Fig. 199.—Robinson's micrometer.
Fig. 200.—Webs of micrometer.
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488.—Relieving Apparatus.—It will be readily understood that the moving parts of so large an instrument must necessarily be very heavy. In this case the telescope, vertical circle, pillars, elliptical table, horizontal micrometer arms, and vertical axis socket weigh nearly 300 lbs. It would of course be impossible to take horizontal angles with so much friction on the flange of the vertical axis as this weight would produce, hence the necessity for some form of relieving apparatus. That employed in this case is a system of forty spiral springs, each of a definite length, which when adjusted support about 6·25 lbs. The spiral springs are mounted on a flat ring in two circles with projecting pins to keep them in position. The upper ends of the springs support a steel ring with a circular groove on its upper surface, between which and a corresponding groove in the outer part of the vertical axis socket three equidistant, nearly frictionless steel rollers run; so that by this means about 250 lbs. weight is taken off the flange of the vertical axis, the remaining weight being sufficient to allow of the instrument moving with the necessary freedom, and at the same time giving all the stability requisite for accurate levelling.
CHAPTER XI.
MINING SURVEY INSTRUMENTS—CIRCUMFERENTORS—PLAIN MINER'S DIAL—SIGHTS—TRIPOD STAND—ADJUSTMENTS—HENDERSON'S DIAL—LEAN'S DIAL—ADJUSTMENTS—HEDLEY'S DIAL—ADDITIONAL TELESCOPE—IMPROVED HEDLEY TRIBRACH AND BALL ADJUSTMENT—REFLECTORS—CONTINENTAL FORMS—THEODOLITE SOUTERRAIN—TRIPOD TABLES—STANLEY'S MINING THEODOLITE—PASTORELLI'S AND HOFFMANN'S ADJUSTABLE TRIPOD HEADS—MINING TRANSIT THEODOLITES—STANLEY'S PRISMATIC MINING COMPASS—HANGING DIAL—HANGING CLINOMETER—SEMI-CIRCUMFERENTOR—MINING LAMPS.
489.—Miner's Circumferentor.—In the original form of theodolite, as it was at first designed by Digges, open sights took the place of the telescope. The sights in this case were extended on arms. The compass-box, afterwards added, was placed over the axis and made as free from obstruction as possible, so that the needle, upon which general surveying formerly depended, could be read correctly by placing the eye vertically to the plane of the horizontal circle of division against which the needle read. After the introduction of the telescope to the theodolite this old form of instrument took the general designation of the circumferentor; and subsequently, being best adapted to underground surveying, it became, with some slight alterations, the miner's dial.
490.—Upon this original circumferentor improvements have been made in the various mining dials we possess, in all of which the large open compass is still preserved. This prominence of the compass does not indicate that the modern scientific mining engineer has any desire to depend upon it for taking horizontal angles, but that in close and tortuous workings it provides the nearest and often the only possible means of taking angles having regard to the extreme difficulties of observation of any kind. Where workings are open and fairly plane the telescope and circle with vernier reading can be used, so that at the present time the better instruments possess the means also of taking observations of angular direction by vernier reading. Several other very important factors specialize mining from ordinary surveying instruments, which may be stated as follows:—1. That there shall be means of shortening the tripod for work in strata of small depth. 2. That the instrument shall be low and compact in itself, that the head of the surveyor may be placed above it if possible, even in shallow workings. 3. That great extent of adjustment of the compass-box to horizontality shall be given in the fittings of the instrument, on account of the difficulty of extending the legs at all times for tripod adjustment and from the extreme inclination of the floor of the working in some cases. 4. That it is desirable in mining survey instruments that the telescope, if there is one, shall take sights at all angles upon the surface of the earth in the locality in which the instrument is used, as also about a vertical position, so as to be able to sight lines from the top to the bottom of the shaft, or vice versa, to set off angles in the same azimuth as those taken at the surface by direction of stretched wires or otherwise. This last contrivance will also give the means of sighting a perfectly vertical point beneath the centre of the instrument placed at the top of the shaft, to make a concurrent station below during ventilation, when the plummet would be disturbed. The devices by which these various requirements have been met more or less perfectly will go far to explain the specialities of construction found in mining surveying instruments, which will now be described, commencing with the oldest and most simple specialised form upon which improvements have been made in many directions.