arc, arck, ark

[In Fr. are an arch, an arc; Prov. arc; from Lat. arcus a bow, anything arched, a mathematical arc. E entially the same word as the Eng. ARCH (q. v.).] A. Ordinary Language: 1. An arch. IMG:content-0697.png:[blocks in formation] 1. Geom.: A portion of the circumference of a circle, cut off by two lines which meet or intersect it. Its magnitude is stated in degrees, minutes, and seconds, which are equal to those of the angle which it subtends. Hence, counted by degrees, minutes, and seconds, the arc of elevation and the angle of elevation of a heavenly body are the same, and the two terms may be used in most cases indifferently. The straight line uniting the twc extremities of an arc is called its chord. "Their segments, or arcs, for the most part, exceeded not the third part of a circle."-- Newton : Opticks . Equal arcs must come from circles of equal magnitude, and each must contain the same number of degrees, minutes, and seconds as the others. Similar arcs must also each have the same number of degrees, minutes, and seconds, but they belong to circles of unequal magnitude. Concentric arcs are arcs having the same centre. 2. Math. Geog. An arc of the earth's meridian, or a meridional arc, is an are partly measured on the surface of the earth from north to south, partly calculated by trigonometry. Such arcs have been measured in Lapland; in Peru; from Dunkirk, in France, to Barcelona, in Spain; at the Cape of Good Hope, and other foreign parts; and in our own island, from Shanklin Down, in the Isle of Wight, to Balta, in Shetland. It was by these measurements that the earth was discovered to be an oblate spheroid. ( Airy's Pop. Astron., and Herschel's Astron.) 3. Astron. (For arc of elevation , see ANGLE. For Diurnal Arc, Nocturnal Arc , ., see DIURNAL, NOCTURNAL, .) 4. Mech. Phil. Arc of vibration (in a pendulum): The arc in which it vibrates. 5. Electricity. Voltaic arc: A luminous arc, which extends from one pencil of charcoal to another, when these are fixed to the terminals of a battery in such a position that their extremities are one-tenth of an inch apart. (Ganot: Physics, transl. by At kins on, 3rd ed. § 718.)