Arc
in geometry, a portion of the 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 circumference of a circle, cut off by two lines which meet or intersect it. Its magnitude is stated in degrees, minutes, and of a heavenly body are the same, and the by degrees, minutes, and seconds, the arc two terms may be used in most cases inof elevation and the angle of elevation differently. The straight line uniting the two extremities of an arc is called its chord. Equal arcs must come from circles of equal magnitude, and each must conchord. Equal arcs must come from circles of equal magnitude, and each must contain the same number of degrees, minutes, belong to circles of unequal magnitude. and seconds as the others. Similar arcs Concentric arcs are arcs having the same must also each have the same number of degrees, minutes, and seconds, but they center.
In mathematical geography, an arc of the earth's meridian, or a meridional arc, is an arc partly measured on the surface culated by trigonometry. Such arcs have of the earth from N. to S., partly calbeen measured in Lapland; in Peru; from of the earth from N. to S., partly cal- Spain; at the Cape of Good Hope, and from Shanklin Down, in the Isle of Wight, measurements that the earth was discovered to be an oblate spheroid. In electricity, a voltaic arc is a luminous are, which extends from one pencil of charcoal to another when these are