The Topographer, His Instruments and Methods; Designed for the Use of Students, Amateur Topographers, Surveyors, Engineers, and All Persons Interested in the Location and Construction of Works Based Upon Topography (Paperback)

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. 1883 Excerpt: ...The screw pitch is usually such that one turn moves the horizontal wire over of a foot of space at a distance of 100 feet, and as there are then 50 divisions on the wheel, one of these would be Tji of a foot on the rod, equivalent to one foot of horizontal distance. Some micrometer heads are graduated in 100 parts. To determine the distance to the rod in any position, read the micrometer wheel after setting it upon some convenient unit of the rod, as 3 feet, then turn the wheel some integral number of times, preferably two, since two revolutions cover one foot at a distance of 100 feet. The second reading is then taken on the rod, and their difference multiplied by IOO will give the required distance. If in this case, the second rod reading were 5.62, the result would be 262 feet. The gradientor is, in fact, a combined transit level and stadia, and may be used by a party of only two persons with great economy in preliminary surveys. It is subject to precisely the same corrections for elevation in topography as the stadia. THE PLANE TABLE. 131. Although much -has already been written on this instrument, describing its uses and adjustments, the introduction of stadia wires in the telescope have so greatly modified this practice as to render a few words on the subject appropriate. The principal parts of the instrument are the board, tripod head and legs, and alidade. The board B (PI. 10, Fig. 1) is composed of thoroughly seasoned pine, framed together so as to prevent shrinkage and warping. It is usually made two feet square, but sometimes oblong, 24X30 inches. It is connected with the tripod by a long and stiff spindle, which projects from the disc C, Fig. 2, to which it is attached by the screws D. The levelling is accomplished by means of the parallel ...