535.—Miner's Transit Instrument.—This is the théodolite souterrain of the French, and is of a construction very general throughout the Continent—Fig. 223. The compass is placed clearly in view. The vertical axis has a clamp and tangent motion to bring the compass to exact bearing if desired, or to permit surveying with the compass only. The axis has also a clamp and tangent screw to the exterior divided circle, which reads with two verniers. The telescope is placed on the side of the instrument, and has clamp and tangent motions to read the vertical circle which the vernier traverses in transit. All the divisions are made strong to be read clearly by lamp-light, either to 1′ or 3′ by the vernier, as desired. A second level is generally placed on some part of this instrument at right angles to the one shown. The instrument is balanced by a counterpoise weight to keep its vertical axis in equilibrium. The height of an instrument with 5-inch needle is about 6¼ inches; the weight without the tripod table is about 14 lbs. The tripod table is constructed in various ways by different makers.
536.—The value of the transit principle applied to mining instruments, for taking back and fore sights for hanging lines in undulating strata, by simply turning the telescope over on its axis, cannot be overrated for exact work such as the telescope alone can perform. Further, with this construction the inclination and difference of hypotenuse and base for correction of the chain measurements may be taken. But it is important in the use of this instrument to observe the side upon which the telescope is situated at the time of observation, right or left. For this a column should be placed in the field-book. As a rule fore sights are taken with the telescope left; back sights with the telescope right, remembering that in plotting all angles are taken eccentrically from the axis of the instrument, that is, tangential to a small circle which represents the eccentricity of the telescope according to the scale used in plotting.
537.—The Tripod Table of a superior class of Continental instruments, whether this is used for surface or mining surveying, is usually made with some form of adjustment to bring the upper surface approximately level before setting up the instrument. In this case the table is made a combination of wood and metal; and the only difference between mine and surface tables is that in the former case there is a jointed arrangement for shortening the legs, but not in the latter. The table surface for superior work is generally adjusted to approximate level either by a ball and socket joint or by a pair of knee joints placed at right angles to each other, with clamps to hold it firmly when adjusted. Radial V-grooves are commonly made for the points of the tribrach, and a hole is sometimes made in the centre of the table for suspending a plummet from the axis of the instrument. There are many forms of tripod table in use, a modified form of one of which in metal will be described further on in the chapter on plane tables. There are certain merits in this table arrangement over connective stands, as the table is convenient to set up fairly level, and the instrument need not be exposed until the operation is complete. On the other hand there is more risk of upsetting and injuring the instrument by accident when loosely placed on the table. There are, however, schemes more or less complicated to prevent this, as by a screw fixed in the tripod head acting against a spring which draws the instrument constantly down when attached, and other contrivances, none of which is perhaps equal in simplicity to Everest's arrangement for the tribrach, [Fig. 191], p. 273, on this particular point.
Fig. 224.—Stanley's improved mining survey transit.
Fig. 225.—Stand for the same.
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538.—Improved Mining Survey Transit.—The author has modified the form of instrument last illustrated, retaining the general principles. In Fig. 224 the compass is made larger and reads in the inside of the step as well as upon the surface, which is the only way in many cases that it can be read in a close working. The reading of the horizontal circle is placed nearly vertical, so that it may be seen clearly when the instrument is near the roof of the mine. The vertical circle is made smaller than the horizontal, as this circle, as a rule, is of less importance, and it can generally be read more exactly from its convenient position. The arrangement also permits greater freedom for the use of the tribrach. The telescope is made with a much larger object-glass than is usual, to take a wide field of view; therefore it forms a good level.
Fig. 226.—Stanley's miner's dial sight.