A treatise on land-surveying; comprising the theory developed from five elementary principles; and the practice with the chain alone, the compass, the transit, the theodolite, the plane table, &cIllustrated by four hundred engravings, and a magnetic chart . e placed over the other end, to avoid the errorwhich might result from the parallax, or apparent change ofplace, of the end read from, when looked at obliquelv. CHAP. III.] The Field Work. 139 Tlie bearing is read and recorded by noting between what lettersthe end of the needle comes, and to what number; naming, orwriting down, firstly, tha
A treatise on land-surveying; comprising the theory developed from five elementary principles; and the practice with the chain alone, the compass, the transit, the theodolite, the plane table, &cIllustrated by four hundred engravings, and a magnetic chart . e placed over the other end, to avoid the errorwhich might result from the parallax, or apparent change ofplace, of the end read from, when looked at obliquelv. CHAP. III.] The Field Work. 139 Tlie bearing is read and recorded by noting between what lettersthe end of the needle comes, and to what number; naming, orwriting down, firstly, that letter, N or S, which is at the 0^ pointnearest to that end of the needle from which jou are reading;secondly, the number of degrees to which it points, and tldrdly^the letter, E or W, of the 90° pomt which is nearest to the sameend of the needle. Thus, in the figm-e, if when the sights W3redu-ected along a hne, (the Northpoint of the compass being mostdistant from the observer), theNorth end of the needle was at thepoint A, the bearing of the linesighted on, would be North 45°East; if the end of the needle wasat B, the bearing would be East; ifat C, S. 30° E ; if at D, South; ifat E, S. 60° W; if at F, West; ifat G, N. 60° W; if at H, 164. —-B (236) We can now understand why W is en the right hand ofthe compass-box, and E on the left. Let the direction from thecentre of the compass to the pointB in the figure, be required, andsuppose the sights in the first placeto be pointing in the direction of theneedle, S N, and the North sightto be ahead. Wlien the sights (andthe circle to which they are fasten-ed) have been turned so as to pointin the direction of B, the point oftlie cucle marked E, wiU have come round to the North end of theneedle, (since the needle remains immovable,) and the reading willtherefore be East, as it should be. The efiect on the readingis the same as if the needle had moved to the left the same quantitywhich the sights have moved to the rig
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Keywords: ., bookcentury1800, bookdecade1870, booksubjectsurveying, bookyear18
