the curvature measured as above is equal to the reciprocal of the radius of the circle. If we consider any small portion of any curve whatever, it may be approximately taken as an arc of a circle, the approximation being closer and closer to the truth as the portion considered is smaller and smaller, and by taking it small enough we may make the approximation as close as we please. The curvature is then the reciprocal of the radius of this circle. The circle which coincides more nearly than any other with the arc at any point of any given curve is generally found by means of the methods of the differential calculus. Such a circle is called the circle of curvature, and sometimes the osculating circle. The radius of it is called the radius of curvature of the curve at the point considered; and the centre of this circle is called the centre of curvature. See CALCULUS, THE INFINITESIMAL. CIRCLEVILLE, Ohio, city and countyseat of Pickaway County, on the Scioto River, the Ohio Canal and the Cincinnati and Muskingum Valley and the Norfolk and Western railroads, 28 miles south of Columbus. It derives its name from a circular earthwork built by some ancient people, and which is the site of the present city. Circleville contains a county infirmary, a children's home, a home and hospital for old ladies, numerous churches, three banks and several daily and weekly newspapers. The surrounding country is very fertile and the city has extensive strawboard works, canneries and flour and corn-meal mills. It also has manufactures of agricultural implements and furniture. Settled in 1806, it was incorporated as a village in 1814, and as a city in 1853. It is now governed under the Ohio Municipal Code of 1902 by a mayor, elected for two years, and a council. One of the city's historic places is the Logan Elm, where John Logan, the Indian chief, made his celebrated speech. A park has been laid out here. Pop. 6,744. CIRCUIT COURT. See COURTS. CIRCULAR NOTES, in commerce, notes or letters of credit furnished by bankers to persons about to travel abroad. Along with the notes the traveler receives a "letter of indication" bearing the names of certain foreign bankers who will cash such notes on presentation, in which letter the traveler must write his name. On presentation the foreign banker can demand to see the letter of indication, and by causing the presenter to write his name, can compare the signature thus made with that in the letter, and so far satisfy himself as to the identity of the person presenting the note. CIRCULAR NUMBERS, numbers whose powers end on the same figure as they do themselves: such are numbers ending in 0, 1, 5, 6. CIRCULAR PARTS, Napier's Rules for. Rules invented by Baron Napier of Merchiston, near Edinburgh, for the solution of all cases of right-angled spherical triangles, eminent for comprehensiveness and utility in extensive surveys, navigation and practical astronomy. They were first published by him in the Descriptio,' in 1614. See TRIGONOMETRY. CIRCULAR POLARIZATION OF LIGHT. Plane-polarized light is altered into circularly polarized light by passing in a par ticular direction through a Fresnel's rhomb. This is a parallelopiped of glass with its faces set at certain angles depending on the refractive power of the glass. The usual experimental rhomb is a bar of thick plate glass, the ends of which are ground to an angle of 54° and then polished. The light entering one base of the rhomb is twice internally reflected before it emerges at the opposite base; and while common unpolarized light passes through the rhomb without suffering alteration, plane-polarized light has its properties in general completely altered. The final result depends on the inclination of the plane of polarization of the incident light to the plane of the internal reflections. In two cases, namely, when this angle is 0° or 90°, the emerging light is still plane polarized; when the angle is 45° the light is circularly polarized; in every other case it is elliptically polarized. In the first case, as will be understood from consulting the article on POLARIZATION OF LIGHT, the analyzer, on being applied to test the beam, shows in one position bright light, and on being turned round the principal axis through 90°, total darkness. In the last case that of elliptic polarization- the analyzer shows, on being turned round, a beam of varying intensity, but never complete extinction. In the case of circularly polarized light the analyzer, on being turned round, shows a beam of the same intensity in every position of the analyzer, and, in fact, does not at first sight differ from ordinary unpolarized light. When, however, it is examined-not with a Nicol's prism direct, but after a second Fresnel's rhomb has been interposed-it is found to differ very materially from unpolarized light. The latter is, as we have remarked, unaffected by the rhomb; the circularly polarized light emerges from the second rhomb plane polarized. It is thus shown how to produce and how to recognize circularly polarized light. We now give a few of its most notable properties. The light, as we have said, that emerges from the second Fresnel's rhomb is again plane polarized, but it does not emerge precisely as it entered. For, except in one particular position of the two Fresnel's rhombs, the light that emerges from the second rhomb has its plane of polarization changed; the plane is turned round, in fact, through an angle depending on the positions of the two rhombs with regard to the original plane of polarization; and it may be turned round either in a right-handed direction, that is, as the hands move on a clock, or in a left-handed direction, that is, a movement opposite to clockwise. We might arrange a set of pairs of Fresnel's rhombs, it is evident, in such positions that each pair should give the plane of polarization of the ray passing through it a farther twist in the same direction, and we might turn it thus through any angle whatever. Such a power as we have imagined in a set of Fresnel's rhombs is possessed naturally by quartz and by a considerable number of solutions of organic substances and it is known as the power of rotating the plane of polarization. When a beam of homogeneous light has passed through the polarizer, and the analyzer is placed in the position of total extinction of the ray (see POLARIZATION OF LIGHT), on introducing a plate of quartz the light reappears; but on turning the analyzer round, either in a right handed direction or in a left-handed direction (whence the names), extinction is again obtained. Quartz is named right-handed quartz or left-handed quartz according to the direction in which the analyzer must be turned. The difference between right-handed and left-handed quartz is due to the fact that the right-handed circular component travels faster in the former and slower in the latter. The amount of the angle through which it must be turned depends on the thickness of the plate of quartz. If, instead of using homogeneous light, as we have been supposing, plane-polarized white light is employed, it is found that the different rays are differently deviated. The effect on the more refrangible rays is greater than on the less refrangible, and the plane of polarization of the blue rays will thus be turned through a greater angle than that of the red rays. It will be perceived from this that having arranged the polarizer and analyzer, and inserted a plate of quartz, as described above, on rotating the analyzer in the direction, right-handed or lefthanded, that corresponds to the nature of the plate of quartz, we shall not arrive at a position of total extinction, but we shall see a most beautiful play of colors changing in order from red to yellow, then to orange, green and blue. These phenomena are among the most beautiful and most striking of all the marvelous phenomena of light. It has been remarked above that certain organic liquids and solutions have this rotatory power. Among these may be mentioned turpentine and the essential oil of anise as instances of left-handed rotatory substances, oil of lemon and oil of caraway and solutions of sugar, as right-handed rotatory substances, and solutions of tartaric acid as showing both_rotations, as explained below. This fact is taken advantage of in Soleil's saccharometer, an instrument for determining the value of canesugar in a liquid. We have spoken above of the right-handed and left-handed properties of quartz; a discovery of Haüy leads us here to the very threshold of the molecular structure of crystals. We may yet hope for discoveries in this direction. On comparing crystals of quartz that give us right-handed and left-handed polarization, it is found that a very remarkable property connects their forms. The crystals that give righthanded and left-handed polarization are of an unsymmetrical construction, such that either viewed in a looking-glass gives an image of the same form as the other. Pasteur, examining the crystals of the two varieties of tartaric acid whose solutions have opposite rotational powers, but whose chemical properties are very nearly the same, showed that the same law holds for them; and, having crystallized what is known as neutral tartaric acid, was able, by picking out the crystals by hand, to separate it into equal portions of lævo tartaric acid and dextro tartaric acid. But we must refer the reader to the special articles on the chemistry of this substance. One of Faraday's most brilliant discoveries was the rotatory power of glass under the action of a powerful magnet. The reader is referred for an account of it to the article POLARIZATION OF LIGHT. CIRCULATING LIBRARY. BRARIES. See LI CIRCULATING MEDIUM. See MONEY. CIRCULATION. See BLOOD, CIRCULA TION OF. CIRCULATION IN PLANTS. See SAP. CIRCULATORY SYSTEM, general name for the group of organs which convey fluids and solids from one part of a body to another, the matter carried being usually of an alimentary nature. Such a system develops when an organism becomes specialized in structure, so that the organs requiring alimentation are more or less removed from the regions where it is supplied to the organism. For the circulatory systems of the unicellular and other animals see BLOOD, CIRCULATION OF, and the articles on Crustacea, Arthropoda, Mollusca, Protozoa, etc. For a description of the circulatory systems of plants see SAP. CIRCUMCELLIONS, sér-kŭm-sěl'i-onz, CIRCUMCELLIONES, or CIRCELLIO. NES, members of bands, probably of fanatic monks, partisans of Donatus (q.v.) and the Donatists in Numidia and Mauretania who, avenging the condemnation of Donatus by the Synod of Arles (314) and the Council of Nicæa (325) and the condemnation of Donatus and his followers by Constantine and Constantius, roamed throughout those provinces pillaging and wrecking the churches and other religious establishments of the Roman Catholics and proclaiming the downfall of the empire itself. Their name probably is formed from circum, round about, and cella, cell, a monk's cell: if the derivation is correct these Circumcellions are of the same class of vagabond monks complained of by the father of western monachism, Saint Benedict, as gyrovagi, circulating vagabonds. CIRCUMCISION, an amputation of the foreskin, or labia minora of the human organs of generation; principally performed on males, but sometimes on females. It has been practised in all ages and by both civilized and savage races, as Arabs, certain African tribes, Christian Abyssinians, Australian "blacks," Malays, some North American Indians, Aztecs, Mayas, Caribs, South American Indians, Jews, Mohammedans, Fijians and Samoans. In the male, the operation consists in removing a section of the prepuce. The original object was religious and symbolic and its antiquity is lost in the shade of mythology. The institution of the rite among the Jews is recorded in Gen. xvii. Here it is stated that Abraham, then 99 years old, was himself circumcised, with his son Ishmael, 13 years old, "and all that were born in his house and all that were bought with his money, every male among the men of Abraham's house." But by the terms of the covenant every man-child among the Hebrews was to be circumcised on the eighth day after birth, and this rule, peculiar to the race, is adhered to with such rigor that even the Sabbath observances are not allowed to interfere with the ceremony. Other Eastern nations have practised circumcision on various days—the Arabs 7, 14, 21 or 28 days after birth, though Josephus states that in his day the Arabs circumcised after the age of 13 on account of the circumcision of Ishmael, their progenitor, taking place at that age; the Mohammedans of Persia circumcise in the third or fourth year; Fijians and Samoans in the |