Arc — Chambers's Encyclopaedia

Arc (Lat. arcus, 'a bow') is any part of a curved line. The straight line joining the ends of an arc is its chord, which is always less than the arc itself. Arcs of circles are similar when they subtend equal angles at the centres of their respective circles; and if similar arcs belong to equal circles, the arcs themselves are equal. The length of an arc is readily found if the angle which it subtends at the centre of the circle is known, and also the length of the whole circumference. Let the whole circumference be 100, and the angle of an arc 50°, the length of the arc is

360^\circ : 50^\circ :: 100 : \frac{100 \times 50}{360} = 14 \text{ nearly.}