Surveying and levelling instruments; theoretically and practically described, for construction, qualities, selection, preservation, adjustments, and ... by civil engineers and surveyors in the field

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1901 Excerpt: ...It may be read at both ends, and if the 0 and 108 points cut the line fairly it is considered correct; if not, the mean of the difference may be taken. In some German instruments a microscope is mounted over the needle point that the needle may be adjusted to a web; but British surveyors seldom feel confident of surveys by the magnet, and generally prefer for triangulation to employ a certain number of distant fixed points, the bearings of which are at first as accurately ascertained as possible, for referring objects, rather than to refer frequently to the magnet. When the magnet is out of use the needle should always remain lifted off its centre. When the instrument is put by for a long period it is better to place it in a vertical position and free the needle, so that it rests in the magnetic meridian, in order to preserve its magnetism as much as possible. 464.--Use of the Theodolite.--In setting up a theodolite, place the tripod nearly over the position in which it is to be used. This is frequently the socket hole formed in the earth by the removal of a ranging pole or picket, to be described Chapter XVI. Then suspend the plummet from the hook, which will be found inside the head of the tripod, after it is set up. If the ground is solid and level, then by shifting the toes of the tripod slightly, and firmly pressing them down one by one, the centre of the plummet may be brought easily within about 25 of an inch of its true position. The theodolite is then placed on its tripod, observing that the telescope is in a position easy to be used. The centre of the pickethole, when this is used for a station, is generally taken Dy guess-work, which is considered near enough. It may be taken with a little more refinement by placing a short false picket of nine i...