528.—In Fig. 217 an improved Hedley's dial is mounted upon an improved form of Hoffmann's head. The whole arrangement is very compact, rigid, and rapid in action. The height of this dial is 9½ inches; the weight 8 lbs. for a 6-inch instrument, in aluminium 5 lbs.

529.—Hedley's Dial with Cranked Rocking Centre.—One defect of the Hedley's dial, which in certain cases makes Lean's preferred, is that with the rocking ring the sights cannot be brought vertical for looking up or down a shaft. The author has devised a means of getting over this difficulty by making the ring of cradle form, thus throwing the bearing surfaces to sufficient height to cause the ring, when the arc is raised to about 90°, to fall under the compass-box and its adjustments, Fig. 218. This dial presents possibly the greatest refinements of the Hedley principle at the time of its patent, No. 9134, 1898. Since this date the reviser has introduced a few further refinements as illustrated at Fig. 219.

This instrument has tribrach levelling with quick-setting spherical lower plate, a sliding tribrach for centring over any desired spot, and full clamp and tangent motions to both horizontal centres. The dividing is upon silver on a 6-inch covered limb reading by two verniers to single minutes, folding sights interchangeable with telescope Y's, and this dial may be used upon any staging without its stand. The somewhat peculiar shape of the cranked rocking ring is necessitated by the movement of the sliding tribrach, which it has to clear in all positions for reading vertical sights.

530.—Accessories Common to Hedley's Dials are a vertical reflector and a diaphragm illuminator.

Reflecting Cap.—One of the disadvantages of Hedley's dials over Lean's was pointed out to be the impossibility of vertical sight where the two last described dials are not used. Some years ago the author devised a plan of obtaining this vertical sight by reflection by means of a reflecting cap, Fig. 220, placed over the end of the telescope. The cap is formed of a tube which fits the outer surface of the object end of the telescope. This is prolonged sufficiently to lock it by a dowel in correct position against revolution when the points that are used for index in the diaphragm of the telescope are vertical. The tube is cut in two and hinged to turn up, as shown in two positions H and H′. When turned up it leaves the tube open for direct vision. A reflector R is placed in the cap, and there is an opening below it equal to the full aperture of the telescope. It is easy to see that by this means a pair of lights or a line may be sighted up or down a shaft, and the azimuth of its direction be reflected to follow a line by slightly rocking the telescope upon its pivots. This may be done, however, with more refinement if there is a clamp and tangent motion to the vertical arc, which is placed only on first-class instruments.

531.—Illumination of the Diaphragm for observing the webs or a point, may be conveniently effected underground by employing a conical ring reflector in front of the object-glass. The aperture through the cone leaves the field of the object-glass nearly free, as it is only necessary that the cone should project in front of this for a very small distance. This reflector is placed over the object end of the telescope when it is required, just the same as the ray shade. The vertical reflector, Fig. 220, goes on the same fitting. The reflector Fig. 221 R may be made of silver or platinum. A light placed anywhere opposite this, and perpendicular to the axis of the telescope will throw sufficient light to show the webs or point. Sometimes a simple, plain mirror placed on an arm bent over to the centre of the front of the object glass, in which the mirror stands at 45° to the axis, is used; but this plan is not so good as that shown Fig. 221, as the light has to be brought to face the mirror quite perpendicular to the axis of the telescope, and this process is frequently difficult to accomplish underground.

532.—Continental Forms of Miner's Dials.—On the Continent generally sights have been abandoned for miner's dials. The telescopes are usually of short form, with large object-glass and wide field of view. The telescope is generally placed eccentrically, which permits the instrument to be made of very low form. There is a certain amount of disadvantage in the eccentricity of the telescope, as angles cannot be taken direct from the centre of the instrument but this is compensated for in the plotting by making each station a small circle equal to the amount of the eccentricity of the instrument to scale, and setting off angles tangentially to this, which may be done with a little more trouble than that of plotting the angle from a point.

533.—French Miner's Compasses.—Fig. 222 shows the simpler form of this instrument. The needle is open and quite free from obstruction. The telescope is centred about level with the compass-box. The vertical axis has clamp and tangent adjustment. The transverse axis is set entirely by hand as with the plain dial. The instrument is set up level by its tribrach adjustment. The height with 5-inch needle in a level position, without tripod head, is about 5 inches; weight about 11 lbs. without the tripod table. The extremely squat form of the instrument permits its use in very close workings, with a short tripod, if the workings are fairly level. It is used also as a cheap form of surface surveying instrument, consequently it is not generally very carefully made. As a good instrument of the class it cannot compete with that to be next described.

534.—It will be seen by Fig. 222 that the instrument has no direct connection with its stand or tripod. This is general with all French and German instruments, even with theodolites and surveying levels, it being the rule that the top of the tripod should form a kind of table upon which the instrument is set up. The table is almost uniformly made of wood, and is somewhat bulky and clumsy in construction, therefore not very well adapted to mining surveying, particularly in wet mines. Neither is the tribrach system of adjustment, unless it is supplemented by some form of ball and socket arrangement, or with adjustable stand. This subject will be further discussed in the description of superior instruments presently.

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.

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.

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.