Forms: 4–7 ark(e, 6 arcke, 6– arc. [a. OF. arc:—L. arcum (nom. arcus) bow, arch, curve.]
1. Part of the circumference of a circle or other curve.
1570. Billingsley, Euclid, III. Introd. Right lines subtended to arkes in circles.
1750. Phil. Trans., XLVII. 64. Any arc described by the revolving body.
1871. C. Davies, Metric Syst., I. 18. The French Government … measured a degree of the arc of a meridian on the earth’s surface.
b. transf. or fig.
1643. Milton, Divorce, vi. (1851), 33. One of the highest arks that human contemplation circling upwards, can make.
1805. Southey, Madoc in Azt., VII. The Britons shrunk Beyond its arc of motion.
1871. Tyndall, Fragm. Sc., II. vii. 97. The circle of human nature, then, is not complete without the arc of the emotions.
2. spec. in Astr. The part of a circle which a heavenly body appears to pass through above (diurnal arc) or below (nocturnal arc) the horizon. The earliest use in Eng. Also fig.
c. 1386. Chaucer, Merch. T., 551. Parfourmed hath the sonne his ark diourne. Ibid. (c. 1391), Astrol., II. vii. Tak ther thin ark of the day. The remenant of the bordure vnder the Orisonte is the ark of the nyht.
c. 1430. Lydg., Bochas, I. xx. (1554), 39 a. As faire as Phebus shineth in his arke.
c. 1590. Marlowe, 2nd Pt. Tamburl., III. ii. The stars fix’d in the Southern arc.
1787. Bonnycastle, Astron., 428. Nocturnal arc is that space of the heavens which the sun apparently describes from the time of his setting to the time of his rising.
1878. Geo. Eliot, Coll. Breakf. P., 572. Say, the small arc of Being we call man Is near its mergence.
3. A band or belt contained between parallel curves; anything presenting this form optically or superficially, e.g., the rainbow (F. arc-en-ciel), the brass arc of a quadrant on which the degrees are marked off, etc.
1642. H. More, Song of Soul, II. I. III. xx. The higher causes of that coloured Ark.
1768. Wales, in Phil. Trans., LX. 120. The quadrant … we found much tarnished, especially the arc.
1831. Brewster, Newton (1855), I. vii. 161. He describes the arcs and circles of colours.
† 4. An arch. (Cf. Fr. arc de triomphe.) Obs.
1563. Shute, Archit., E iiij a. The arke triumphant of Seuerus.
1671. Milton, P. R., IV. 37. Statues and Trophees, and Triumphal Arcs.
1731. Pope, Mor. Ess., II. 30. Turn arcs of triumph to a garden-gate.
5. Electr. The luminous bridge formed between two carbon poles, when they are separated by a small air space, and a powerful current of electricity is sent through them. Often attrib.
1821. Sir H. Davy, in Phil. Trans., 427. The poles were connected by charcoal so as to make an arc, or column of electrical light.
1882. J. Gordon, Electr. Lighting, 62. In arc lamps … the resistance which converts the current into heat, is that of the heated air between the ends of two carbon rods.
6. transf. in Phys. Circuit, round.
1855. H. Spencer, Psychol. (1872), I. I. ii. 42. The nervous arc … consists of the afferent nerve … the ganglion corpuscle to which its central extremity runs, and the efferent nerve thence issuing.
