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Fig. 318.—Alidade to plane table.
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702.—Lateral Adjustment to the Alidade.—The author's plan of obtaining this is to increase the practical width of the rule by giving it an extended point of support on one side so as to set the telescope in azimuth. For this construction the telescope is mounted upon a plate with an arm extending outwards upon the back of the rule. This has a milled-headed screw placed at the near extremity of the arm. The screw is inserted in a deep bush for wear; this attachment is shown in section Fig. 318. The adjusting screw A′ has a collar fixed upon its point which is centred upon a tight screw tapped into the milled head. This collar, as it does not turn with the milled head, does not abrade the surface of the paper by contact with it. A small cross level B is put upon the arm between the milled head and the standard of the telescope. The under side of the rule is cut away or placed obliquely to the surface, so that it bears on the outer ruling edge only. The milled-headed screw being at its normal position and the table level, less than half a turn one way or the other will bring the small cross bubble to its centre in a few seconds for any average irregularity of the surface of the table, and by this means cause the telescope to move correctly in azimuth.
Fig. 319.—Subtense points.
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703.—The Telescope Arranged for Subtense Measurements.—Where a stadium, [art. 556], is to be used for estimating distances from station to station, when an ordinary telescope is used, the author places two platino-iridium points vertically from top and bottom of the diaphragm, and adjusts these by a screw until a subtense angle upon the stadium of 1 foot cuts the point at a distance of 100 feet, or according to the measurements to which the land is taken. In this case it is necessary to have an altitude arc to the telescope, as shown upon the alidade in Fig. 317. This has a degree scale reading by vernier to about 3 minutes.
704.—Various Devices for Fixing the paper on the Surface of the Table have been made. Many prefer simply pinning it with drawing pins on a quite plain pinewood surface. In this case the table is better slightly sunk round the edges with a rabbet of the depth of the thickness of the head of the pin, so that the alidade may rest firmly even over the pin heads. The French plane tables have very generally rollers at each end of the table, upon which a long slip of paper is rolled, sufficient for twelve or more stations. The rollers for small tables are made of brass tube about 5/8 inch in diameter. They commonly move with a turn-key which is inserted in a square fitting in the end of the roller. The rollers keep the paper tight by means of ratchet wheels and spring pawls at their ends. This plan is very convenient for topographical work, as for instance a river may be followed from station to station right down its course and appear on a single slip, its bearing being indicated by the compass north line. Fig. 320 shows the manner in which the author has made this plane table.