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.
491.—Plain Miner's Dial.—The original simple form of specialized miner's dial is shown in Fig. 201. It consists of a compass, divided to single degrees, read by a finely pointed edge-bar needle mounted on a jewelled cap. The needle has a sliding rider placed upon it, art. 130, so that it may be carefully balanced to horizontality in any locality in which it is used. The divided compass is raised on a step, and the upper surface of the needle is made to be quite level with the division when the compass is horizontal. In erecting the instrument with the needle correctly balanced, the compass may therefore be brought to horizontality by the coincidence of the upper surface of the needle with the plane of the divisions, without the necessity of having spirit levels.
Fig. 201.—Mining dial.
Fig. 202.—Cover to the same.
Fig. 203.—Sight.
Fig. 204.—Section of ball and socket joint.
492.—The compass-box is extended in one meridian, north to south, by strong arms that carry a pair of sights hinged to turn down to the surface of the cover for portability. The compass-box and arms together are termed the limb. The limb of the instrument is mounted upon a ball and socket joint to be described. The socket is slotted down on one side to permit the limb to be turned to a vertical position. In this position the level shown on the front of the instrument is used for levelling by means of the sights: this level is not, however, put on all plain dials.
493.—The cover of the compass-box, Fig. 202, is fixed on the box to a given position by a stud and slot. It has an arc divided upon its outer surface, which is centred from a small hole placed near the outer edge. A line from the centre of the hole to the zero of the arc is made perpendicular to the central indices of the sights. A piece of silk or a horse-hair carrying a small plummet is fixed to hang from the hole. By this means when the limb is turned down in the slot of the socket and the silk or hair stretched by the plummet to permit it to hang in front of the arc, it will then cut the divisions, and thus form a reading index to the arc, giving thereby approximately the vertical angle at which the sights are set to degrees.
494.—The instrument is mounted on a simple jointed tripod to be described. It will be seen by the above description that this instrument is cheaply made, and is not designed for very exact work. It is now giving way for more exact instruments, but it forms the groundwork on which mining survey instruments are most generally constructed. The height of this dial with sights erect is 11 inches; weight, 6 lbs. Some of the separate parts above enumerated, which are common to many other forms of mining instruments, will now be more particularly described.
495.—Sights, one of which is shown separately, Fig. 203, are common to mining instruments. They are constructed essentially in two parts, technically termed the slit and the window. The slit A is a narrow parallel cut made through the metal upon the inner surface of the sight, which is turned towards the centre of the instrument. The thickness of the metal is hollowed away on the outer side which comes next the eye, so as to present a thin edge only for the sighting slit, as shown in section at A′. In some instruments the slit is formed of two thin plates fixed to the sight by screws in slots, which render it adjustable both to width and position; this is the better way if machinery be not used for cutting the slit. The window B is an oblong opening, across which a hair wire or a thin plate placed edgewise is fixed in line with the slit. The hair or wire is laid in a deeply engraved line, so that it is in the same plane as the centre of the slit. The ends of the hair are held firmly by drawing them through small holes and fixing them therein by means of dry, conical, pinewood pins pressed tightly in the holes. When a thin plate is used edgewise, this is soft-soldered into the top and bottom of the window. In the pair of sights the window of one sight is placed at the lower position and the slit in the upper. In the fellow sight the positions of these parts are reversed, the observation being always taken from the slit through the window. The duplication of parts in each sight permits it to be used in either direction.
496.—In the use of the Sight the point or object to be observed from the slit should appear to be bisected by the hair in the window at the same time that it appears to the eye to stand in the centre of the slit. For this reason it is not necessary that the slit should be very narrow. It is generally more comfortable to take the sight with the eye at the distance of 10 to 12 inches in front of the slit to obtain clear vision of it. In this case if it be made too narrow it shuts out the field of view.
497.—It is not quite certain that the old slit and window is the best form. Many mining engineers prefer a pair of equal slits, one of which replaces a window. In this case, instead of the wire covering the object sighted in the use of the instrument, the object is made to appear in the centre of the forward sight slit. In this construction the sight apertures are made much narrower so that they do not cover too much of the field of view. Excellent work is done with this open form of sight, and its construction is much more solid than that of having loose hairs.
498.—Universal Sight, termed technically hole and cross sight, consists of a small hole C′, Fig. 203, on the inner side of one sight that is hollowed away on the outer side which comes next the eye, so as to present a thin edge of the hole only. The fellow sight C has a hair cross placed centrally in a circular window. This is of occasional use for sighting angles approximately in altitude and horizon simultaneously; but the cross occupies so much of the sight space that observation with it cannot be depended upon.
499.—Ball and Socket Joint.—This is shown in elevation Fig. 201 at F, and in section Fig. 204 F, D. It is one of the oldest forms of adjustment, and is common to many dials. When the clamping screw G is released the ball is free in its socket F to move about its centre, to the extent of the opening at the top of the socket, in any direction. A plug E, which really forms the lower half of the socket, is screwed into the part F′ at the lower part of what is technically called the socket-piece. The plug is turned upwards by its screws so as to tighten the ball by means of a tangent screw G which works in a rack thread cut in a part of the circumference of the plug, thus forming a screw and cross screw, which, as the construction indicates, clamps the ball with great rigidity. There are several other ball and socket arrangements; these will be discussed in describing the special instruments to which they are affixed. The only objection to this form is that it elevates the dial very much more than others.
500.—The Tripod Stand of an Ordinary Miner's Dial.—The upper part is shown in Fig. 205. This form of tripod is common to many dials. The legs are made about 1¼ inches in diameter. The heads of the legs are fitted directly without brasswork between the book-plates A, to which they are held by cross screws or bolts which form the joint on which the legs move for extension. Unless the head be worked out of the solid, the book-pieces are screwed to a plate that carries a male plug centre to which the dial is fixed by a milled-headed screw shown at Fig. 201 L. The plug is grooved at the position of the point of the screw so as to permit rotation of the instrument when the screw is slightly released. This tripod head remains permanently fixed to the legs. Each leg is jointed to part in its centre by unscrewing, to present when disjointed a metal point to hold the surface of the ground, to form a short stand. The usual height of the full tripod legs is 5 feet; the upper part only 2 feet 6 inches. The usual form of joint is shown in detail in section Fig. 205. C the male screw, which is fitted to the woodwork by a socket and cross pinned to it. This piece has a point at its lower end. D the socket-piece is screwed over the point to extend the leg when the tripod is required of full length. The woodwork of this lower piece has a conical metal point to bite the ground when it is set up in use. Occasionally for close work shorter legs are provided, or the legs are jointed in three parts. In the common dial shown, the legs are left exposed when out of use; with superior instruments they are packed in a deal case that protects the socket fitting to which the instrument is attached. Another much better form of tripod will be discussed further on with the instrument to which it is attached.
Fig. 205.—Jointed tripod legs of a miner's dial.
501.—Examination and Adjustment of the Plain Miner's Dial.—The tripod should be first set up to full length and each length separately twisted to right and left to see that its socket fittings are good and free from shakiness. The legs should each be separately pressed in and out at its centre to see that the screws clamp the parts firmly and are free from shakiness. The instrument should then be set up and its socket fitting be felt to see that it is free from shake, and also be turned round to see that it moves freely. The ball fitting should be clamped and its rigidity be tested by fair pressure on the two ends of the limb separately. The sights should be examined to see that they are quite linear with hair and slit. The compass-box should be levelled by the coincidence of the upper surface of the needle with the plane of the division, and be reversed in every direction by turning the compass-box, the reading being observed with the N. point of the needle at N. E. W. S. to see that it bisects the graduation by angles 180° apart. The compass-box being level, the sights should be ranged with an external object at a distance—a plumb-line is best—a piece of string suspending a stone answers—to see that they are vertical, and that they cut the same line with the position of the sights changed fore to back. If the sights are coincident, but do not range with the plumb-line, the needle is out of balance, and this may be corrected by shifting the rider.
502.—Henderson's Dial.—This is an improvement upon an old form of circumferentor,[19] in which four sights are centred in opposite pairs so as to revolve about the vertical axis, so that one pair of sights may take any angle to the other pair. In Mr. J. Henderson's dial the improvement consists in making the compass larger, the needle being made to read by a vernier placed upon one end to 3′ of arc. Mr. Henderson prefers plain slit sights instead of slit and window sights, as before stated, which avoids the accidental derangement of the horse-hair.[20] The instrument combines some of the parts of Lean's dial, to be next described. Illustration of this instrument is given in Mr. B. H. Brough's Mine Surveying.
503.—Lean's Dial.—The inventor of this instrument was Mr. Joel Lean, a Cornish mine manager, who was well known at the end of the 18th century for his important improvements in mining apparatus. This dial is still popular in Cornwall and other mineral districts. In general construction the sights and limb on which they are mounted are the same as in the plain dial just described, art. 491. The legs are also
the same—other parts are additional or modified. In the engraving, Fig. 206, the sights and vertical arc with its telescope are shown mounted together on the limb. This is done to show the relative position of these parts: they could not in practice be used simultaneously upon the instrument. They are separately attached to the limb by the same pair of milled headed screws. As a general rule the telescopic arrangement, which will be described further on, is used above ground and the sight arrangement below. The details of construction are as follows:—
Fig. 206.—Mining circumferentor or Lean's dial.
504.—The Tripod—of the mining circumferentor, in common with many other forms of dial, has the legs fitted directly between book-pieces, which are fixed to the lower parallel plate, as shown Fig. 206, thus dispensing with the separate tripod head, common to levels and theodolites. Otherwise the parallel plates are similar to those described for levels and theodolites, art. 193, and are used in the same manner. The upper parallel plate in this dial carries the male axis, which fits into a socket attached below the centre of the limb in the manner just described for plain dial. The tripod stand, with its parallel plates attached, is generally packed in a pinewood case when out of use. The reason for attaching the legs directly to the lower parallel plate instead of having a tripod head is that it saves the extra elevation of the instrument by the depth of one screw fitting. At the same time it must be observed that it exposes the axis to the air by separating the instrument at this part when it is put by, which renders the axis difficult to be kept lubricated and in smooth working order. On the Continent and in America it is general to detach the legs only, on a plan shown, Fig. 85, p. 140. This keeps the axis attached, and is probably the better plan, although it may be found a little more troublesome to erect the instrument.
Fig. 207.—Section of compass-box and axis of Lean's dial.
505.—Revolving Compass forms a part of Lean's dial and many other dials. It is shown in section Fig. 207. As the axis is constructed in this instrument, the socket-piece A is ground to fit the male axis S, and at the same time it is shouldered to fit the surface of the parallel plate T to prevent excess of friction on the axis fitting, so that it may move easily to set the needle to magnetic north of the compass-box if desired. The socket-piece is attached to the compass-box through a collar. The compass has a step D which is divided to degrees on its inner edge to read to the point of the needle, and similarly to degrees on its outer edge to read with a vernier scale, shown D to 3′. The vernier is set off on each side of the zero line in ten divisions, which are figured 30, 45, 0, 15, 30, art 322, p. 184. The upper surface of the needle is made level with the upper surface of the step. The bottom plate of the compass-box is divided to 10°: in some difficult positions in the use of the instrument this last is the only reading that can be sighted. The compass-box, which carries the vernier B, is fixed centrally on the arm plate. The arm plate is centred upon a step fitting between the compass and the socket-piece, so that it carries the whole superstructure of the instrument around the compass, its relative position being read by the vernier. The edge of the compass plate is formed into a toothed wheel, as shown in section in the figure on the right-hand side, into which a small wheel or pinion R is fixed in a box upon the arm plate that works by means of a large milled-head screw P. By means of this milled head the instrument may be rotated about the compass, so that the line of division on the compass step reading into the vernier performs the functions of the horizontal limb of a theodolite. In this manner angles may be taken by means of the vernier, quite irrespective of the reading of the needle. When the compass is set to the zero of the vernier at north (360°) it may be fixed in this position by means of a pin fitting in opposite holes to the arm plate and bottom plate of the compass, not shown; and when thus fixed the needle only is used as in the plain dial. Between the collar-piece C and the socket-piece A a wedge-shaped lift raises the needle off its centre by pressing in a slide shown at L.
506.—The Vertical Arc is erected upon the limb as close as possible to the compass-box, so as to leave room for a level to be placed between the seatings of the arc and sights. The axis of this arc is a simple hinge joint, brought down nearly to the surface of the cover which protects the glass of the compass-box: this is done to keep the instrument as low down as possible. The telescope, which is of the same kind as that used for the theodolite, traverses the arc tangentially, permitting it to be adjusted for reading the arc by its vernier by means of a clamp and tangent motion at any position. The arc is divided on one side into degrees, and reads by the vernier to 3′ in the same manner as the horizontal circle. On the opposite side it is divided with a percentage scale of difference of hypotenuse and base which reads to an index line. A spirit level is placed under the telescope, in line with its axis, to which it is adjustable by means of capstan-headed screws. The telescope when fixed is placed just sufficiently above the arc to permit it to be brought to a vertical position at 90°, or a degree or two over this, with the full aperture of the object-glass beyond the extreme edge of the horizontal circle. By this construction a bearing may be taken of any object upon the surface from the top of a shaft, and a line may be sighted to the bottom of the shaft in exact azimuth with this without changing the horizontal adjustment of the instrument. In the same manner, if the vertical axis be perfectly adjusted by the level on the vernier plate, the telescope at 90° + n will indicate a perfect vertical to the station of the instrument above, the + n being the allowance to be made for the eccentricity of the telescope, provided the collimation is perfect. If this is not perfect, the vertical may still be taken accurately by means of three observations taken from equal division of the entire horizontal circle, say at 360°, 120° and 240°.
507.—It will be noticed that the vernier to the compass circle comes directly under the vertical arc, therefore it can only be read obliquely when this arc is mounted: with open sights the vernier can be read directly. This is a defect in this instrument, as the vernier is mostly required for exact work when the telescope is used.
508.—Lean's dial possesses the qualities 1 and 4, pointed out in art. 490 as important to dials; in 4 the power of setting the telescope to the vertical with great facility being the most important. This quality has kept the dial a favourite with many mining engineers in mineral districts for many years. Otherwise for general work the compass is most inconveniently obstructed by the arc above it, and the instrument, although, of course, of less height than the theodolite, some of the functions of which it performs indifferently, is too high to be used in shallow workings. The height of a 5-inch Lean's dial to the central apex of the telescope is 9½ inches; to the top of the sights placed in a level position, 8 inches; weight of instrument only, 6½ lbs. The 6-inch instrument is about 1 inch higher, and weighs 1 lb. more.
509.—A number of variations have been made in Lean's dial; but none that the author is aware of has proved successful. In an instrument of this class, designed by Mr. J. Whitelaw,[21] the vertical arc is brought down to the compass-box by placing pivots on each side of the box after the manner of Hedley's dial, to be next described. This lowers the instrument about an inch, and is an improvement; but this is effected at the expense of placing a striding bar across the compass box, which is a great impediment to the clear sighting of the compass.
Messrs. Newton & Son have made the telescope to detach from the arc of Lean's dial to be placed directly upon the limb. In this way they claim for it that it combines a miner's dial and dumpy level. The arrangement appears to the author to make the instrument top heavy as a dial, and to give too little power for a good level, added to which it costs about the same as the two separate instruments of equal quality. Of course any telescopic dial may be used as a level by clamping it at zero. Practical surveyors generally object to compound instruments that entail many loose pieces. These were a fashion in the middle of the nineteenth century.
510.—Examination of Lean's Dial.—As regards the stand, sights and parallel plates, particulars have been given upon the plain dial just described. The revolving compass should be turned round by the milled head P, Fig. 207, of the pinion wheel R to see that the compass-box revolves steadily at all points without disturbance of the needle. It may also be particularly observed that the needle does not oscillate at any part of the circle, to be sure that the compass-box is quite free from iron. The vernier should be examined at four opposite positions of the needle to see that the needle is truly centred and is in accord with the vernier. The lifter should be tried to see that it lowers the needle gently on the centre, and that it holds the needle firm off the centre. The telescope should be set up and directed to an object, and all parts of the instrument clamped and the needle observed. The telescope should then be detached and the sights set up, to see that they range fairly with the telescope. If they do not do so the difference should be noted and treated as a constant in any case of change from telescope to sights on the same survey. The difference ought to be very small, otherwise the instrument should be returned to the maker.
511.—The Adjustment of Lean's Dial is the same as that of the plain theodolite, so far as this can be carried out; but generally the adjustment is depended upon as it leaves the manufacturer. For the general use of this and other dials some notes will be made further on, but as regards vertical position and the taking of azimuth angles, for which this dial is specially adapted, notes may be made here.
512.—To set a line in Azimuth with one taken above Ground.—This is necessary where there is local attraction to the needle below, or there is a suspicion of this, so that the needle cannot be depended upon with certainty. The instrument is placed on staging over the pit and a vertical is taken to its centre either by the means briefly discussed art. 506 by the instrument, or by suspending a plummet, a ball, or a bullet from the centre of the instrument by a thread and burning the thread when the ball is free from vibration. The ball is allowed to fall upon a smooth horizontal surface formed of earth or otherwise, in which it makes a dent which will be vertical to the axis of the instrument if the ball has not been deflected by ventilation currents. Two lights, as distant as possible to be seen to range in line with the dent, are placed at the bottom of the pit. The lights, if thought desirable, may range north and south with the needle; but in whatever direction this may be set the correct azimuth of this may be taken by cutting them by the webs of the nearly vertical telescope of the dial; and this azimuth may be correctly set out on the surface by a pole or other station mark, or its true direction by a pair of these, one on each side of the pit's mouth, the second station mark being set out after a shift of the horizontal vernier exactly 180° on the circle. A straight-edged flooring board painted white may be made to cut the line from light to light, which is more definite for bearing than the lights themselves.
513.—Hedley's Dial, the invention of John Hedley, H.M. Inspector of Mines, in 1850, has now become the most popular form of miner's dial, modified, however, from its original form in various ways. The peculiar feature of this form of dial is that the sights move upon a framework centred upon a horizontal axis, so that they may by a rocking motion take horizontal angles within a wide azimuth without obstruction to the sight of the compass.
Fig. 208.—Hedley's dial.
514.—For consideration of the general features of Hedley's dial, the tripod and the ball and socket are the same as that described for the plain dial; but the socket is not cut down on one side to change the position of the axis, as the compass-box in this instrument is required to be kept uniformly level. The general appearance is shown Fig. 208. For districts in which the working strata are fairly level, parallel plates are put to this instrument in place of the ball and socket joint. The compass-box revolves, as that described for Lean's dial; but it is more general in this instrument to have a clamp and tangent motion, as in a theodolite, than the rack and pinion motion. Two levels for setting the compass horizontal are sunk into the plate of the compass-dial low enough to miss the edge-bar needle. The step of the compass is divided into degrees and the plate of the dial to 10°. The vernier, which is placed on the opposite side of the box to the vertical arc, reads to 3′, as described for Lean's dial.
515.—The Rocking Centre forms the peculiar feature of Hedley's dial. From opposite points of the under side of the compass two pivots are projected. These are set perpendicular to the vertical axis, which is placed above the ball and socket. The pivots are placed central with the vernier and in line with E. to W. of the compass when this is set to zero (360°). The pivots form the axes of a stout ring—rocking ring—which surrounds the compass-box, with space sufficient to clear it when the ring is rocked about its axis. The ring has two extended arms which carry sights as shown. These turn down upon the compass-box when out of use. One of the pivots is prolonged for about ¾ inch beyond the outer circumference of the ring. The prolongation is made generally of triangular section. This forms a fitting to the vertical arc, which is attached by a milled-headed screw when required, the arc being an encumbrance when this dial is used for making horizontal plans only.
516.—The Vertical Arc, with its index arm, forms a separate piece. The arm is centred upon the arc with a ground fitting, which is retained in its position by a collar fixed with three screws. The arm-piece forms the axis, through the centre of which a triangular hole is made to fit the triangular prolongation of the pivot, so that the index arm remains fixed, and the arc moves with the rocking ring, to which it is held by a pair of dowels. The arc is divided into degrees on the outer edge of its surface, and a scale of difference of hypotenuse and base upon its inner edge. The graduations read to a single index line upon a fiducial edge carried down from an opening in the index arm.
Hedley's dial can be locked by a pin, which is attached to the under side of the compass-box, so as to work by the compass only. The ring can also be locked level with the compass by a sling latch-piece so as to convert it into a plain dial.
517.—The great merit of Hedley's dial is that the rocking centre permits a greater range of open sighting than any other; and the instrument is very low, permitting its use in shallow workings. Further, that it is a very strong instrument to resist accidents, and is very portable. The height of a 6-inch Hedley's dial above the tripod head, in a level position, is 9 inches to the top of the sights. Weight of instrument, 7 to 10 lbs.
Fig. 209.—Hedley's dial with ball clamp.
518.—In the author's simple dial, Fig. 209, which is of a modern form, the ball is clamped by a capping-piece over it moved to clamp by two stout pins. This form gives a little less height and still holds the dial firmly. The horizontal axis moves rather stiffly, so that no clamp to the arc is required. It is a very cheap form of dial, but substantially made. It answers for a small mine survey.
519.—There have been many variations made and proposed for Hedley's dial. Mr. Casartelli, of Manchester, places the arc over the centre of the compass-box.[22] This plan is intended to make the rocking centre firm; but the arc interferes a little both with the sights and the view of the compass box. Messrs. Davis and Son connect wheel-work with the arc, so as to magnify the scale of motion. Other less important variations in Hedley's dial are common.
520.—Examination and Adjustment of Hedley's Dial.—The general examination of the stand and of such parts of the instrument as correspond with Lean's dial is the same as just given. The rocking ring should be lifted and pressed down at each end alternately to see that there is no loss of time on the axis. The arc should be examined in like manner. The dial should be set up in front of a plumbed line to see that its sights range properly when the instrument is set level by its bubbles. A point should be observed, say through the hole and cross webs at the top of the sight; and with this point kept in view the rocking ring should be moved upwards or downwards so that the point traverses the plumb-line to the extent of the rocking motion. If it does not do so, possibly the transverse level in the plate of the compass-box may be adjusted to make it do so; but in this adjustment it must be particularly observed that the balance of the needle remains so that it still reads the graduation with its upper edge, and that the sights traverse the same plumb-line when turned about, as it is possible to set the level right with one pair of sights and throw other parts out. There are no simple means of adjustment provided, so that if the instrument is not accurate it should be returned to the maker for correction.
521.—Improvement in Hedley's Dial, by Addition of Telescope.—Surface work being generally performed with the theodolite, surveying with open sights following this cannot be effected with sufficient accuracy; therefore there becomes a necessity for the use of the telescope, which was first placed on this instrument by the author at the suggestion of Mr. W. Preece, C.E. In mines, also, although sights present often the only possible means of directing angular positions in cramped and tortuous workings, on the other hand, better work can very often be done and the telescope be conveniently used. Under these conditions, this addition forms an important improvement in the instrument, to be at hand to apply when desired. The telescope of this instrument detaches exactly as with Lean's dial, but the sights are made with an angle piece, so as to extend them to a distance of about 12 inches apart for sighting. Fig. 211 is of one cranked sight. The instrument illustrated Fig. 210 has parallel plates, art. 193, p. 99, suitable for fairly level workings. A ball and socket joint is sometimes fitted to this instrument in place of these.
522.—The Telescope is placed on Y's, and is of exactly the same form as that described for a plain theodolite. The Y's in this instrument offer a great convenience for reversing the telescope for back sights in range when the vertical axis is fixed. The level under the telescope is sufficiently good to convert this instrument into a level for drainage, etc., when the rocking ring is locked with the compass. Its examination and adjustment are the same as those last given, except for the telescope, which is the same in all particulars as that of a 5-inch plain theodolite.
Fig. 212.—Improved miner's dial.
523.—Improved Miner's Dial.—The illustration given, Fig. 212, is of the form of dial introduced by the author, a part of the arrangement only being of his own design. The telescope with Y supports is the same as that just described, and the sights, not shown, are cranked in the same manner as shown Fig. 211. The horizontal circle, instead of being in the interior of the box, is placed on the exterior rim, and reads with two verniers—not for correction, but for convenience of reading in different positions. The compass is divided upon the upper surface of the step to degrees, and in the same manner on the interior cylindrical surface of the step. This last often permits the compass to be read in a close working when the upper surface could not either be lighted or sighted. This plan was used on old circumferentors.[23] The plane of the compass is divided to 10° as usual. The compass adjusts by clamp and tangent motion. The axis of the instrument is supported upon a ball and socket arrangement designed by the author for roughly bringing the compass to level, and a parallel plate adjustment for final setting. The ball is fixed by clamping a pair of plates together by a thumb-screw. Each plate is hollowed in the centre to hold nearly half the ball. When fixed, the instrument is found to be very rigid.
524.—A plan of clamping designed by the author to meet the conditions of the tribrach system of adjustment of equal rigidity to that above described, is shown in elevation, Fig. 213 B. In this the upper half of the socket is screwed down outside the lower half socket by means of three projecting handle pins. This is a somewhat neater arrangement than that shown in Fig. 212. Either of the above-described ball arrangements elevate the instrument, and are better omitted for close working if there is a special adjustment in the tripod attached to the instrument, as that to be described presently, which will be found sufficient in most cases. The height of the instrument from the tripod is about 6½ inches; weight, 11 lbs. for both parallel plate and tribrach adjustments.
525.—Adjustable Tripod for Dials.—The author's improved form of tripod is adjustable to all heights between 30 inches and 57 inches, Figs. 213, 214. Each leg is formed of two stiff bars of mahogany, shown in detail, Fig. 214 G of section, about 1¼ inches by 5/8 inch, and a third bar or leg G′ of about 1¼ inches square, which slides between the other two. The sliding surfaces are grooved and tongued together in V grooves in the solid. Two strap-pieces of brass SS′ are fixed near the ends of the bars. One of these S′ is firmly soldered to a boss-piece that takes a thumb-screw, which has quite sufficient power to hold the leg G′ firmly at any position of extension. It is a rigid stand, which may leave the tripod head nearly vertical upon any inclination of the floor surface.
Fig. 213.—The author's adjustable ball joint and socket tribrach stand.
Fig. 214.—Adjustment to leg of tripod.
526.—Hedley's Dials, with Pastorelli's and Hoffmann's Ball Arrangements.—By either of these arrangements the ball and socket is brought down close into the parallel plate adjustment so that the dial is of less total height. Hoffmann's is now becoming the most popular system, as practice has shown it to be the most perfect for mining survey. By either of these arrangements the ball and socket is clamped by the same screws that bring the instrument to final position. In Pastorelli's arrangement[24] the socket is drawn down upon the ball by the adjusting screws. In Hoffmann's[25] the ball is pressed up into the socket, which is the exact mechanical equivalent. When the screws are lightly clamped the ball can be moved with moderate force, or even quite loosely by careful adjustment; and in either case, when the ball is once set, care must be taken to keep pressure constantly upon it during the final adjustment by the screws. The general arrangements are shown in two Figs. 215, 216, which are taken from the drawings of the respective patents. In Fig. 215, va, the axis of the instrument terminates in a ball e which works in a cup f. The axis has also a portion of a ball of greater radius b concentric with the lower ball e. The upper parallel plate d is cupped over this ball. When the parallel plate is moderately free on b, the axis va may be set to any angle within the range of the central opening of d; and as the friction upon bd is greater than that upon fe, the axis moves by the adjustment of the parallel plate screws aa. In Fig. 216 the action is precisely the same, except that the pressure is upwards instead of downwards. In Fig. 215 there are springs s under the parallel plate screw heads to keep contact when the screws are loosened. In Fig. 216 the spring is a plate under the screws s, the action being the same in both cases.
Fig. 215.—Pastorelli's ball and socket adjustment.
Fig. 216.—Hoffmann's ball and socket adjustment.
527.—Some objections have been made to this class of arrangement, over the simpler one of clamping the ball independently and then adjusting by the screws, as being more complex. On the other hand this compound arrangement has the merit in underground instruments of being lower and more compact, which is very important. The author has somewhat modified the arrangements of Hoffmann's head, as shown in the engraving on next page, to render it still more compact for mining instruments.