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539.—Two pairs of sights are placed upon the telescope, either for roughly sighting an object or station, or to be used in difficult positions. These are made on a new principle, shown Fig. 226. The sights are placed in two windows, each of which is formed of a needle point of platino-iridium. In sighting, the points are brought over each other, the distant lamp or object appearing between them. A sharp point gives much clearer definition than a hair, as it subtends of itself no angle to the axis of the eye. ab represent the pair of sights, c as they appear superimposed. This instrument is very conveniently fitted with subtense points in the telescope, by which distances may be taken with the author's staff, [Fig. 105], p. 158, without actual measurement, for the particulars of which see next chapter. The subtense points are arranged to measure the staff either vertically or horizontally. As a rule it will be found with this instrument better to take rough positions first with the points, and afterwards by the telescope. The instrument cannot be recommended universally for underground surveying, but it is valuable under certain conditions in close strata. Its height is 6 inches and weight 13 lbs.
Fig. 225 is an ordinary tripod, like that used with a level. This is preferred by many mining engineers as being firmer than any jointed arrangement, and is sufficient for working in a seam of fairly equal thickness. The legs vary from 9 inches to the full height, 5 feet 4 inches. An ordinary set of three tripods would be 1 foot 6 inches, 3 feet 6 inches, and 5 feet 4 inches.
Fig. 227.—Stanley's underground theodolite.
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540.—Mining Theodolite.—This theodolite is of the most convenient form for underground railways, Fig. 227. The telescope transits on its axis to be brought to a vertical position. The vertical axis is pierced so that about 10° of angle may read below the vertical most conveniently by means of a diagonal eye-piece. The centre is supported upon a sliding fitting so that it may be displaced about 1¼ inches about the centre of the tripod and be clamped to its position. The horizontal axis is pierced to permit the diaphragm to be illuminated by a lamp. The tripod stand is fitted with sliding legs, if it is to be used for mine survey, to adjust for irregularity of surface of the ground and for low workings. The form of the instrument is very compact, rigid, and portable.
Fig. 228.—Stanley's prismatic mining compass.
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