62. Mine-chambersto 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 largeenough 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.
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 toprevent 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 Fuzesare 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 smallcylindrical block of insulating material,B, and terminating about1/16inch above its end. A very fine platinum wire,C, about1/1000inch in diameter and1/8inch 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 wiresbefore 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 thedesired 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.
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 somewhatthroughout, 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.
76. Organization of Mines.—Underground warfare is conducted in the dark, in bad air, with constant danger of caving earth, suffocation by noxious gases, destruction of men and galleries by intentional explosions of hostile mines or accidental ones of our own, in addition to the usual dangers and difficulties of opening and supplying the mines under the close fire of the enemy.
These considerations necessitate the rejection of all complicated systems in the attack, and in the work carried on by the defence during the siege.
Ignorance of the point to be selected for attack, and the great expense of permanent countermines, also require those prepared beforehand by the defence to conform to simple and economical systems.
For this reason it is not necessary to give in detail the systems proposed by the older writers. They are described in most of the extended treatises on military engineering.
77. The Attack.—The object of the attack is to advance his galleries in the most rapid manner possible, with the best available system of ventilation, and to place his mines in such position as to break up the galleries and destroy the men, materiel, and works of the defence, both above and below ground; or toform connecting craters which may be occupied and converted into parallels, trenches, etc.
78.To accomplish this, when no natural ravine exists, a deep trench or “lodgment” is made, usually connecting the entrances of all the galleries and serving as a communication between them, and as a depot for such supplies as must always be at hand.
From this lodgment the galleries are started by a shaft, blinded descent, or mining-gallery; the method depending upon the depth to be reached and the thickness of cover required.
The entrance of each gallery is protected from horizontal and vertical fire, and from splinters, by a bomb-proof cover and traverses of sufficient thickness and strength.
The galleries are generally driven in lines nearly parallel, and at such distance apart that the hostile miners working at any point between them will be heard, either from the main galleries or from returns called “listening-galleries” or “listeners.”20
Depending upon the depth at which they are placed and other circumstances arising in different cases, the main galleries in various sieges have been placed at distances apart varying from about 8 to 30 yards.
These galleries are connected at intervals by “transverse galleries” or “transversals,” which assist the ventilation very much and give additional communication between them.
Branches for placing mines are driven in prolongationof the gallery or obliquely to the right or left, and, when the gallery is at a low level, inclining upward so as to shorten the line of least resistance, economize powder, and diminish the injury to the gallery and branches, resulting from the explosion of the mine.
79.When the hostile miners come within striking distance of each other, each strives to run his galleries directly toward the other in order to avoid exposing its flank to the hostile mine; thus diminishing as much as possible the injury resulting from its explosion.
80.The mines of the attack are generally overcharged in order to do the greatest possible injury to the mines of the defence, and to open large craters, but undercharged mines and camouflets are also used at times.
81. The Defence.—The object of the defence is to retard or stop the advance of the attack, by the destruction of his mines and miners, without forming craters which will assist him in making his parallels and approaches.
82.For this purpose his galleries must satisfy nearly the same conditions as those of the attack. They usually start out from the counter-scarp gallery or from a parallel gallery a little in advance of it, and extend to a greater or less distance from the work, according to the time and expense allowable for their construction. For permanent works they are frequently prepared in time of peace, and lined with masonry. It is particularly for this class of countermines that many elaborate systems have been designed for completely covering the ground, and for throwing up the same earth several times by mines placed at different depths and exploded in succession. For reasons previously given, these cannot be recommended.
83.A simple system of galleries placed as far below the surface as practicable, parallel or slightly diverging, connected when necessary by transversals whose lines prolonged pass inside the enceintes, and with branches fulfilling the same conditions driven out for listening-galleries, will, under the direction of an energetic officer, fulfil the conditions of defence as well, probably, as any that can be devised.
The branches leading to mine-chambers can be driven out from the main galleries, transversals, or listeners, as may be desired; and if the hostile miners obtain possession of any part of the system and blow it up, the lines of craters formed will be so swept by the fire of the work that they can hardly be occupied by the enemy.
84.As a rule, the mines of the defence will be undercharged or camouflets, to avoid the formation of exterior craters, but the rule is not without exceptions.
85. Shaft Mines, mines placed in vertical shafts, are used by both attack and defence for destroying galleries, etc., in their vicinity. By the attack they are usually placed in craters already formed, or in other places protected from hostile fire. A shaft is sunk rapidly, generally “à la Boule,” heavily “overcharged,” filled up with earth, and fired.
The defence may use the same method or may sometimes prepare them beforehand, tamping them and leaving a tubular opening through the tamping for loading and firing them.
86.The tactics of mine warfare result directly from the consideration above given. The special details ofattack and defence vary in each particular case. The reports of mining operations in different sieges21supply precedents and give suggestions for future operations of a like character.
87. Todleben’s Rules.—The general principles of mine tactics have been laid down by General Todleben from his experiences at Sebastopol (in Royal Engineers Occasional Papers, vol. i., 1877). They may be summarized as follows:
88. The Attack.—The besieger should advance by several galleries, securing those on the flank by listeners. He must be active and persistent, as the enemy will use every available moment to develop his countermines. When he receives the first camouflet of the defence he must hasten to fire his overcharged mines in the uninjured branches, in order to destroy the hostile countermines. He will generally suffer losses more or less heavy from this epoch forward, but must submit to them; since too much circumspection and delay will almost always result in complete failure.
Before firing the overcharged mines he must have everything in readiness to occupy and intrench himself in the craters formed; to open communication from the trenches to the craters either by sap or by forming a line of connecting craters; and for constructing shelters for the party occupying the craters and holding them against the sorties of the defence.
After occupying the craters, he should drive forward his galleries from them at once, unless the besieged has anticipated him and surrounded the craters with branches—which may be assumed to be the case if any delay has occurred in occupying it. In this case he should sink shafts à la Boule, heavily overcharged, and fire them, and immediately occupy the new crater and push out from it; and thus progress as rapidly as possible, by constantly placing and firing overcharged mines, whose craters will, with little alteration, form both communications and parallels.
When the fire of the defence upon the crater is so severe that a deep shaft cannot well be sunk, a shallower one with correspondingly small charge is first sunk and fired, and a deeper one is sunk from the crater thus formed.
The overcharged mines should be well tamped when time permits. If not well tamped the charge should be increased (or high explosives used.—J. M.).
89. The Defence.—The defence should push out his galleries as far as possible and at the earliest practicable date, connecting them by transversals for ventilation, and holding them at a level below any likely to be reached by the attack.
When near the enemy, he should stop work several times a day and listen for sounds from the hostile miners which will locate their position.
Hearing the sound of the enemy’s miner, he may work toward him noiselessly, or prepare and charge a chamber and await the approach of the miner toward it, listening at the point where the hose trough (tube for fuse wires.—J. M.) comes through the tamping until the enemy is near enough to justify firing. Judgment asto distances must be formed from practice obtained while driving the countermines.
To avoid forming craters on the surface, and to do the greatest possible damage to the besieger’s works, the besieged should not fire his mine until the enemy’s distance from it is less than the line of least resistance reckoned toward the surface.
When this condition is fulfilled, he may give to his camouflet a charge of from3/10to4/10the charge for a common mine placed at the same depth, since the charge will produce its principal effect upon the enemy’s gallery, and but little upon the surface.
Special care must be exercised by the defence to avoid premature explosions, since a mine fired at too great range damages only his own branch, and may make a crater; thus working directly to the advantage of the attack, who may prepare an overcharged mine or sink a shaft à la Boule in the crater thus made.
As successive explosions of necessity damage the branch in use, to avoid falling back, another one should be prepared as a reserve before the first is disabled, and at a little distance from it.
After the attack has fired his overcharged mine, the defence, by a strong fire of canister, musketry, etc., should prevent him from occupying the craters, and if he takes possession, should drive him out by a continuous mortar fire, keeping him from completing his communications by fire from guns.
The defence should push forward branches and establish himself under the slope of the craters, in front and on both flanks, and by exploding camouflets prevent the attack from driving galleries or sinking shafts à la Boule.
When the nature of the soil admits, many of the camouflets will be placed by boring. Should the defensive measures above and below ground not debar the enemy from establishing himself in the crater, the defence may establish overcharged mines immediately in its front, with a view to destroying the advancing galleries of the attack, blowing up the men and their lodgment in the crater, and opening up the latter to the fire of the work.
Shafts à la Boule being very dangerous for the countermines, the defence should do his best to prevent their use, by artillery and musketry fire above ground, and by camouflets placed by boring under ground. In addition, he must take advantage of every favorable opportunity to delay the progress of the attack by sorties from the works.
90. Remark.—In underground warfare the besieger has a decided advantage, but the besieged, by a cool consideration in handling his mines, and by persistently holding back the attack, foot by foot, may very greatly retard it, or even cause such losses and delays as to lead to its being abandoned.
91.The attack having reached the scarp of the work, mines are prepared for breaching the counter-scarp and scarp.
Experience shows that the charges are best located in rear of the counterforts when they exist, or at equal intervals along plain walls. The charge should not be placed immediately in contact with the masonry, but in the earth behind it, and at a depth below the top ofthe wall equal at least to 1½ the L. L. R., measured to the face of the wall.
The charge should be estimated by the rules already given, and increased by 20 to 30 per cent, so as not only to throw down the walls, but also to break up the earth and form a practicable breach.
92.The galleries for placing the chambers behind the counterscarp are branches from the gallery of descent into the ditch; those behind the scarp may branch out from a gallery driven under the ditch, when water or rock do not forbid, or from a gallery driven through the scarp wall after crossing the ditch by sap or by a bridge.
93.To start the gallery through the scarp wall, a miner is “attached” to the wall by protecting him from fire along the ditch, from sorties, and from loaded shells, etc., rolling down upon him from the parapet by suitable traverses and splinter-proof.
This operation is of course very dangerous, and is generally impossible unless the fire of the defence along the ditch is previously silenced. To expedite the work of the miner a gun is sometimes brought down through the gallery, and the face of the wall is shattered by its fire before the miner is “attached.”
94. Blastsare small mines used generally for breaking up rocks, or masonry in demolitions.
95.Holes for placing the charges are drilled usually withdrill-barsorchurn-drills, known also asjumpers.
These are steel bars sharpened to a chisel edge. Thedrill-baris usually held by one man and struck with a hammer by another; it is turned slightly after each blow in order to make a round hole.
For small holes the driller holds the drill in one hand and strikes with the other.
Thechurn-drillis a longer bar, generally sharpened at both ends and enlarged in the middle. It is used for drilling vertical holes by raising and dropping it in the holes, turning it slightly after each blow.
96.Thechargemay be gunpowder or high explosive. If the former, it must be thoroughly tamped. If the latter, tamping will greatly increase its effect; but it is in some cases preferable to obtain the desired effect by increasing the charge and saving the time taken in tamping.
97.Blasts are fired by electric fuses, Bickford fuse, firing-tubes, needles, etc.
The fuses have been already described.
Thefiring-tubeis a very small iron pipe, which is insertedin the powder charge and the tamping rammed around it.
After the tamping is finished the tube may be filled with fine powder poured in it if the hole is vertical or inclined downward, or straws filled with powder may be inserted if it inclines upward or is horizontal. A “squib” of wet powder is also sometimes placed in the tube and ignited, when it passes down the tube like a rocket and fires the charge.
Theneedleis a smooth copper wire, longer than the depth of the hole. It has a ring handle, by which it may be turned around and withdrawn. It is inserted in the charge, the tamping is well rammed around it, and it is withdrawn, leaving a pipe in the tamping, through which fire may be communicated, as described for the firing-tube.
98. Tamping.—The best and safest tamping is perfectly dry silicious sand, poured in the hole so as to fill it completely, but not rammed. It cannot be used in holes which incline upward nor when the needle is used.
In such cases moist clay, brick-dust, etc., are used. The first layers are pressed in upon the charge, and the subsequent ones thoroughly rammed down with a copper tamping-bar. A hammer is used with the bar, when necessary, in deep holes and hard rock.
With high explosives no tamping should be used except dry sand or water. Holes which incline upwards should receive an extra charge and be untamped.
99. Determining the Charge.—The charge of gunpowder or high explosive required for any particular hole in ordinary blasting can be best estimated by an experienced blaster. If one is not to be obtained,an approximate estimate for the first experiment may be made from the formulas (counting the high explosives about four or five times as strong as gunpowder for ordinary use), and the charges for subsequent blasts may be estimated from the effects of those first fired.
100. Precautions.—If atampedhole misses fireit should never be cleared out for recharging. A new hole should be drilled near, but not breaking into it.
Electric fuses or Bickford fuses (with blasting-caps for high explosives) should always be used when they can be obtained.
101. Deliberate Demolitions, such as the destruction of walls, casemates, etc., in time of peace, or at a distance from the enemy in time of war, should be so made as to economize powder and work. To accomplish this, the mines and blasts should be located where they will produce the best effects attainable, and the charges should be proportioned to the work required from them.
The table previously given (p. 124) will serve as a guide for computing the first charges used, and from the results of these the charges of subsequent ones may be determined.
Judgment must be used in placing the charges, so that, when possible, they will destroy the supports and allow the superstructure to break up by falling.
The charges will usually be placed in chambers under or hollowed out in the masonry. Sometimes they are more advantageously placed in a trench outside and close to the foot of the walls. They should always be well tamped: when in mine-chambers, by methods previouslydescribed; when in trenches, or laid along the exterior of walls, by loading them with earth, etc., until the line of least resistance passes through the wall to be destroyed.
102. Hasty Demolitionsare made when the time available for the work is limited.
The structures usually destroyed are houses, walls, stockades, bridges, tunnels, canal-locks, railroads, rolling-stock, etc., etc.
The time does not usually allow the charge to be placed in the most advantageous position or to be properly tamped. For this reason the high explosives are best suited for this kind of work, and large charges are a necessity.
103. Houses and Magazinesare best destroyed by placing several charges with connecting trains inside and along the walls, laying strong timbers upon them, with struts from the timbers to the floors and roof above; barricading the doors and windows from within, and firing the powder from a safe distance without.
104. Walls.—A wall not exceeding 3 or 4 feet in thickness may be breached by charges of gunpowder placed at intervals along it. Calling the thickness of the wall in feett, the charge in pounds may be 3t3, placed at intervals of 2t.
For gun-cotton the Woolwich rule calls for charges in pounds of from ⅓t2to ½t2per running foot. Experiments made in New York with dynamite indicate that the charges should be at least ½t2per running foot, and for very good masonry should exceed this.
A charge of dynamite of ½t2per running foot will be given by a cylindrical cartridge whose diameter ininches equals the thickness of the wall in feet.22The effect of the charge will be very much increased by throwing over it even a very light tamping of earth or sand.
105. Stockades.—A strong stockade or palisade may be broken down by charges of from 40 to 60 lbs. of gunpowder placed in contact with it, and preferably covered with sand-bags. 10 or 15 lbs. of high explosive should produce about the same effect.
106. Bridges.—Arched bridges are best attacked in the piers if high and thin, or at the haunches and crown of the arch. Two or more charges in the length of the pier, or width of the roadway, will be more effective than the same amount in a single charge at the middle.
The charges should be placed in chambers cut in the piers or down through the roadway to the back of the arch.
The abutments of single-span arches are generally very strong, and the haunches well covered with earth and masonry. In hurried work, therefore, the crown will generally be selected, a trench dug down to it across the roadway, the charge placed in the trench, tamped if possible, and fired.23
High explosives, from their shattering effect, are perhaps most advantageously used by suspending them beneath and in contact with the arch at the crown and haunches. The plank or timber upon which they areplaced should be as heavy as possible, in order to act as a partial tamping, and should be drawn up so that the explosive will be in actual contact with the soffit of the arch.
Under these circumstances they should produce as great an effect as four or five times their weight of gunpowder.
Iron and wooden truss-bridges should be thrown down by breaking the main braces near the piers, or the chords near the centre, by charges placed in a joint if possible. High explosives are particularly valuable for this purpose.
In wooden bridges they may be placed in auger-holes bored for them, and in iron bridges inside the hollow members, between eye-bars, or in other similar places.
107. Tunnels, Canal-locks, and similar constructions must be attacked with large charges, so placed as to temporarily or permanently disable the work, as may be considered necessary. The location of each charge should be determined and its amount computed from these considerations before the destruction is attempted.
A temporary obstruction is frequently all that is necessary or desirable for these works, and the damage done to them should be carefully regulated with a view to their subsequent repair and use.
108. Railroads.—Railroads are temporarily disabled by tearing up the track, making hot fires of piles of ties, placing the rails upon them so that they will heat and bend by their own weight; or, better still, twisting the rails while hot by suitably-shaped steel hooks and wooden levers of the kind devised by General Haupt (Pl. XII, Fig. 33). Rails so twisted cannot be again used until re-rolled.
109. Rolling-stock.—Railroad cars may be disabled by breaking one or more wheels with sledges, or may be destroyed by burning. Locomotives may be disabled temporarily by carrying away the smaller parts of the mechanism, or permanently by breaking the engine-cylinders with sledges; bursting the boilers or burning out their fire-boxes by drawing out nearly all their water, fastening down the safety-valves, and building a hot fire in the furnaces; or by making a hot fire under them so as to heat and thus bend or warp the reciprocating parts of the machinery.
110.In all hasty demolitions with explosives the charges should be well in excess of those computed by the ordinary rules: first, because the explosives will not be so placed as to act to the greatest advantage; and, second, because the demolition should beimmediateandcomplete.