a curve on which the doctrine of pendulums, and time-measuring instruments, in a great measure depends. Mr Huygens demonstrated, that from whatever point or height, a heavy body, oscillating on a fixed centre, begins to descend, while it continues to move in a cycloid, the time of its falls or oscillations will be equal to each other. It is likewise demonstrable, that it is the curve of quickest descent, i.e., a body falling in it, from any given point above, to another not exactly under it, will come to this point in a less time than in any other curve passing through those two points. This curve is thus generated: suppose a wheel or circle to roll along a straight line till it has completed just one revolution; a nail or point in that part of the circumference of the circle, which at the beginning of the motion touches the straight line, will, at the end of the revolution, have described a cycloid on a vertical plane.

CYCLOPÆDIA, or ENCYCLOPÆDIA, denotes the circle or compass of arts and sciences. A cyclopedia, say the authors of the French Encyclopédie, ought to explain as much as possible the order and connexion of human knowledge. See ENCYCLOPÆDIA.