Chapter XI (2)

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.

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.

541--=Prismatic Mining Survey Compass.=--This arrangement is designed by the author for very close workings. The entire depth of the instrument being only 4 inches, any reading may be taken from one point of view simultaneously with the observation. The 5-inch compass, Fig. 228, has a floating ring divided to half degrees, and the reading of this is reflected through a prism so that it appears directly under the fore sight, to be seen at the same time. The prism has a slight magnifying power, so that by estimation a bearing may be easily taken to ¼ degree or nearer. The principle of the compass is described art. 148, the prism art. 55; but in this case the prism is raised and has a second lens under it, so that it forms a kind of prismatic Ramsden eye-piece. This elevation of the prism permits sighting under a certain amount of downward inclination, regulated by the height of the prism and the length of the back sight, as well as the upward inclination which is common to the use of prismatic compasses. The most important feature in this compass is the mode of lighting, which is effected by means of a large prism, Fig. 229 _R_, placed under the compass-box in a square tube, and a small movable lamp to throw light into it, Fig. 228 _L_. The floating ring, Fig. 229 _C_, is made of celluloid, quite transparent, so that the divisions upon it are clearly read through the small window in the cover of the compass-box. The fore sight _W_ is jointed in two folds _jj_, so that it extends the distance of sights to about 10 inches apart in use, and yet folds away closely to the compass for portability when out of use. On the near sight a cut is made transversely to the slit. A second similar cut on the fore sight is made level with this to take levels roughly. About 20° are set off on each side of the cut on the fore sight, so that angles of altitude may be approximately taken--although the instrument is not well adapted to this. Two levels set at right angles to each other, to be used in setting up the instrument, are fixed under the compass-box. Weight of instrument, 4¼ lbs. without the tripod stand.

542.--=Hanging Compass.=--A very general method of underground surveying in mineral districts upon the Continent is by means of the _hanging compass_; this instrument is therefore generally found in catalogues of surveying instruments in France, Germany, and Italy. The original hanging compass was invented by Balthasar Rössler about 1660.[26] It appears to the author to be a valuable instrument for surveying in tortuous mineral veins where sighting is difficult. The measuring line upon which it is used is either a hempen or copper cord or a chain. The compass is hung upon the cord or chain, which may be stretched to any point out of sight, and the compass will then indicate the bearing of the line. In Germany two instruments are used simultaneously--the hanging compass for taking the bearing, and a clinometer, composed of a light brass semicircle graduated to degrees, with a small plummet for taking the inclination.

543.--=Hanging Dial.=--Fig. 230 represents a modification of the hanging compass designed by the author, by which inclination may be taken simultaneously with bearing, if the dial can be suspended near the centre of the line or chain where the catenary curve is parallel with its points of support.

544.--In the construction of the instrument a circle of brass about 6 inches diameter, ½ inch wide, and 1/8 inch thick, has two arms extended to 12 inches at the upper part, on the end of each of which a hook is formed for hanging the instrument upon a cord or chain. Upon the lower part of the circle a _fork-piece_, with a bearing clipping the circle, is attached by two screws. The fork-piece is constructed to support two axes concentric to the vertical circle, in which the compass-box is suspended much above its centre of gravity, so that it falls by its own weight in use to a level position. Upon the edge of the compass-box an index is brought up nearly to the interior surface of the vertical circle, which reads into graduations upon this circle into degrees and half degrees.

545.--=A Light Hanging Clinometer=, Fig. 231, shows the kind that is used in Germany, of 5 inches diameter, graduated to degrees, made of thin brass. It is packed in the case with the hanging compass, described art. 542. The ends of the semicircle are formed into hooks for hanging on the line. The plummet has a horse-hair line, which cuts the degrees. The clinometer may be used only when the hanging dial Fig. 230 cannot be suspended near the centre of the line, in which case this light semicircle will cause less deflection of the line, and give the inclination approximately. For further details of the use of the hanging compass the reader is referred to Mr. B. H. Brough's admirable work on _Mine Surveying_.

546.--=Semi-circumferentor.=--This simple instrument can scarcely be enumerated with mining surveying instruments, as it is much more used for surface work; but being of the class of circumferentors to which miners' instruments generally belong, this is the most convenient place for its description. It has very general use on the Continent. Its construction is very simple, Fig. 232. It is supported on a ball and socket joint. The socket is formed in two pieces, which are clamped together to hold the ball by a winged-headed screw. One pair of sights is mounted upon the extreme ends of lugs upon the limb. The limb is divided to half degrees. When the ball is loosely clamped the fixed pair of sights may be adjusted to cut any desired object. A second pair of sights is jointed upon an axis to move centrally between the first pair. These are made shorter to pass within the first pair to any angle around the arc, except the small angles with which the sights themselves interfere when they are superimposed. The movable sights carry verniers to read on the limb to 2′. There is a small compass attached to the limb. As a cheap instrument for taking angles approximately it is very useful, particularly for workmen employed in carrying out work from drawings plotted from a survey by a better instrument. The weight of the instrument with 6-inch circle is about 2 lbs.; height above tripod, 7 inches.

547.--The tripod of this instrument is made of wood. The head is shown Fig. 233. The legs are simply extensions of the upper parts, which are shown attached with bolts. The point of each leg has a steel shoe to prevent it slipping in use. The head is turned to a cone, which fits into the socket-piece of the instrument and permits it to be rotated with moderate friction. The head is made of triangular section that the legs may be clamped firmly to it. When used for underground work a separate set of short legs is provided, which attach to the head by the same bolts.

548.--=Lighting Underground.=--The old underground station, formed of a lighted candle or lamp, is not now considered good in practice where surface land is exactly defined by boundaries held by legal clauses and rights. The system of underground surveying now very generally followed is that first recommended by Mr. Thomas Baker, C.E., and afterwards fully developed by Mr. H. Mackworth,[27] by which a station taken for angular directions is formed by the position of the centre of a tripod. For this system three tripods are provided for each instrument, with head adjustment complete. These tripods are made in such a manner that the instrument can be placed on any one of them in a level position. Two lamps are provided, the flame of either of which will take the position of the vertical axis of the instrument when the lamp is placed upon the tripod formerly occupied by it. It is easily seen that by this system fore and back sights or angular positions can be extended with all the accuracy that the uniformity of the flame of the lamp will permit.

549.--=Mining Survey Lamp.=--The author constructed this lamp from an idea given to him by Mr. Geo. Kilgour, C.E., Fig. 234. It is somewhat different from the ordinary form. Its accuracy does not depend upon the regularity of the flame. A vertical axis is formed under the lamp, which is made to the same fitting on which the mining survey instrument is placed. The lamp is placed entirely eccentric to the vertical axis in such a manner that a vertical line formed by a wire upon its face may stand central and linear with the axis. A cross line is also placed at the same height above the tripod head as the centre of the axis of the telescope or cross sight. By this means, although the lamp throws its light broadly in one direction only, the cross is a perfectly defined object, easily picked up and brought to exact bearing in the instrument when placed upon another tripod. In converting this lamp from a fore to a back sight it has simply to be turned half round on its axis, which is done without any displacement of the relative position of the cross in vertical or horizontal directions. Where this lamp is required in mines liable to fire-damp, it is made on the safety principle of the Davy lamp.

Electricity has been applied to lamps for surveying. This plan has been found successful where a secondary battery is used that can be charged by a dynamo upon or in the mine, or with some of the modern dry batteries.

550.--=Mining Targets.=--The three tripod system has been much improved by the introduction of accurate targets made specially for the instrument used, and interchangeable with the instrument on either stand. The reviser has designed several forms of these. They are generally used with a mining theodolite for high-class mine surveying, and the lower part is similar to the lower part of the theodolite they are used with. Instead of a horizontal circle, they simply support a plate carrying cross levels, and a pillar carried up to bring the target level with the optical centre of the telescope of the theodolite; this part is made to fit the outer centre of the lower part into which it is held by a special clamp. The theodolite is made with a double outer vertical centre, and this is held to the lower part similarly clamped, so that the theodolite and targets all lift out of their centres and interchange with each other. A complete mining set of this description is shown at Fig. 235. This forms a very complete mining outfit. It consists of a highest-class tacheometer with quick setting spherical lower plate, mechanical centring stage, auxiliary top and side telescope, illuminated axis, striding level, also two targets with quick setting spherical lower plates, mechanical stages, cross levels, and swivelled sighting crosses. All three are made with lift-out centres, which are interchangeable, and all have base plates permitting their use on any staging or fixing without their stands. The targets are sometimes made to hold candles instead of the swivelled cross, and sometimes with plain steel points only.

The auxiliary telescope is the special form designed by Mr. Dunbar Scott, and it embraces all the advantages and eliminates all the disadvantages of all other types.

The particular feature is its interchangeability with top or side positions, and the means provided to ensure perfect adjustment with the minimum of trouble, thus forming a mining transit which will perform with exactness all the complex functions in mine surveying and requiring no correction for eccentricity.

The auxiliary telescope is provided with a centre that may be screwed to the threaded extension of either the transverse axis or the vertical pillars of the main telescope. In either position it is clamped firmly and ranged quickly into alignment with the main telescope by two opposing screws. The diaphragm of the auxiliary telescope has one web only, so placed that it is vertical when on the top and horizontal when at the side.

The observation of steep horizontal angles is made only with the auxiliary on top, and of precipitous vertical angles with the auxiliary on the side. A counterpoise is provided, which exactly balances the auxiliary, so that there is no strain upon the instrument.

For vertical sighting it is also most useful and accurate, as by transferring the lines of both positions of auxiliary two lines are transferred down a shaft, at right angles to each other, which, if produced, will intersect each other exactly under the centre of the instrument, and no allowance or calculation whatever has to be made to ascertain the centre.

The whole attachment adds very little to the weight, the greater part being of aluminium, and it is packed separately in the case so as not to interfere in any way with the instrument when not in use.

In Fig. 235 the auxiliary telescope is shown at top; Fig. 236 shows it attached at the side.

551.--=Pocket Instruments.=--A very light pocket instrument has been designed by Mr. D. W. Brunton, which will be found useful; he terms it a pocket mine transit, but of course it has nothing to do with a transit. It is designed for roughly taking horizontal and vertical angles, and answers the purpose of a prismatic compass, clinometer and Abney level, and is very portable, made in aluminium, and weighing only 8 oz. It is shown at Fig. 237.

The cover is provided on its inside with a mirror, and this acts as a back sight; it is opened out to an angle which reflects the fore sight, and the object sighted and the reading of the needle is then taken. It is necessary to hold the instrument firmly against the body and see that it is level sideways by placing the spirit level across the box and bringing the bubble to the centre of its run, while any turning movement should be made by turning the body from the hips. For vertical sighting the fore sight is used as the back sight, and the mirror in the lid moved to reflect the bubble, the back sight being formed by the hole in the mirror seen at the bottom of the centre line, the clinometer bubble is then moved till the air bell is seen in the centre of its run and the vernier reading taken.

552.--=Dip Compass.=--This consists of a magnetic needle suspended between centres so as to move readily in a vertical plane, and is shown at Fig. 238. When in use the ring is held in the hand and the compass-box by its own weight takes a vertical position; it must then be held in the plane of the meridian. In this position the needle when unaffected by the attraction of iron assumes a horizontal position. When brought over any mass of magnetic iron ore it dips, and thus detects the presence of such ore with certainty.

If held in a horizontal position it serves as an ordinary pocket compass and thus indicates the magnetic meridian in the plane of which it should be held when used to ascertain dip.

FOOTNOTES:

[19] Plate xiv., fig. 5. _Geometrical Essays_, Geo. Adams, 1803.

[20] _Proc. Min. Inst._, Cornwall, 1883, vol. i. p. 317.

[21] Patent No. 1592, April 1878.

[22] Patent No. 1857, J. L Casartelli, May, 1874.

[23] Illustrated plate xv. Fig. 1., _Geometrical Essays_, John Adams, 1803.

[24] Pastorelli's patent, No. 2714, 1863.

[25] Hoffmann's patent, No. 2084, 1878.

[26] _Geometria Subterranea_, Voitel, 1686.

[27] _Subterranean Surveying_, Thos. Fenwick, Lockwood.