8. Description of the Common Gunners' Quadrant.
This instrument, represented in Fig. 14, consists of two branches made of wood or brass, one of which is about twelve inches long, eight lines broad, and one line thick ; and the other four inches long. Between these branches is placed a quadrant, divided into 90°, the di visions, commencing at the shorter branch, which is fur nished with a thread and plummet.
The use of the quadrant is to point cannons, mortars, Etc. which is dune by placing the longest branch in the mouth of the piece of ordnance, and by elevating the piece, till the plumb line rest at the degree of elevation required.
Description of Mr. Irving's substitute for Me ner's Quadrant.
As one of the objects of the Gunner's Quadrant is to point guns precisely in the same manner when the visual line does not bear on the object, and when the object aimed at is hit by the gun at a particular eleva tion ; the following instrument, which answers the pur-' pose much better, was contrived by Alex. Irving, Esq. when first Lieutenant of the Edinburgh Corps or Artil lery. It is represented in a front and back view in Plate CCCCLXXVI. Fig. and 16. The following is Mr. Irving's own description of it. " Breadth of the aper ture, one inch ; length, one and a half, which is divided by a scale, into 18 parts, each of which, when the length of the gun (this calculation applies to the ordinary light six-pounders) is made radius, will be equal to five mi nutes. The Nonius dividing plate which is fixed to the horizontal moveable wire, being divided into five parts, which altogether are equal to four divisions of the scale, will give a division into minutes.
The two feet of the instrument rest upon the upper part of one of the muzzle mouldings of the gun, on which it is kept by a steel spring. The spirit level must be parallel to the line joining the feet, and at right angles to the vertical wire. The view of the back part will show the mode in which the Nonius and horizontal wire are raised and lowered by means of a screw.
The aperture is bisected vertically by a black wire, which is cut at right angles by a horizontal wire. The latter, however, must not be a wire, but a thin plate set edgewise, that it may bear being raised or lowered.
When it arrives at the opposite side of the instrument, it is flattened in a contrary direction, and kept close to the limb of the instrument by a slip of metal, which, however, allows it to move freely up and down.
In using the instrument, says Colonel Macdonald, the horizontal wire can be depressed or elevated till its intersection with the perpendicular one cuts some point on the object, and, by bringing the intersecting point on the same part of the object in all succeeding shots, the gun will be always similarly pointed. The angle of elevation of fin d-pieces seldom exceeds the degree which that instrument is capable of ascertain ing, and it might easily be rendered capable of mea suring larger angles than are usual in field service. See Colonel Macdonald's work On Projectiles, Fuzes, Z.7e. pp. 59, 60. Lond. 1819.
For further information on the subject of Quadrants, see Bion on the Construction and Principal Uses of Ma thematical Instruments ; Smith's Optics, vol. ii. chap. vii. p. 332; " The Method of Constructing Mural Quadrants," published by order of the Commissioners of the Board of Longitude, in the year 1763 ; Vince's Treatise on Practical Astronomy ; Grandjean de Fou chy's Machine for Managing a Quadrant, in the Mem. Acad. 1740, p. 468; or Machines Approuvees, vii. p. 47. A quadrant with a reflecting Ttlescope is described in the Mem. Acad. Par. 1746, Hist. p. 121; Gersten's Quadrantie Muralis Idea .Nova, in the Phil. Trans. 1747, vol. xltv. p. 507; Fondly on Converting a Quadrant into an Azimuthal Instrument, in the Mem. Acad. Par. 1781, p. 259; Caesaris de Quadrante Mu rali Mediolanensi Ramsdeni, 8vo. See SEXTANT.
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