Page:The American Cyclopædia (1879) Volume V.djvu/609

CYCLAMEN CYCLOPAEDIA 605 CYCLAMEJV, a genus of plants of the order pri- mulacea, distinguished by a tuberous, orange- shaped root, heart-shaped leaves, flowers (ap- pearing in autumn) of one petal divided into five oblong segments, and by the peculiar flower stalk, which after flowering twists into a spiral and turns over toward the ground, so that the seeds, ripening in this position, fall and produce new plants. The common cyclamen, G. Euro- Cyclamen Europseum. pceum, is very hardy, and flourishes in several countries of the European continent, especially in Italy, where it is called paneporcino, from the fondness of swine for the tuberous root. C. Persicum is a very ornamental species, and is a favorite garden plant in southern Europe. CYCLOID (Gr. /ci>/cAo?, a circle, and eldos, form), the curve described by a point on the circum- ference of a circle when the circle rolls along a straight line. A nail in the tire of a wagon wheel, as the wheel rolls along the street, describes a cycloid. It is usually described upon paper as follows : Fasten a bit of black lead to the edge of a coin, for example a cent, so that when the coin is laid flat upon the paper and moved the lead may trace a mark ; lay a ruler upon the paper and place the edge of the coin where the lead is attached against the edge of the ruler ; roll the coin along the edge of the ruler, and the lead will describe a cycloid. When the coin has rolled a distance equal to one half its circumference, the lead will be at a distance from the ruler equal to the diameter of the coin ; when it has rolled a distance equal to the circumference of the coin, the lead will again be next to the ruler. The lead will thus describe an arch which to the eye appears like the half of an ellipse or oval, but is in reality an entirely different curve. If the coin continue to roll along the ruler, an- other arch exactly like the first will be de- scribed, and so on indefinitely. In mathema- tics these arches are all regarded as forming one curve, of which each arch is a branch. As they are all merely repetitions of the first one, the investigation of the properties of one is sufficient ; and unless the contrary is stated, the word " cycloid " is generally understood as meaning a single branch. The straight line along which the circle rolls is called the base of the cycloid, and its length equals the cir- cumference of the circle. The length of the cycloid is four times the diameter of the circle ; and the area of the surface included between the cycloid and its base is three times the area of the circle. If a cycloid be inverted so that its concave side is upward, then a body rolling down the curve will reach the lowest point in the same time from whatever point of the curve it starts. The cycloid is also called the brachystochrone (Gr. fSpd^ioro^ shortest, and %p6vos, time), or the curve of swiftest descent, because a body starting from any point of the curve, when in its last mentioned position, will reach its lowest point in less time than it would had it rolled from the same starting point along any other curve or straight line. The cycloid is one of the most important curves in the the- ory of mechanics, and the investigation of its properties was among the earliest applications of the differential calculus. CYCLOIDS, an order of bony fishes, estab- lished by Agassiz. See COMPAEATIVE ANATOMY. CYCLONE, a storm of wind moving in im- mense whirls, and at the same time sweeping onward over the surface. Cyclones are usual- ly 200 to 300 m. in diameter, sometimes more than 1,000 m. Their central point is calm, and this moves forward from 2 to 40 m. an hour. These storms originate outside the equatorial belt between the tropics, and move toward the poles. In the southern hemisphere the rotation is in the same direction with the hands of a watch placed with the face upward ; in the northern, the direction is reversed. (See HURRICANE.) CYCLOPEDIA, or Encyclopaedia (Gr. wido?, a circle, and iratdeia, education), originally the cycle of the seven liberal arts and sciences which constituted with the ancients the course of education for the higher class of citizens, viz. : grammar, arithmetic, geometry, music, astronomy, dialectics, and rhetoric. Quintil- ian mentions it as the orb or full circle of learning : Orbis ilia doctrines quam Greed eynvichoTrcuSeiav vacant. In its modern accepta- tion the word commonly designates a summary of human knowledge, either in one or in all departments, arranged either systematically according to the logical connection of topics, or lexicographically according to the alphabet- ical succession of terms ; and therefore distin- guished as either general or special, systematic or alphabetical. Speusippus, the nephew and disciple of Plato, is usually accounted to have written the first cyclopaedic work, under the title Aidhoyot TUV irepi rr/v TlpayfjiaTeiav '0/^otwv, which has not been preserved. The work of Aristotle on the sciences (ILepi 'ETncrfauv), the lost books of Varro entitled Rerum Hu-