Geometrical and Graphical Essays; Containing a General Description of the Mathematical Instruments Used in Geometry, Civil and Military Surveying, ... Illustrated by Thirty-Four Copper Plates - Tapa blanda

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This historic book may have numerous typos, missing text or index. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. 1813. Not illustrated. Excerpt: ... DESCRIPTION OF A NEW POCKET BOX SEXTANT, AND AN ARTIFICIAL HORIZON,* BY THE EDITOR. Fig. 11, plate 19, is a representation of a very convenient pocket sextant, and contams a material improvement on the reflecting cross staff before described, see Jig. 4, plate 14. In military operations, as well as trigonometrical ones, it has been found of very essential service. A B a round brass box three inches in diameter, and one inch deep. AC is the index turning an index glass within the box: a, a, are the two outside ends of the screws that confine an horizon glass also within the box. An angle is observe^ by the sight being directed through an hole in the side of the box about D, upon and through the horizon glass and the second opening at E, and the angle is read off to one minute by the divided arc and nonius F, G, H. By sliding a pin projecting on the side of the box, a dark glass is brought before .the sight hole, not shewn in the figure; by pushing the pin at b, a dark screen for the sun is interposed between the index and horizon glasses. I is an endless screw, sotnetimcs applied to give a very accurate motion, like the tangent screw to the index of a sextant. Or a racked arc and pinion may be applied at about r, d, which I think in some respects better. The following table is sometimes engraved upon the cover that goes over the box when shut up. By the sextant being set to any of the angles contained in this table, an height or distance of accessible or inaccessible objects is obtained in a very simple and expeditious manner. * Plate 19, fig. 13, represents a portable and universal brass stand for supporting occasionally a sextant or reflecting circle, which with the horizon packs into a flat portable case. Make a mark upon the object, if accessible, equal to th...