Chapter II: Practical Operations and Details

=18. Tools and Appliances.=--The different operations of mining are carried on by the use of picks, shovels, bars, saws, axes, hammers (large and small), chisels, wheel and hand barrows, windlasses, ropes, wooden or leather buckets, gauge-sticks, mason’s levels (Pl. XI, Fig. 4), plumb-lines, candles, closed lanterns, tin pipes, rubber and canvas hose, canvas, nails, etc., etc., of the kinds in common use; and the following special tools and appliances, viz.:

The _Miner’s Pick_. Smaller and lighter than the common pick. Neither its head nor its handle exceeds 2 feet in length.

The _Miner’s Shovel_. Similar in shape to a common shovel, but not exceeding 2 feet in length.

The _Push Pick_ (Pl. XI, Fig 5), which has a lance-shaped blade about 3½ in. wide and 6 in. long attached to a handle about 2 feet long.

The _Field Level_ (Pl. XI, Fig 6), which consists of three strips of wood about 2" × ½", arranged as shown. The strip _A_ is 4' between centres of pins; _B_ and _C_ are 2', 9-15/16"; the angle at a = 90°. A spirit-level is inserted in piece _C_, and a plumb-line attached as shown. The markings on _A_ are used for gentle slopes, those on _B_ for steep ones.

The other sides of _B_ and _C_ are divided into degrees of arc, the centre being at the middle point of the outside edge of _A_.

The _Slope Block_, which is a wooden cube used in connection with a mason’s level for fixing slopes.

_Angle Templets_ (Pl. XI, Fig. 7), making a definite angle, used in laying out galleries.

The _Miner’s Truck or Car_. A small, four-wheeled wagon with fixed axles and very short wheel-base; exterior dimensions about 20" wide, 18" to 20" high, and 30" long. Used for carrying earth through galleries, and usually hoisted up the shaft and dumped outside, replacing the buckets used in sinking the shaft.

The _Miner’s Bellows_ (Pl. XI, Fig. 8). A leather bag with wooden top and bottom, provided with inlet and outlet valves, from the latter of which the air is led off in pipes or hose. In using the bellows the miner stands upon the lower handles and works the bellows with his hands by the upper ones.

This is frequently replaced by a common blacksmith’s bellows or the rotary blower from a portable forge; and sometimes by an improvised air-pump, consisting of a large open cask filled with water and another smaller one, with one head removed and the other provided with outlet and inlet valves and an air-tube, inverted over the large cask, supported by a spring-pole and worked up and down by hand in the water of the lower cask.

The _Miner’s Candlestick_ (Pl. XI, Fig. 9), which holds a candle, and may be driven into the side or bottom of a gallery.

_Miner’s Lamps_ (Pl. XI, Fig. 10), can be used only when the ventilation is good, as they give off more smoke than candles.

When an electric-light plant is available, incandescent lamps will be used for mining.

_Earth Augers_, similar to those used for boring post-holes, but of different diameters, are sometimes used for placing camouflets. Their shanks are made in short lengths, which can be joined together to allow of boring a deep hole from a narrow shaft or gallery.

GALLERIES AND SHAFTS.

=19. The Dimensions of Galleries and Shafts= are determined by the use to be made of them, the necessity of ventilation, and the minimum space in which a man can work.

They are usually about as follows, viz.:

Height, Width,
feet. feet.

1. Great or grand galleries 6 7
2. Common galleries 6 3½
3. Half galleries 4½ 3
4. Branches 3½ 2½
5. Small branches (_rameaux de combat_). 2½ 2

Shafts vary in size--from the smallest in which a man can work (about 2' × 4'), to any size that may be required, seldom exceeding 10' × 10'.

Great galleries are used for descent into a ditch, and when it is wished to pass cannon through them.[13]

Common galleries are used for descent into a ditch, and for communications. Troops can pass through them “by twos.”

Half galleries answer for general purposes of attack. They allow the miner to work freely in different positions without being cramped, but are small enough to admit of rapid driving. Branches and small branches are driven out from the galleries to the mine-chambers, etc. They can be driven rapidly for short distances (10 to 20 feet); but when of greater length the earth is removed from them with difficulty, they are not easily ventilated, and are too small for use as communications.

=20. Shaft and Gallery Linings.=--In very firm soil it is sometimes practicable to drive small shafts and galleries short distances without lining them; but if these are to stand for any length of time, there is always danger of their falling in, particularly if shaken by the explosion of mines in their vicinity. When it is considered safe to use them, however, the shafts should be elliptical in plan, and the roofs of the galleries should be pointed arches. As a rule, however, both shafts and galleries should be lined. Those which are permanent in their character--as the main galleries of the countermines of a permanent work--are lined with masonry. Masonry linings may be of brick, stone, or concrete walls and arches. The smaller galleries constructed during a siege, and all the shafts and galleries of the attack, are lined with wood. Wooden linings are of two general types, known as _cases_, and _frames and sheeting_.

=21.= _Cases_ (Pl. XI, Figs. 11 and 12) are made of plank, from 6" to 12" wide, each case consisting of a cap-sill, a ground-sill, and two stanchions. The cap and ground-sills are cut to a length equal to the clear width of the shaft or gallery plus twice the thickness of the stanchions; a rectangular notch is cut in each end to receive a corresponding tenon cut on the stanchion. The length of the stanchions between shoulders is equal to the clear length of the shaft or height of the gallery. The length of the tenons is generally equal to the thickness of the cap and ground sill (usually 2´´), and their width about three inches. Notches are cut in the sides of all the pieces of the case, as shown in the figure, for convenience in handling them.

In grand galleries the tenons at the top of the stanchions are usually shorter than the thickness of the cap-sill, and those at the bottom, as well as the mortises in the ground-sill, are omitted. The stanchions are kept from collapsing by blocks nailed to the ground-sills. These blocks are 2" thick, and wide enough (about 9´´) to so guide the wheels of a gun-carriage as to prevent the axle striking the stanchions.

In cases for smaller galleries also the tenons are sometimes omitted at the bottom of the stanchions, the mortises in the ground-sills cut an inch or two deeper, and the stanchions kept from collapsing by keys driven in the mortises (Pl. XI, Fig. 13).

=22. Shaft and Gallery Frames= (Pl. XI, Figs. 14, 15, and 16) are made of scantlings, halved together at the ends, as shown in the figures. _Sheeting_ is made of boards or planks of the necessary thickness, sawn to proper lengths, and bevelled at the ends. When sawn lumber is not available, the frames may be made of saplings, and in some cases poles may be used for sheeting.

The middle of each cap and ground sill, both in frames and cases, is distinctly marked by a shallow saw cut or otherwise.

=23.= The following table gives the dimensions, in inches, usually adopted for the pieces of cases, frames, and sheeting, for galleries of different sizes, viz.:

----------------+------------------------------------+---------------------------------------------
| Cases. | Frames and Sheeting.
+------------+-----------+-----------+------------+-----------+---------+----------
|Ground-sill.|Stanchions.| Cap-sill. |Ground-sill.|Stanchions.|Cap-sill.|Sheeting.
----------------+------------+-----------+-----------+------------+-----------+---------+----------
| In. | In. | In. | In. | In. | In. | In.
Great galleries | 3 | 4 | 5 | 6 × 4 | 6 × 6 | 6 × 9 | 2
Common galleries| 2 | 2 | 2 | 6 × 3 | 6 × 6 | 6 × 8 | 1½
Half galleries | 2 | 2 | 2 | 5 × 3 | 5 × 5 | 5 × 7 | 1½
Branches | 1½ or 2 | 1½ or 2 | 1½ or 2 | 4 × 3 | 4 × 4 | 4 × 5 | 1 or 1½
Small branches | 1 to 2 | 1 to 2 | 1 to 2 | 3 × 3 | 3 × 3 | 3 × 4 | 1
----------------+------------+-----------+-----------+------------+-----------+---------+----------

The cases of branches and small branches are sometimes made very strong, with a view to resist rupture by the explosion of neighboring mines. For this purpose cases made of oak plank 4" thick are used, and the branch near its end is packed full of scantlings provided with rope-handles at their ends for withdrawing them after the mine is fired. This packing is, however, of doubtful utility, since a compression of the case sufficient to call the resistance of the packing into play is very apt to produce a permanent deformation of the cases, which will jam the scantlings and prevent their removal. For convenience in use the pieces of cases should be of uniform width.

=24. Relative Advantages of Cases, and Frames and Sheeting.=--In favorable soil, cases, when they can be obtained, allow of more rapid progress and give a lining with a smooth interior. In very bad soil they cannot be used for the larger galleries.

Frames and sheeting can be used in all soils which admit of mining operations, and can usually be improvised from materials found in the vicinity.

=25. Sinking a Shaft by Frames and Sheeting.=--The size and position of a shaft are usually determined by the character and direction of the gallery which is to start out from it. It is evident (Pl. XI, Fig. 17) that the clear width of the shaft must be enough greater than the outside width of the gallery to allow the side sheeting of the gallery to be freely inserted outside the frames of the gallery and inside those of the shaft; also, that the shaft frames must be so spaced as to leave a clear space at the bottom for the gallery. This space must be equal to the clear height of the gallery, increased by the thickness of the cap-sill, the sheeting, and one or two inches for easy working. This and the thickness of one frame being deducted from the depth of the shaft, the remainder may be divided up into a number of equal or unequal parts called _shaft intervals_. In order that the sheeting may not yield under the pressure of the earth, these intervals seldom exceed 4 feet.

The length of the shaft must be great enough to allow the miners to work freely, and to insert the sheeting for the first gallery interval.

The sheeting for both shafts and galleries is cut in lengths about 1 foot greater than the interval between frame centres.

=26.= The size and position of the shaft having been fixed, the top frame (Pl. XI, Fig. 15) is placed in position and secured by pegs, and the direction of its axis is accurately fixed by the score marks at the middle of the end pieces. The side and end pieces of this frame are respectively about 3 feet longer than those of the other frames, and are so halved together as to make of their ends four projecting _horns_, 1½' long, which keep the frame in place during the excavation of the first interval.

This frame is usually placed with its top flush with the surface of the ground. The miner proceeds to excavate the shaft with pick and shovel, making it large enough in plan to admit the sheeting outside the frame. Usually the first interval can be excavated without supporting the earth at the sides, which are vertical or slightly undercut, so that at the bottom of the interval the shaft will be large enough to admit the second frame, the sheeting of the first interval, and a system of wedges which hold this sheeting out from the second frame a distance somewhat greater than the thickness of the sheeting of the second interval. The verticality of the sides is determined by the plumb-line, and the size of the shaft by two gauge-sticks cut respectively to the outside length and width of the excavation, and distinctly marked at their centres.

To avoid the inconvenience of working under the top frame, the first interval is frequently marked out and excavated before the frame is fixed in its position.

When the shaft is deep enough the second frame is put in place and nailed together; the notches in the ends of the side pieces turned upward and those of the end pieces downward. The top and second frame are connected by nailing to them four battens of proper length (two on each side), which suspend the second frame from the top frame at the established interval. The second frame is placed vertically below the top frame by using the plumb-line and the scores in the frames.

The sheeting is inserted outside the top frame, bevelled end first, bevel outside, and pushed down until its top is flush with the top frame. The lower end of the sheeting is held out from the lower frame by suitable wedges, and the excavation of the second interval is commenced.

In ordinary soil the sides of the shaft will now require support. Sheeting is therefore introduced and pushed down as the excavation proceeds, and the wedges previously placed are removed to make room for the sheeting.

=27.= If the pressure of the earth becomes great enough to spring the sheeting-planks inward, an _auxiliary frame_ is introduced. This is a frame similar to the shaft frames, but from 4 to 6 inches larger in outside dimensions.

The sheeting rests directly against the outside of this frame, and is thus held out far enough to allow the third frame to be placed and the wedges to be inserted as before.

The auxiliary frame is then removed and used in the next interval.

=28.= Successive frames are placed in the same manner until the one directly over the gallery is reached. Great care is taken to place this frame at exactly the right height, and the shaft is then continued to the required depth. A frame is placed at the bottom with its top at the level of the floor of the gallery, and the sheeting is allowed to rest directly against the outside of this frame. When the soil will allow it, the sheeting is omitted wholly or in part over the portion of the shaft which is to form the gallery entrance.

=29. Precautions.=--In sinking shafts especial care must be taken to make the excavation no larger than is required for placing the lining, since if a vacant space is left outside the lining the sides of the shaft may give way through its entire height, and fall against the lining with a blow which will crush it in.

_This is often the cause of fatal accidents both in shafts and galleries._

=30. Partly-lined Shafts=, i.e., those in which the sheeting-planks are separated from each other by greater or less intervals, should only be used for small depths and when they are expected to stand for a very short time.

They are a constant menace to the miners, owing to the danger of their caving in, and in a much greater degree to the probability of stones, etc., falling from the unprotected parts and seriously injuring or killing the men at the bottom.

=31. Driving a Gallery with Frames and Sheeting.=--The direction of the gallery has already been marked by the scores on the shaft frames; but it must be verified by plumb-lines, and two small pickets be driven on the line of its axis, which is located exactly by small nails, one driven in the head of each picket.

Two gauge-rods are prepared, giving the extreme height and breadth of the excavation, i.e., the height of the frame plus two thicknesses of top sheeting, and the breadth of the frame plus four thicknesses of side sheeting. The middle of each gauge-rod is also plainly marked.

A gallery frame is set up against the side of the shaft (Pl. XI, Fig. 17), its ground-sill flush with the bottom frame of the shaft; or its stanchions may rest upon the shaft frame as a ground-sill.

The gallery frame is carefully located and fastened in position with battens and braces. The shaft sheeting is then forced down two or three inches with a bar, and the top sheeting of the gallery inserted and driven in until its end is supported by the earth. It is given the proper upward pitch by a scantling laid across it and secured to the shaft frames. The shaft sheeting is forced further down, the earth at the top excavated, and the top gallery sheeting advanced. As this work proceeds the side sheeting-planks are successively inserted and driven forward.

In this way the gallery is advanced one gallery interval, usually about 4 feet, when a second frame is placed. Its position is verified by the score marks; for direction, by a line; for grade, by a spirit, mason’s, or field level; and for verticality, by a plumb-line. It is then secured in place by nailing battens to it and the preceding frame. Wedges are inserted between the frame and the sheeting, and the gallery is continued by the same methods (Pl. XII, Fig. 19). When the sheeting is advanced only by hard driving the frames are slightly inclined to the rear at first, and are afterwards driven forward until vertical.

=32.= If, while advancing the sheeting, the pressure upon it becomes so great as to spring it, a _false frame_ (Pl. XII, Fig. 18) must be used. This consists of a cap-sill, ground-sill, and two stanchions, connected by mortises and tenons. The stanchions have tenons and the sills mortises at each end. The cap-sill is usually rounded on top and, for facility in setting up and removing, its mortises are longer than the width of the tenons. The latter are held in place by wedges when the frame is in position. The false frame is usually made of the same height as the common frames and, when side sheeting is used, wider by twice the thickness of this sheeting. When side sheeting is not used, its outside width may be equal to the clear width of the gallery.

In using the frame (Pl. XII, Fig. 19) the ground-sill is first placed accurately in position at a half interval in advance, the stanchions are set up, and the cap-sill placed upon them and wedged. The whole frame is then raised about 2 inches by folding wedges placed under each end of the ground-sill, and is secured by battens. The sheeting will now rest directly upon the cap-sill and stanchions, and have the proper inclination to clear the next frame by its own thickness, as is required.

The next frame is then set up, the wedges driven under the sheeting, and the false frame removed; which is easily done, owing to its construction.

=33.= When the soil is very bad a _shield_ (Pl. XII, Fig. 20) is used to prevent the earth in front and above from caving into the gallery. In starting out from the shaft the following method is adopted: As soon as the top sheeting is sufficiently advanced and the shaft sheeting is forced down about 1 foot, the top plank of the side sheeting is inserted and driven forward about 2 feet, and the earth at the top of the gallery is excavated for from 6 inches to 1 foot in advance. A piece of plank a foot wide and in length equal to the width of the gallery is then placed directly under the top sheeting and against the face of the excavation, and is held in place by braces at its ends secured to the shaft lining. The shaft sheeting is then lowered another foot, the next plank of the side sheeting inserted, the earth excavated, and a second plank of the shield placed in the same way as before. This is continued until the entire face is covered. The top and side sheeting are then driven forward, and the top plank of the shield is removed and replaced in advance; after which each plank is removed and replaced in succession, as above described.

=34. Inclined Galleries.=--_Method of fixing the slope._--If the gallery instead of being horizontal is _ascending_ (Pl. XII, Fig. 21) or _descending_ (Fig. 22), the proper slope is obtained by using a _slope-block_ whose edge is equal to the rise or fall of the gallery in one interval. This is placed upon the lower of two consecutive ground-sills, and the proper height of the other is determined by a mason’s or spirit level (Fig. 21). If a field-level or a mason’s level properly marked for the slope is used, the slope-block may be dispensed with (Fig. 22).

=35. Position of Frames.=--In driving _descending galleries_ better progress will be made and less material used if the frames are set at right angles to the axis of the gallery (Pl. XII, Fig. 22); and this is the usual custom. In driving _ascending galleries_ this is impracticable, and the frames are set vertically (Fig. 21). In all other respects inclined galleries are driven in the same manner as horizontal ones.

=36. Partly-lined Galleries.=--In very firm soil side sheeting may be omitted entirely, and in that less firm the side planks need not be in contact. When the side sheeting is omitted the width of excavation may be reduced to the clear width of the gallery, and the stanchions be let into the side wall flush with its surface. In this case the ground-sills are frequently omitted, the stanchions resting upon wooden blocks, stones,or directly upon the earth.

To save material, the planks of the top sheeting are sometimes more or less separated also. This can only be recommended when rapid and temporary work is required with limited materials; and in these cases the earth between the planks should be supported by a packing of sticks, brush, etc., etc.

=37. Change of Direction in Galleries.=--In changing the direction of a gallery, the new direction is laid off by using a carefully made angle-templet (Pl. XI, Fig. 7) or slope-block, field-level, etc., and it is prolonged in the new direction by the methods already described. When the soil is firm enough to stand safely while excavating and lining one gallery interval, even if somewhat short, no difficulty exists in changing the direction of a gallery in either a vertical or horizontal plane, since the excavation in the new direction may be made so large that the miner working in it can place the new frames and introduce the sheeting and wedges. The gallery can then be carried on without diminution in size.

When the soil is bad, however, special arrangements must be made for introducing the sheeting.

=38. Change of Slope.=--To pass from a horizontal to an _ascending_ gallery (Pl. XII, Fig. 21) it is only necessary to give the top sheeting the proper angle by holding down its back end with a piece of scantling placed across the gallery for that purpose; and, to give the side sheeting the proper inclination, cutting trenches in the bottom of the gallery for the lower pieces, if necessary.

In passing from a horizontal to a _descending_ gallery (Pl. XII, Fig. 22) the roof may be carried forward horizontally, and the floor given the desired pitch by increasing the height of the consecutive frames, until enough head-room is obtained to allow the top sheeting for the descending gallery to be inserted at the proper height and angle. The frame at this point is made with a cap-sill (upon which the sheeting rests directly), and a second cross-piece below it, serving as a cap-sill for the descending gallery. From this point forward the frames may be set perpendicular to the axis of the gallery, as previously stated.

If the descending gallery is very steep and the horizontal pressure of the soil great, it may be necessary to strengthen the stanchions of the last two or three vertical frames by cross-pieces near their upper ends.

=39. Change of Direction Horizontally.=--Slight changes of direction of narrow galleries, either to right or left, may be made in a manner entirely similar to that above described for descending galleries, by widening the frames until the side sheeting can be inserted at the required angle, and strengthening the cap-sills, when necessary, with additional stanchions.

When the gallery is wide or the changes of direction abrupt, however, it is customary to drive the gallery entirely beyond the turning-point, and then break out a gallery in the new direction from the side of the original gallery.

=40. Returns.=--A gallery starting out from the side of another is called a _return_, and is _rectangular_ or _oblique_ according to the angle made by its axis with that of the original gallery, which is called the _gallery of departure_.

That the return may be broken out, the interval between the frames of the gallery of departure at this point must be such as to admit between the stanchions a frame and the side sheeting of the return (Pl. XII, Fig. 23). This part of the gallery of departure is called a _landing_, and its floor is made horizontal.

If the return is oblique (Fig. 24), its width measured along the gallery of departure will be determined by an oblique section, and may be so great that the strength of the lining of the gallery of departure will not allow the necessary length of landing. In this case a short rectangular return is first broken out from the side of the gallery of departure, and the new gallery is broken out from the side of this return (Fig. 25). The latter method diminishes the length of the landing when the change of direction is less than 45°.

=41.= The floor of a return is started at the level of the floor of its landing. In firm soils which will stand for a short time without support the first frame may be set up entirely outside the gallery of departure (Pl. XII, Figs. 24 and 25) and may be of the same height in clear as this gallery. When the soil is bad, however, and side sheeting is required in the gallery of departure, the first frame of the return must be set up against this sheeting in the interval between the stanchions of the landing (Fig. 23). This makes the clear height of the return at this frame less than that of the gallery of departure by a little more than the thickness of the sheeting.

The first frame of an oblique return should be so made that the sides of the stanchions will be parallel to the side walls of the return, thus giving a good bearing to the side sheeting.

In very bad soil, the first few frames of a return must be firmly braced to resist the backward thrust of the earth, by battens connecting them together and by struts across the gallery of departure. The latter are removed when the return is sufficiently advanced.

=42. A Complete Map= must be made of every system of mines, showing the centres of shafts, and the axes and slopes of all galleries; giving also the references and lengths of all landings, and the locations, references, and dimensions of all mine chambers.

=43. Working Drawings= must also be made from which sheeting can be cut to proper lengths and angle-templets, etc., cut and framed.

These can be easily and accurately drawn by remembering that, to allow the miners to insert the sheeting, every return must have such dimensions outside its sheeting that, if it were free to move, its lining could be slid back across its landing as a drawer slides in a table. The size of the landings and dimensions of frames having thus been determined, the parts of the galleries between them may be divided into intervals, which, for convenience, should be equal, since this will allow the sheeting to be cut to a uniform length.

=44. Sinking a Shaft with Cases.=--(Pl. XII, Figs. 26 and 27.)--A case of the required size is put together and accurately placed upon the site of the shaft, whose dimensions are marked upon the ground outside it. The case is then removed and the earth excavated to the depth of the case, which is placed in the excavation with its top flush with the surface of the ground. Its position is carefully verified, and it is secured in position by packing earth around it. The excavation is then continued for the depth of another case, which is put in place as follows, viz.:

One end piece is placed in position, the tenons of the two side pieces are inserted in the mortises at its ends, and the side pieces are pushed back into position; a pocket-shaped excavation is made with a push-pick beyond the end of one of the side pieces and running back three or four inches into the side wall; the remaining end piece is inserted in this far enough to allow the mortise at its other end to slip over its corresponding tenon; it is then drawn back, and the tenons at both ends fitted into their mortises. The notches cut in the sides of the pieces allow them to be easily handled.

The next case is placed in the same way, care being taken not to excavate two consecutive pockets at the same angle.

When practicable, it is well to fill up these pockets by stuffing in sods from below before placing the next case.

Some miners prefer to place one tenoned piece first, then the two mortised pieces, leaving a wedge-shaped opening behind one of them, and insert the other tenoned piece last, drawing the mortised piece forward upon its tenon.

When the sides of the cases are tenoned at one end only and secured by wedges at the other, they are easily placed in position without cutting out behind them.

=45.= Upon reaching the level of the top of the gallery, the pieces on the gallery side of the shaft are omitted if the ground is firm, but if it needs support these pieces are put in place and secured by cleats or braces, but the tenons are not inserted in the mortises.

=46. Driving a Gallery with Cases= (Pl. XII, Fig. 27).--This is practicable only when the soil is somewhat firm. In breaking out from the side of the shaft, a frame is first placed inside the shaft to support the ends of the shaft cases resting against the pieces which are to be removed. The latter pieces are then taken out and grooves are cut in the earth for the ground-sill, stanchions, and cap-sill of the gallery, and these are put in place in a manner entirely analogous to that described for sinking a shaft. This case is set flush with the inside of the shaft and supports the side pieces, whose tenons rest upon its stanchions. The projecting earth is then cut away and grooves are cut for the next case, which is placed in position and the excavation continued as before.

=47.= When the earth shows a tendency to cave, which it frequently will in great galleries, the cap-sill must be put in position and supported while the miner excavates the grooves for the ground-sill and stanchions.

To support the cap-sill, two crutches are used. A _crutch_ (Pl. XII, Fig. 28) consists of an upright piece of timber carrying a cross-piece whose length is equal to the width of two cases. The upright piece rests upon the ground-sill of the case already placed, and is raised to the proper height by wedges. The part of the cross-piece which projects in advance is made 2 inches higher than the rear part, to support the cap-sill somewhat above its final level, so as to allow the tenons of the stanchions to be easily inserted. The rear part of the cross-piece is attached to the upright by an iron rod or short chain.

So soon as the case is set and adjusted to position the crutches are taken down by removing the wedges, and are replaced under the next cap-sill.

=48.= In very firm soil shafts and galleries are frequently driven with cases not in juxtaposition, but separated by greater or less intervals. Pieces of planks (which may be parts of cases) placed vertically and resting against the sides and ends of the cases in shafts, or horizontally and resting upon the cap-sills in galleries, and somewhat separated from each other, are used to support the earth between the cases.

The same remarks apply to this construction as to the similar one sometimes used when mining with frames and sheeting.

=49. Change of Direction in Galleries Lined with Cases.=--Slight changes in direction in a horizontal plane can be easily and gradually made by setting each case a little obliquely to the one preceding it, and separating the stanchions on one side while they touch on the other, supporting the roof in the wedge-shaped openings, if necessary, with pieces of wood, etc. For an abrupt change, it is better to break out a rectangular return from the side of the gallery and pass from this into the required direction by gradual change.

If the return is to be of the same height as the gallery of departure, the cap-sills of the latter, for a distance equal to the width of the return, are lifted off the tenons of the stanchions by struts and wedges, and the first case of the return is set as in breaking out from a shaft; the ground-sill is, however, narrowed by the thickness of the stanchions of the gallery of departure so that the face of the case of the return is flush with the inside of the gallery of departure, and the ends of the cap-sills of the latter rest upon the cap-sill of the first case of the return.

=50. Change of Slope.=--In passing from a horizontal to a _descending gallery_ the change may be made gradually, in a manner similar to that described for a change in horizontal direction, and the cases remain normal to the axis of the gallery. (Pl. XII, Fig. 31.)

To pass to an _ascending gallery_ by the method above described would require the earth at the face of the gallery to be undercut in order to introduce the case, and this undercutting would be continued so long as the cases were normal to the axis of the gallery. This construction is, as a rule, impracticable. In ascending galleries, therefore, the cases are set with their stanchions vertical, while their cap and ground sills lie in the slope of the roof and floor of the gallery.

=51.= To conform with this requirement, and for convenience in setting up, the ends of the stanchions receive the proper bevel, while the sides of the tenons and mortises are made parallel to the sides of the stanchions. (Pl. XII, Fig. 12.)

=52. Shafts à la Boule.=--In order to place a charge of explosive directly under the ground occupied, or for other reasons, it is frequently necessary to sink a small shaft in the least possible time. For this purpose a modified form of cases is sometimes used, in which the ends are halved together instead of being tenoned and mortised. (Pl. XII, Fig. 29.) They are spaced at greater or less distances apart, according to the nature of the ground, and are connected together by battens. Stones, pieces of wood, etc., etc., are driven between them and the sides of the shaft to support the latter.

This construction is called a shaft à la Boule. It is expected to stand for a few days at most. Many other extemporized linings may be used for similar purposes.

=53. Blinded Galleries.=--Galleries cannot be successfully driven with less than 3 to 3½ feet of undisturbed earth over their sheeting. In making a descent into a ditch, or in pushing forward an approach in siege operations, it is often impracticable to lower the bottom of the trench of departure sufficiently to give the requisite cover for starting a gallery at once. In these cases _blinded descents_ or _galleries_ may be used, the tops and sides of which are supported by _blindage-frames_, or _blinds_, each of which consists of two side parts of 4" × 6" scantling 9' long, united by two cross-pieces of the same section 3' 8" long, which are mortised or halved into them, leaving horns at each end 1' long. (Pl. XII, Fig. 30.)

=54.= The galleries are constructed as follows (Pl. XII, Fig. 31): A double sap with a width of 8' is broken out from the side of the trench in the direction required and is driven forward in the usual manner, but with a continual increase in depth, at a slope not exceeding 1/4. The side slopes are as steep as the earth will allow. Two blindage-frames are set up vertically on the sides of the sap, 7' apart in clear, with their tops at the level of the tops of the trench gabions, their bottom horns resting in holes dug for them. These frames are prevented from falling inward by another frame placed crosswise upon them, with its horns resting on their cross-pieces. The side of this top frame toward the front may be held up by a stake or crutch, and the second pair of frames be placed at such an interval that their horns will interlock with those of the top frame. Successive frames may be placed in the same manner. The covering or “roof” is formed by three or four layers of fascines placed across the trench on top of the frames, and covered with earth thrown back upon them as the work proceeds. The sides of the gallery are held up by fascines, etc., laid along outside the frames.

As soon as the bottom of the blinded gallery has reached the proper depth a mine-gallery may be started and carried forward.

=55.= The blindage-frames described above give to the gallery a clear width and height of 7'. For smaller galleries the blinds may all be made shorter and of lighter scantling; or, if desired, those for the sides may be of a different length from those for the top.

=56. Rate of Advance of Galleries.=--The following table gives an estimate of the men and tools required for shafts and galleries, with the probable rate of advance in good soil:

KEY:
NC O = N. C. Officer.
M = Miners.
P = Picks.
MP = Miners' Picks.
P-p = Push-picks.
Sh = Shovels.
MS = Miners' Shovels.
MT = Miner’s Truck.
F-l = Field-levels.
M-r = Measuring-rod 6´.
T-l = Tracing-line.
M S = Mauls or Sledges.
CB = Canvas Buckets.
R-l = Rope-ladder.
W-b = Wheel-barrows.
MB = Miners' Bellows.
Pr. = Progress, ins. per. hour.
-------------------+-----------+----------------------------------------------------------------
| Men. | Tools.
Kind of Gallery, +----+------+---+---+---+---+---+-----+---+---+---+---+---+---+---+---+------
etc. |NC O| M | P | MP|P-p| Sh| MS| MT |F-l |M-r|T-l|M S|CB |R-l|W-b|MB | Pr.
-------------------+----+------+---+---+---+---+---+-----+----+---+---+---+---+---+---+---+------
Great gallery or } | | | | | | | | | | | | | | | | |
Blinded gallery } | 1 |12[14]| 4 | 2 | 2 | 8 | | | 1 | 1 | 1 | 1 | | | 4 | | 12
Common gallery | 1 | 4 | | 1 | 1 | 2 | 1 | 1 | 1 | 1 | 1 | 1 | | | | 1 | 12
Half gallery | 1 |4[15] | | 1 | 1 | 2 | 1 | 1 | 1 | 1 | 1 | 1 | | | | 1 | 16
Branch gallery | 1 |4[15] | | 1 | 1 | 2 | 1 | 1 | 1 | 1 | 1 | 1 | | | | 1 | 24
| | | | | | | | | | | | | | | | |{ 30
Small branch | 1 | 3 | | 1 | 1 | | 2 |1[16]|[17]| 1 | 1 | 1 | | | | 1 |{ to
| | | | | | | | | | | | | | | | |{ 36
| | | | | | | | | | | | | | | | |
Shaft | 1 |4[18] | | 1 | 1 | 2 | 1 | | 1 | 1 | 1 | 1 | 1 | 1 | | |{ 18
| | | | | | | | | | | | | | | | |{ 24
-------------------+----+------+---+---+---+---+---+-----+----+---+---+---+---+---+---+---+------

VENTILATION OF MINES.

=57.= The gases resulting from firing mines and from the lamps, bodies, and candles of the miners so vitiate the air in galleries that, unless means for ventilating them are adopted, the miners must eventually abandon them or become asphyxiated.

In ordinary circumstances, when no powder gases are present, a gallery cannot be driven safely more than 60 feet without ventilation.

The measures adopted for ventilating galleries consist, 1st, in forcing in fresh air; 2d, in drawing out foul air; and, 3d, in assisting the natural diffusion and circulation of the air through them.

=58.= The first is accomplished by forcing air through pipes, which may be of tin, wood, or common hose, leading to the point where ventilation is required. The air is forced in by the use of the miner’s bellows or other apparatus already described. This method is simple in its application and places the fresh air where it is needed, but drives the foul air back into the galleries occupied by other miners. It is the only practicable method of ventilating single, long, narrow galleries and branches.

=59.= The second method may be applied to a system consisting of a number of galleries connected by transversals, by so placing an exhausting fan as to draw the air out through one gallery, while by light wooden or canvas doors and screens the other galleries are so arranged that the fresh air, entering from the exterior, sweeps through the galleries occupied by the miners, and escapes through the unoccupied gallery leading to the fan, carrying the gases with it.

In this method a single large gallery may be ventilated by using a canvas partition placed near the top or on one side, so that the fresh air will go in on one side around the end of the partition and back by the other side to the fan.

This method has the advantage of carrying the gases away from the galleries occupied by the men, and supplying fresh air throughout those which are occupied.

The exhaust may be produced by a fire constantly burning at the foot of a shaft instead of by a fan.

The method is, however, complicated in its application, and can seldom be used for military mines.

=60.= The third method, or assisting natural ventilation, is carried out by cutting numerous cross-galleries connecting those which are near each other, by making air-shafts and bore-holes connecting the galleries with the surface of the ground, and, when practicable, by placing the openings of the shafts and galleries at different levels. This method will serve for a few men working leisurely in preparing countermines before an attack, but is entirely inadequate during active mining operations.

=61.= By the use of masks covering the face, and supplied with fresh air either through hose or from a reservoir of compressed air carried with him, a miner may work in galleries in which the air is irrespirable. The advantages which may frequently result from the time thus saved justify providing apparatus of this kind for use in mining operations.

MINE-CHAMBERS.

=62. Mine-chambers= to contain the charge of explosive are preferably nearly cubical in form, and if not charged at once, or if of large size, must have sufficient lining to support the roof and sides.

When they are above the level of the gallery they are arranged to drain into it. They are made large enough to contain the receptacle for the charge and to allow the charge to be placed in it. They are, as a rule, placed in short returns at one side of the branch or gallery, but may be at its end, above or below it. The mine-chamber frequently consists of so much of the end of the gallery as is necessary to contain the charge.

LOADING AND FIRING MINES.

=63. Preparing the Charge.=--The weight of the charge necessary to produce the desired effect is determined by the rules previously given. Its volume, if of powder or compressed gun-cotton, may be found by allowing 30 cubic inches to the pound; and if of dynamite, about 20 cubic inches.

If the mine-chamber is perfectly dry, and the mine is to be fired at once, a layer of straw may be placed upon the floor of the chamber and the charge contained in canvas bags laid upon it. When the ground is more or less wet, or when the mine is not to be fired immediately, the charge should have a water-proof covering, which may be a thoroughly calked and pitched box, an ale-barrel or beer-keg, the metal barrels in which powder is shipped, or India-rubber or pitched-canvas bags,--depending upon the amount of moisture present and the time that the charge is to remain in place. Many of the high explosives are not affected by dampness, and but little if any by water; but to secure the fuse and its connections from injury, and to remove all danger of misfires, the explosive should in all cases be protected from water if practicable.

=64. Distribution of Fuses in the Charges.=--Gunpowder will explode with full effect if ignited, but to prevent the explosion of the central part of a large charge scattering the exterior portion before it is ignited a number of fuzes should be used. They may convey fire only, but must all be ignited by the firing apparatus, and simultaneously. One fuze to each 100 lbs. of powder is not too great an allowance; but when lack of time or appliances does not admit of placing a number of fuzes, the desired effect may be obtained by increasing the charge of powder and using one fuze. (See Abbot, Prof. Papers Corps of Engineers, No. 23, 1881, p. 62, for number of fuzes needed; and pp. 244-51 for simultaneous ignitions.)

The high explosives detonate with full force only when exploded with a detonating fuze. Under favorable conditions one fuze will detonate a very large charge, but cases arise in which a portion of the charge explodes and the remainder does not. To insure the best results, therefore, it is desirable to distribute fuzes throughout a large charge, at the rate of perhaps one fuze to each 50 lbs. These fuzes should contain from 20 to 30 grains of fulminate of mercury, which is itself very sensitive to shock, and has in a high degree the power of detonating the other explosives. One fuze only (or, for safety against defects, two or three) need be connected with the firing apparatus, the others serving to reinforce and carry on the wave of explosion after it is started--differing in this respect from their use with charges of gunpowder.[19]

=65. Character and Construction of Fuzes.=--Formerly, for firing mines, trains of powders put up in linen tubes, quick-match, and other similar devices were used. Electric-blasting apparatus is now in such common use that it will always be available for any extended mining operations. For single mines with small charges it may, however, be necessary sometimes to resort to the older method of firing, the apparatus for which can be readily improvised. But even in these cases “Bickford” or “Safety” fuze will usually be available, and may be used alone for firing gunpowder, or with a common fulminate-of-mercury “blasting-cap” for high explosives. It burns at the rate of about 4 feet per minute. Very quick-burning fuzes are also made which may be used at times (e. g., _Bickford Instantaneous_, which burns at the rate of 120 feet a second; _Gomez Lightning_, which burns so rapidly that it may almost be said to detonate; etc.) Great care must be taken not to mistake them for the common Bickford.

=66. Electric Fuzes= are made of three general classes: First, those which are fired by a spark from a high-tension machine; second, those which are ignited by a current from a battery or “dynamo;” third, those which can be fired by either. (Abels, etc.)

=67.= Those of the second class are manufactured in large quantities, and, in connection with a portable dynamo or “blasting-battery,” are almost universally used for blasting operations throughout the United States.

These fuzes (Pl. XII, Fig. 32) are made up of two insulated copper wires, _A_, _A_, passing through a small cylindrical block of insulating material, _B_, and terminating about 1/16 inch above its end. A very fine platinum wire, _C_, about 1/1000 inch in diameter and 1/8 inch long, connects the ends of the insulated wires. Surrounding the platinum wire is a small quantity of gun-cotton, mealed powder, or fulminate of mercury, _D_. A copper capsule containing 15 to 30 grains of fulminate of mercury, _E_, is pressed down over the cylindrical block far enough to bring the fulminate in contact with the material surrounding the platinum wire, and the whole fuze is then coated with a water-proof composition. The insulated copper wires are cut to various lengths for convenience in connecting with the conductors or lead wires from the battery.

=68.= Fuzes of the first and third classes are now but little used. Many of them are unsatisfactory and dangerous. They differ in construction from those of the second class principally in that the platinum-wire bridge is omitted, and the exploding spark or current passes from one insulated copper wire to the other through a material which is ignited by it.

=69. Placing the Fuses in the Charges.=--A certain number of cartridges or packages should be selected, each fuse inserted and well packed in the explosive, and the wires or free end of the safety fuse brought out through the opening, which should be made water-proof, if necessary, by securely closing and thoroughly pitching it. The wires or exterior part of the fuse should then be securely fastened to the outside of the cartridge, so that an accidental strain upon them will not break the waterproofing or move the fuse from its place. They are then coiled up and remain so until the cartridge is placed in the general charge of the mine.

=70.= Several of the high explosives congeal at a temperature above the freezing-point of water, and in this state are less sensitive to shock, and explode with difficulty if closely packed in cartridges as usually delivered from the factories. They explode more readily when in the form of a powder. When using them in cold weather, therefore, each fuse should be put in a cartridge loosely filled with the powdered explosive, or with some high explosive not affected by cold. Others need special primers to cause detonation. The fuses should, of course, be placed in these primers.

=71. Placing the Charges.=--The charge is placed in the mine-chambers, either in the dark, by light reflected through the galleries, by closed lanterns carefully placed and guarded, or, when practicable, by incandescent electric lights. It is carried through low and narrow galleries on men’s backs or in miner’s cars, and should for this reason be put up in packages not exceeding 50 lbs. in weight.

It is packed in the chamber with great care, and under the immediate supervision of the responsible officer. The packages containing the fuses are distributed uniformly throughout the mass, and the wires uncoiled and led back into the gallery, the free ends of the two wires of each fuse having been previously twisted together for safety against electric currents and for identification.

These wires, which must be long enough to reach through the tamping, are all collected together and led back through it in a wooden or other conduit, which protects them from injury while tamping the mine.

When electric lights are used, great care must be taken to remove the light and all its conducting wires before the wires of the fuses are uncoiled and laid along the gallery.

=72. Tamping.=--Mines are tamped with sods and earth, wood and earth, sand-bags, etc., etc.

When sods are used the branch is filled for about 3 feet with sods carefully laid and packed with the joints filled with earth. About 3 feet of earth is solidly packed against this, then alternate layers of sods and earth until the desired length of tamping is obtained. To tamp with wood and earth or sand-bags, a wooden shield is first placed across the branch and firmly braced; behind this, earth is solidly packed or sand-bags carefully laid until the required length of tamping is obtained. Sometimes a second shield is put up behind the earth tamping, and firmly braced in position. The strength of the tamping is also increased by pieces of timber crossing each other diagonally, with their ends resting against the sides of the branch. Sand-bags make the best tamping, as they offer high resistance and are easily placed and removed.

The tamping should have a length equal to at least 1½ times the line of least resistance of a common mine corresponding to the charge, and if not of the best quality, to twice this line.

=73. Firing Mines.=--If electric fuses are used the main conductors or lead wires coiled upon a reel are taken in and the ends properly joined to the fuse wires; they are then led through the galleries, attached to the battery, and fired at the designated instant. Under no circumstances should the main lead wires be connected to the battery or dynamo until everything is ready for firing.

If a Bickford fuse is used its length is regulated to the desired time of firing from its known rate of burning. The miner lights the end and retires; the explosion takes place approximately at the calculated time. With the “Lightning Gomez” or similar fuses a length reaching to the firing-point may be used. It is lighted at the desired time, and burns with such rapidity that for lengths not exceeding 300 or 400 feet the time of burning is inappreciable.

Instead of using great lengths of these fuses, they may be cut shorter and their ends be brought together and inserted in a little mealed powder which is fired by a piece of safety-fuse, slow match or port-fire, etc., long enough to give the miner time to retire to a safe distance after igniting it.

Bickford fuse is best ignited by a piece of cotton wicking soaked in oil and loosely tied around it. This, when lighted, will burn through the covering and set fire to the composition. By this device many fuses may be ignited in a short time. A slow match or “touch-paper” for igniting quick-burning fuses or powder-trains may be made by soaking common paper in a strong solution of nitre and drying it.

CAMOUFLETS BY BORING.

=74.= In favorable soil a camouflet or small mine may sometimes be placed and fired very quickly by the following process:

A hole 2" to 3" in diameter and of the desired depth is bored in the proper direction with an auger or boring-bar. A cartridge containing from ½ lb. to 2 lbs. of dynamite is pushed down to the bottom and fired. The explosion increases the diameter of the hole somewhat throughout, and obstructs it more or less with loose earth. At the same time it enlarges the part near the seat of the charge into a bottle-shaped cavity, whose size varies with the charge used and the nature of the soil. The hole is rapidly cleared out with a long-handled scoop, the cavity filled with powder, primed, and fired.

The enlargement made by the charges of dynamite above given may contain from 50 to 100 lbs. of gunpowder under favorable circumstances.

=75.= In stony soil this method becomes very difficult if not impracticable; and when it can be used the preliminary explosion of dynamite vitiates to a greater or less degree the air of the shaft or gallery from which the boring is made, and also informs the enemy of the progress and intention of the miner.

To remove the latter objections, the English authorities recommend the use of holes 6" or 8" in diameter, bored with earth-augers, charged to a length of 2 or 3 calibres, and well tamped. When applicable, this method is manifestly a great improvement upon the other; but the auger is so liable to be stopped by stones which a boring-bar might break or push to one side, that it can only be applied in very favorable soil.