is in part as follows:
. Qui in vita sua multo labore circumferentiae circuli proximam rationem ad diametrum invenit sequentem
quando diameter est I
turn circuli circumferentia plus est
quam i x0000 0000000000000000000000000000 et minus
quam 314159z65358979323846264338327950289 I00000000000000000000000000000000000 . .
This gives the ratio correct to 35 places. Van Ceulen's process
was essentially identical with that of Vieta. Its numerous root
extractions amply justify a stronger expression than " multo
labore," especially in an epitaph. In Germany the "Ludolphische
Zahl " (Ludolph's number).is still a common name for the ratio, 5
Up to this point the credit of most that had been done may be
set down to Archimedes. A new departure, however, was made
by Willebrord Snell of Leiden
in his Cyclometria, published
in 1621. His achievement
was a closely approximate
D geometrical solution of the
problem of rectification (see
fig. 9) : ACB being a semicircle
whose centre is 0, and AC the arc to be rectified, he pro
duced AB to D, making BD equal to the radius, joined DC,
1 Vieta, Opera math. (Leiden, 1646); Marie, Hist. des sciences math. iii. 27 seq. (Paris, 1884).
2 Kliigel, Math. Worterb. ii. 6o6, 607.
' Kastner, Gesch. d. Math. i. (Gottingen, 1796—1800).
But see Les Delices de Leide (Leiden, 1712) ; or de Haan, Mess. of Math. iii. 2426.
5 For minute and lengthy details regarding the quadrature of the circle in the Low Countries, see de Haan, " Bouwstoffen voor de geschiedenis, &c.," in Versl. en Mededeel. der K. Akad. van Wetensch. ix., x., xi., xii. (Amsterdam) ; also his "Notice sur quelques quadrateurs, &c.," in Bull. di bibliogr. e di storia delle sci. mat. e fis. vii.
99144.
v1. 13and produced it to meet the tangent at A in E ; and then his assertion (not established by him) was that AE was nearly equal to the arc AC, the error being in defect. For the purposes of the calculator a solution erring in excess was also required, and this Snell gave by slightly varying the former construction. Instead of producing AB
(see fig. so) so that BD was equal to r, he produced it only so far that, when the extremity D' was joined with C, the part D'F outside the circle was equal to r; in
other words, by a nonEuclidean construction he trisected the angle AOC, for it is readily seen that, since FD'=FO=OC, the angle FOB = 3 AOC .6 This couplet of constructions is as important from the calculator's point of view as it is interesting geometrically. To compare it on this score with the fundamental proposition of Archimedes, the latter must be put into a form similar to Snell's. AMC being an arc of a circle (see fig. II) whose centre is 0, AC its chord, and HK the tangent drawn at the middle point of the arc and bounded by OA, OC produced, then, according to Archimedes, AMC
End of Article: CIRCEIUS MONS (mod. Monte Circeo) 

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