Forms: 6–7 theodelitus, 7 theodelite, -dolit, -dilit, 8– dolet, 7– theodolite. [Origin unknown: see Note below.]
A portable surveying instrument, originally for measuring horizontal angles, and consisting essentially of a planisphere or horizontal graduated circular plate, with an alidad or index bearing sights; subsequently variously elaborated with a telescope instead of sights, a compass, level, vernier, micrometer, and other accessories, and now often with the addition of a vertical circle or arc for the measurement of angles of altitude or depression.
The original theodelitus of Digges was for horizontal angles only, and many quots. down to 19th c. use the name in this sense; Digges also describes a compound instrument having also a vertical semicircle for taking altitudes, but he calls that his topographicall instrument, restricting the name theodelitus to the horizontal circle.
1571. Digges, Pantom., I. xxvii. H iij. The composition of the instrument called Theodelitus. It is but a circle diuided in 360 … degrees, or a semicircle parted in 180 portions, and euery of those diuisions in 3 or rather 6 smaller partes…. The index of that instrument with the sightes &c. are not vnlike to that whiche the square hath: In his backe prepare a vice or scrue to be fastned in the top of some staffe. Ibid., I ij. [In the figure] GEFO [is] Theodelitus, GF his Alhidada or index with sightes. Ibid., xxix. I jb. Describing also within the same square the Planisphere or circle called Theodelitus.
1607. J. Norden, Surv. Dial., III. 127. It [Circumferentor] is a new name giuen to the very Theodelite, used in a sort otherwise then the Theodelite.
1611. A. Hopton, Speculum Topogr., vi. 27. The Theodelitus is an instrument consisting of a Planisphere and an Alhidada. Ibid., Table D d 2b. To take a plat at one station by the Theodelite.
1669. Sturmy, Mariner’s Mag., II. 46. Any Instrument, as the Plain Table, the Theodolit or Circumferenter.
1701. Moxon, Math. Instr., 20. Theodolet, a whole Circle made of Brass, containing 360 degrees, diagonally or otherwise divided, with an Index and sights moving on the Center, and a box and Needle in the middle.
1790. Roy, in Phil. Trans., LXXX. 136. It is a brass circle, three feet in diameter, and may be called a great theodolet, rendered extremely perfect.
1833. Herschel, Astron., ii. § 155. The zenith sector and the theodolite are peculiar modifications of the altitude and azimuth instrument.
1842. Penny Cycl., XXIV. 314/2. Theodolet, or Theodolite … the name generally given to the instrument used for measuring horizontal angles. [Ibid., 315/2. The problem is to measure the horizontal angle between two objects. Ibid., 316/2. If the vertical angles are to be measured as accurately as the horizontal angles, the instrument becomes an altitude and azimuth circle.] [Cf. ALTAZIMUTH.]
b. attrib., as theodolite-goniometer, a goniometer with horizontal and vertical graduated circles; theodolite-magnetometer, an instrument for measuring magnetic declination, and for observations of magnetic force; theodolite-needle, the needle of the compass of a theodolite.
1820. Scoresby, Acc. Arctic Reg., I. 333. A theodolite needle … performed ten vibrations in sixty seconds.
1877. Knight, Dict. Mech., Theodolite-magnetometer.
1909. Cent. Dict. Suppl., Theodolite-goniometer.
Hence Theodoliitic a., of, pertaining to, done or made with a theodolite (Webster, 1864).
[Note. The name, alike in the Latinized form theodolitus and the vernacular theodelite (subseq. -dolite), originated in England, and is not known in French and German until the 19th c. Its first user, and probable inventor, L. or T. Digges, has left no account of its composition, as to which various futile conjectures, incompatible with its early history and use, have been offered; such is the notion that it arose in some way out of alhidada or its corruption athelida occurring in Bourne’s Treasure for Travailers, 1578, which an examination of the works of Digges and Bourne, where both words occur in their proper senses, shows to be absurd. Theodelite has the look of a formation from Greek; can it have been (like many modern names of inventions) an unscholarly formation from θεάομαι ‘I view’ or θεῶ ‘behold’ and δῆλ-ος ‘visible, clear, manifest,’ with a meaningless termination?]
