ATTACK
OF
FORTIFIED PLACES.
INCLUDING
SIEGE-WORKS, MINING, AND
DEMOLITIONS.
PREPARED FOR THE USE OF THE
CADETS OF THE UNITED STATES
MILITARY ACADEMY.
BY
JAMES MERCUR,
Professor of Civil and Military Engineering at the United States
Military Academy, West Point, N. Y.
FIRST EDITION.
FIRST THOUSAND.
NEW YORK
JOHN WILEY & SONS,
53 East Tenth Street.
1894.

Copyright, 1894,
BY
JAMES MERCUR,
West Point, N. Y.
Right of Translation Reserved.

PREFACE.

In this work an attempt has been made to give in outline the best modern methods of attack upon a fortified position by assault, surprise, blockade, or siege; and also the detailed constructions of those types of trenches, batteries, magazines, etc., etc., which seem best suited to resist the fire of modern cannon, and to afford cover to a besieging force.

It is not supposed that these types will be exactly copied in all cases of actual practice, but that a wise discretion will be used in modifying or combining them when necessary or desirable.

The constructions given are standard types, which have grown up by combining the suggestions and the experience of the military engineers of all civilized nations.

In selecting them I have drawn freely upon the textbooks of the schools of military engineering at Chatham, Fontainebleau, Vienna, and Berlin, as well as upon that of the late Professor Mahan, and the manuals of Duane and Ernst.

The standard work of Gumpertz and Lebrun is frequently referred to in “Military Mining”; and I am also under obligations to General H. L. Abbot, Corps of Engineers, for the use of his unpublished notes on the experimental mines at Willett’s Point, and the result of his experiments upon the mining effects of shells charged with different explosives.

J. M.

West Point, N. Y.,
October, 1894.

INTRODUCTION.

Modern wars have been marked by sharp aggressive campaigns and great battles in the open field, with few close and long-continued sieges.

The subject of siege-works has therefore attracted less popular attention than was formerly devoted to it.

Fort Wagner, Vicksburg, Petersburg, Strasburg, Belfort, Paris, Plevna, and Géok Tépé have shown, however, that at their respective dates regular siege and mining operations were necessary to reduce either permanent or field fortifications, if well equipped and defended.

The volume of fire delivered by the small arms and machine guns now in use has made an open assault upon a well-supplied and well-defended parapet, under ordinary circumstances, a hopeless undertaking, and has necessitated more deliberate methods of attack.

The increased accuracy and penetration of modern cannon have rendered obsolete many of the older methods of making regular approaches.

The newer constructions described herein, while giving greater protection to the attack, are in general slower in their advance than those previously used. This seems, however, to be an unavoidable evil, which is mitigated only by taking advantage of every opportunity for rapid advance offered by the errors of the defence.

It is not to be inferred that light field works and lines will in the general case require for their attack a system of regular approaches; but trenches and saps may be necessary for placing a battery or parapet in a commanding position or one favorable for enfilade, or for giving a covered approach over an exposed ridge; and their frequent employment may be expected on future fields.

The destructive effect of grenades and Coehorn shells charged with high explosives will doubtless in many cases check or stop the advance of saps and trenches, and necessitate the use of blinded approaches or mining-galleries in stubbornly contested sieges. The successful application of mines at Géok Tépé will doubtless lead to their future employment under similar circumstances. In the close attack upon a shielded casemate or disappearing turret their use seems a necessity, and when these defences are founded on rock or massive concrete foundations, tunnelling operations by drilling and blasting will be required. When practicable they will be expedited by the use of power-drills driven by electricity.

It seems hardly necessary to add, that in sapping and mining operations, as in all other branches of military engineering, all new and improved inventions and methods which are applicable to the work on hand will be used, as a matter of course.

The thickness of cover given in the text is based upon the penetrations of the hostile projectiles.

For ready reference the maximum penetrations obtained in experimental firing up to this date (1894) are given herewith, viz.:

Service bullets, copper or German-silver jacket, of 6.5 to 8 mm. calibre, initial velocity from 2000 to 2550 f. s.:

French authorities give a muzzle penetration of 12 mm. = 0´´.473 in iron plates for the Lebel bullet. No published experiments confirm this.

But few experiments seem to have been made to determine the penetration of the projectile of field and siege guns into earth, and the published data are very meagre and unsatisfactory.

The German Engineer’s Handbook (Pionier Taschenbuch, 1892) prescribes the following thicknesses of parapets for cover against small-arm and cannon fire, viz.:

English authorities report craters of 21 feet length and 8 feet depth blown out from an earth parapet by a single 200-lb. 8-in. howitzer shell. They also state that the projectile of the pneumatic dynamite gun has penetrated 40 feet of earth.

Owing to the rapid development of ordnance the current scientific and military periodicals are in general the only source from which the latest results in penetration, etc., can be obtained.

CONTENTS.

ATTACK OF FORTIFIED PLACES.

CHAPTER I.
THE ATTACK WITHOUT THE USE OF REGULAR APPROACHES.

1. A fortified position may be taken by blockade, surprise, assault, bombardment, or siege.

A blockade consists in so surrounding a place and closing its communications as to keep the garrison from receiving reinforcements, provisions, and supplies sufficient to enable it to continue the defence and to avoid starvation.

The object of the attacking force is, in general, to completely close all communications between the garrison and the exterior; but this is not always possible, nor is it necessary in all cases, since such obstruction of communications as will reduce the incoming supplies below the necessary expenditures of the garrison will ultimately exhaust its stores.

An efficient blockade, continued long enough, will consequently reduce any place.

Whether it is advisable to attempt to reduce a place by blockade will depend upon the time which will probably be taken in its reduction, the force required for surrounding it, and repelling sorties from the interior or beating back a relieving army, and the expense in men and materials of taking the work by other methods. Blockades are more effective in reducing cities and towns than in taking places occupied only by a military garrison, since the presence of a large number of non-combatants in a place rapidly exhausts its store of provisions, renders epidemics more likely to break out, and by the suffering and misery resulting demoralizes the garrison, unnerves the commander, and eventually causes its fall. This justifies the apparent harshness of not allowing non-combatants to leave a beleaguered place.

The steps necessary for establishing the blockade are identical with those taken for the investment in a regular siege and will be described hereafter.

The capture of Paris in 1870-71 is one of the most recent and striking examples of a blockade on a large scale.

SURPRISE.

2. A sudden and unexpected attack made upon a garrison unprepared to receive it is called a surprise.

Formerly these were of not infrequent occurrence, but with modern means of communication and methods of warfare they can hardly be looked for, except in small affairs, where, through the weakness or exhaustion of a garrison or the incapacity of its commander, the necessary and ordinary precautions for their prevention are impracticable or are neglected; or where they are brought about through treachery in the garrison, by which the gates are opened to the attack.

Probably in the majority of cases attempts at surprise will be detected and defeated; but as a success is usually valuable far in excess of the losses suffered in its execution, promising opportunities for their trial should not be neglected.

Surprises, when thought possible, are undertaken under the cover of night, fog, or severe storms. The tactical disposition of the troops is similar to that used in open assault, the columns being preceded by ladder-parties for scaling walls, engineers for blowing down barriers, etc., etc., according to the nature of the case, and followed by a large reserve which is designed to hold any points captured by the advancing columns. It is usually considered best to make simultaneous attacks at several points, in order to confuse and divide the defence, holding the main reserve nearest to the column which is expected to succeed; but making provision also for promptly and fully supporting any other party which may have forced an entrance into the work. An entrance secured, consecutive points should be occupied and held, preserving communication between them, and avoiding too great dispersion of the troops, until a foothold is gained which can in all probability be held against the defence. After this greater boldness may be used in attacking important points within the place.

The complete capture of the work and its garrison cannot ordinarily be expected, however, until daylight allows the systematic movement of the attack throughout the place. In case of failure, any captured gate must be held if possible until all the troops have retreated through it and are covered by the reserves.

DEFENCE AGAINST SURPRISE.

3. The measures necessary to guard a fortified place against surprise are of two classes. First, for its prevention, by use of all the usual outposts and interior guards,—the organization and duties of which need not be repeated here,—and of telegraphic and other signals and communications with the surrounding country by which the approach and movements of any attacking force may be made known before it comes near the work.

Second, for its repulse, by so training and disciplining the garrison that, upon the alarm being given, the parapets, batteries, etc., will be manned and all defensive measures will be taken before the assaulting body can enter the work.

This will be accomplished by so thoroughly drilling the garrison in its duties that each man will go at once to his proper station fully equipped for his duties at any hour day or night, without confusion or unnecessary excitement. The subsequent measures are the same as for resisting any other assault.

ASSAULT.

4. By an assault is meant an open attack upon a position by troops in line or column.

Formerly it was recommended to make assaults at early dawn, in order to have the increasing daylight for securing the results of victory; more recently night attacks have been more strongly advocated in order to diminish the losses from the fire of the defence while making the attack, and the still greater ones which follow a repulse when, the fire of the supports and reserves of the attack being suspended for fear of injuring the retreating troops, the defence pours upon the latter the full close and deadly fire of all its arms. Whether the advantages of a night attack more than counterbalance the dangers resulting from the confusion due to darkness is, however, a question not yet settled.

Open assaults upon fortified positions, well manned and armed, have, since the introduction of firearms, been considered the most bloody, uncertain, and frequently the most unjustifiable operations in war. With the introduction of machine and rapid-fire guns and magazine rifles it may be considered as an established fact that a well-defended line cannot be carried by an assault in front until its fire is overpowered or its ammunition exhausted.

This conclusion, which has been drawn from attacks on field-works, is still more positive in regard to attack upon works of strong profile protected by deep ditches and other obstacles.

DISPOSITIONS FOR AN ASSAULT.

5. When an assault is ordered the tactical dispositions must be so made as to keep the fire of the defence down to its lowest possible limit, until the assailant can close in with the bayonet.

With this end in view, batteries are established sweeping the lines; the assaulting columns, well supplied with ammunition, are formed where protected from fire; working parties are arranged and provided with such tools and appliances as are necessary for removing or overcoming obstacles; and all preparations are made for simultaneous action by the entire force.

It is manifest that to silence the fire of the work the attack must have a marked preponderance of artillery arranged both for enfilade and front fire upon the front of attack and the collateral works; and that the batteries must be established, the fire opened, and the guns of the defence silenced before the assault is made; and that this fire must continue until the assaulting troops are so near the work as to necessitate its discontinuance to avoid injury to them.

The working parties—carrying axes, saws, crowbars, and similar tools which are needed for removing the existing obstacles; explosives for blowing down gates, barriers, etc.; fascines, gabions, hurdles, etc., for crossing ditches, covering trous de loup, and other purposes; and, when necessary, ladders for escalade—move forward with the columns of attack; the latter must be so handled that, when the artillery fire is suspended, they can keep down the fire of the defence with rifle and light machine-gun fire.

Under cover of this fire the obstacles must be removed by the working parties, and the first assault made by the troops detailed for this purpose. With these troops should be a certain number of artillerists provided with lanyards, friction primers, etc., to serve any guns that may be captured, turning them against the defence.

A party of engineers provided with high explosives for blowing down gates, etc., should follow closely behind the advance in case of an escalade; they should also be provided with appliances for blowing up magazines, etc., when possible, in case of a repulse. The gates being captured and opened, the mass of the assaulting troops enter by them and complete the capture of the place.

In case of repulse the retreat of the advanced parties is covered, when possible, by the infantry fire of the reserves, and that of the latter by the artillery, as in the advance.

DEFENCE AGAINST AN ASSAULT.

6. Permanent works being designed to be secure against assaults and surprise, their guns of position are protected as well as circumstances admit against the hostile artillery and infantry fire. During the cannonade preliminary to an assault a wise discretion must be used as to how much ammunition may be profitably expended in replying to it, and how great an exposure of the men to the artillery fire is justifiable. As a rule but little reply is made from the work.

The machine and rapid-fire guns should be withdrawn from the parapets and be protected under bomb-proofs until the relaxation of the hostile fire due to the near approach of the assault allows them to be run out and to open fire. The infantry of the garrison is similarly handled, being held under cover until the proper moment, then manning the parapet and pouring a close, rapid, and deadly fire upon the assault.

The fire of the fronts directly attacked, both machine-gun and infantry, will be directed principally at the assaulting columns and working parties, the collateral works and fronts will, in addition to pouring a cross-fire upon the assaulting columns, direct a large part of their machine-gun fire upon the supports and reserves, while the more powerful guns will generally direct their fire upon the hostile artillery.

The troops not needed for manning the parapets are held under cover in a central position as a reserve, to strengthen the force at any part of the parapet or to meet and drive out any body of the enemy penetrating the work.

Should the attack be repulsed, the most rapid and destructive fire from all arms is directed upon the retreating troops with a view to inflicting the greatest possible losses; but a counter-attack is, as a rule, not attempted. When made, however, it should be limited to making an advance upon one or both flanks to a position giving a more effective fire upon the retreating troops, and retiring from this position to the cover of the work as soon as the main attack is completely repulsed and before the advanced troops become compromised by a close engagement with the enemy.

BOMBARDMENT.

7. By a bombardment, technically speaking, is meant a more or less continuous shell-fire upon a place with a view to destroying magazines, buildings, materials, and supplies of all kinds, in addition to inflicting the greatest possible losses upon the garrison and producing among the inhabitants a state of terror and unrest, frequently extending to mutiny, and finally causing the surrender of the place.

The term bombardment is also frequently applied to a cannonade opened upon a place to silence its artillery prior to an assault or during a siege.

A bombardment promises success when the place is small and not well provided with bomb-proofs, when the garrison is weak or of bad morale, when the inhabitants are numerous and not in sympathy with the garrison, or when the commandant is weak. A well-built and well-equipped modern fort can hardly be reduced by bombardment with any reasonable expenditure of time and ammunition; although the successful use of torpedo-shells charged with high explosives will probably render untenable works not designed to resist their effects.

When it is designed to reduce a place by bombardment a complete investment is, as a rule, necessary only to prevent the withdrawal of the non-combatants (a severe measure, but one frequently adopted), or to insure the capture of the garrison upon the fall of the place.

The disposition of the troops is made for the special object in view. The infantry, cavalry, and field artillery complete the investment, if made; or, when the place is not invested, are concentrated at such points as may be necessary to protect the artillery from any sorties from the place, and to meet and repel attacks from any relieving force.

The artillery of larger calibre used for the bombardment proper should consist principally of rifled howitzers and mortars, which are easier to transport and more suitable for high-angle fire. As it is not intended to dismount or silence the guns of the place by direct or enfilade fire, an artillery duel should be avoided.

The batteries should be located, so far as possible, in places screened from the artillery of the defence by undulations of the ground, etc.; or, if this is impossible, by artificial screens as a cover from sight, and by trenches as a protection from fire.

Considerable latitude is allowed in selecting sites for batteries. For convenience of supply and unity of command they should be collected in groups, the batteries of the groups separated by at least 100 to 200 yards; and the groups should be located, so far as other considerations allow, near the main lines of communication.

If these groups do not entirely surround the place, they should, when practicable, extend at least half way around, so as to bring a reverse fire on all covers.

The fire, once opened, should continue night and day. If a conflagration breaks out, a sharp fire of shells should be directed upon it and its vicinity to prevent its extinction. Special efforts should be made to blow up magazines and destroy shops, storehouses, docks, roads, bridges, or other communications useful to the defence; but, so far as is practicable consistently with these, an attempt should be made to avoid injury to public monuments, museums, antiquities, and works of art.

Bombardments are sometimes commenced and continued for a longer or shorter time without the expectation of reducing the place, but to destroy some of the constructions above mentioned or to prevent the completion or arming of a work which it is intended to attack by other methods. A slow bombardment may also precede the active cannonade which prepares for an assault, or the systematic artillery attack of a regular siege.

DEFENCE AGAINST BOMBARDMENT.

8. The defence against bombardment is frequently, from necessity, strictly passive, and consists in so disposing the troops and materials as to protect them under bomb- and splinter-proofs, repairing damages to the latter and to magazines and parapets as occasion offers; saving the ammunition of the place by firing only such shots as promise to pay for themselves by the effect produced; and reserving all the strength of the place to meet the subsequent attack, if made.

When circumstances admit, a more active defence may be made, by a strong garrison, by well-conducted sorties which may capture and destroy the hostile guns and batteries and defeat and drive off their supports.

Sorties of this kind may sometimes be profitably made against the flanks of the attacking force, or against isolated batteries, even when a general attack cannot be made. Opportunities for their use should not be neglected.

CHAPTER II.
SIEGE OR ATTACK BY REGULAR APPROACHES.

PRELIMINARY CONSIDERATIONS, DEFINITIONS, ETC.

9. By a regular siege is meant a systematic and more or less deliberate attack upon a fortified place, in which the besieger aims to invest the place and capture its fortifications in succession by regular approaches, beginning with the most advanced and ending only with the reduction of the innermost keep and the surrender of the garrison.

The successive steps of a siege are usually the following:

The investment.

The artillery attack.

The construction of parallels and approaches.

Breaching by artillery or mines.

The final assault.

The introduction of modern breech-loading rifled guns, howitzers and mortars, rapid-fire and machine guns, and magazine small arms has brought with it the need of a higher grade of mechanical skill and improved machinery for making the ordinary repairs. This imposes upon both attack and defence the necessity for providing machine shops and tools fitted for work of this kind, with the steam power required to drive them. In connection with these, steam sawmills and other simple wood-working machines should be provided, as well as all other available labor-saving appliances which can be used to lighten the labor of the troops.

Portable tools, such as picks, shovels, crowbars, rammers, axes, hatchets, bill-hooks, gabion-knives, hammers, saws, carpenters', joiners' and blacksmiths' tools, etc., etc., must be provided.

10. The principal special tools and appliances used are the following, viz.: sap-forks, sand-bag forks, scrapers, sap-shields, measuring-rods of various lengths, pocket compasses with attachments for fastening them to measuring-rods, tracing-lanterns, dark and ordinary lanterns, tracing tape or cord, tracing pickets or stakes, fascines, gabions, hurdles, sand-bags, blindage and gallery frames and sheeting, etc., etc.

The sap-fork and sand-bag fork ([Pl. I], Figs. 1 and 2), about 4½ and 4 feet long, respectively, have steel heads with three and four prongs, as shown in the figures, those of the sap-fork being sharp and those of the sand-bag fork blunt.

They are used for handling and placing gabions, fascines, and sand-bags in position when, without their use, the sappers' arms would be exposed to fire.

The scraper ([Pl. I], Fig. 3) is a large hoe, of about the dimensions given in the figure, used for levelling off the surface of parapets, etc.

The sap-shield, introduced by the English ([Pl. I], Figs. 4 and 6), is a flat plate of mild steel 3 feet 6 inches by 1 foot 9 inches × ¼ inch, with two handles on its back as shown. Total weight, about 80 lbs.

It may be used as shown in the figure, and sometimes by small parties as a body-shield in such operations as blowing in gates, etc., etc.

Measuring-rods of rectangular cross-section, straight and divided into feet and inches, are needed for special purposes; but the ordinary rods are cut from round brush wood and to the length required.

Tracing-tape is usually a white tape, about 1½ inches wide, in lengths of 150 feet, marked at equal intervals, usually 5 feet, by short pieces of tape sewed to it. A loop of strong cord is fastened to each end. For convenience in use it is ordinarily rolled into a ball.

Tracing-pickets are about 18 inches long and one inch in diameter. To make them visible in a dim light the bark is removed from them. Ordinary pickets are usually 3½ or 4 feet long, 1½ to 1¾ inches in diameter, sharpened to a triangular point.

The tracing-lantern ([Pl. I], Fig. 5) is a dark lantern with a reflector arranged to throw a light vertically downward.

The other tools, materials, and appliances above mentioned, not of the ordinary commercial patterns, are described in Field Fortifications and Military Mining, q.v.

CHAPTER III.
TRENCHES, APPROACHES, PARALLELS, SAPS, SPLINTER PROOFS, AND PASSAGE OF THE DITCH.

11. Trenches.—A military trench consists of a ditch and embankment affording cover from direct fire. Trenches are used for approaches (or boyaux), parallels, and communications with magazines, etc.

12. Parallels are trenches which take their name from the fact that they usually are located on lines approximately parallel to the general front of attack. In a regular siege at least three and frequently a greater number of parallels are used. The exterior one, which is first made, is known as the first parallel, the next one as the second parallel, and so on. They are used to cover the part of the besieging force known as “the guard of the trenches,” which protects the men making the approaches, etc., and also as “places of arms” for assembling troops for assault or for other purposes.

The trench of a parallel is usually 10 feet wide at the bottom and 4 feet deep, finished on the reverse with a slope and on the front with two steps and a berm, with treads of 18 inches and rises of 15-18 inches ([Pl. I], Figs. 7 to 13).

The parapet of the parallel should not be higher than 4 feet 6 inches. Its upper surface, particularly in the second and third parallels, should be made approximately plain with a scraper, and its interior slope should be finished and if necessary revetted, so as to afford a good infantry-fire. To allow the troops to move out to the front in line, portions of the interior slope should be cut into steps of not more than about 18 inches rise, and be revetted with fascines or other materials ([Pl. I], Figs. 12 and 13). These portions should be 25 or more yards long and near the approaches. If a general assault is to be made, the parallels must be similarly arranged for the necessary length of front.

13. Approaches are trenches leading up toward the fortification on the front of attack; they connect the parallels and give protection to the besiegers in moving back and forth. To avoid enfilading fire they usually run in zigzags ([Pl. VIII], Figs. 80 and 81) across the capitals of the work, with branches seldom exceeding 100 yards in length at the first parallel, and growing continually shorter as they approach the work. Each branch is so directed that its prolongation will pass from 30 to 40 yards outside the most advanced position within effective range held by the defence. At each turning-point of the zigzag the more advanced branch is prolonged from 10 to 20 or more yards to the rear, to cover the angle of the approach. These returns are also useful for storing trench material, etc.

After the return is completed the sharp angle in the trench is rounded off to allow gun-carriages, etc., to make the turn.

Approaches are usually 4 feet deep, 9 to 12 feet wide at bottom, with slopes in front and rear as steep as the earth will stand, and have a rough parapet not less than 4½ feet high, separated from the trench by a berm of 18 inches, or more if necessary ([Pl. VIII], Fig. 82). When drainage requires it, as it very frequently does, the bottom of the trench is sloped from front to rear about 6 inches; a ditch cut along the reverse slope, discharging into the drainage ditches of the vicinity, or into drainage pits excavated in rear of and outside the approaches. These may be lined with a gabion to prevent their sides falling in.

TRACING AND CONSTRUCTING PARALLELS AND APPROACHES.

14. Tracing Parallels.—Parallels are located by engineer officers after careful reconnoissance of the ground. Guiding points and lines are marked so as to be readily found in the dusk, but so that they cannot be seen by the defence. When completely screened from view important points are marked by posting sappers at them.

When no other practicable method can be used, the directions are determined by the use of a pocket compass fastened to a straight measuring-rod.

Tracing is begun as soon as the approaching darkness will conceal the parties from the defence, while close objects are still visible. The length of parallel traced by each officer should not exceed that occupied by one unit, usually a battalion, as a working party. (A battalion of 500 men will occupy 800 yards).

The tracing party consists of one officer, one N. C. officer, and one sapper to each 50 yards of parallel, with one or two extra men.

The officer is provided with a pocket compass and measuring-rod. The N. C. officer has a tracing-lantern and a mallet with muffled head. Each sapper carries a roll of tracing-tape, a tracing-picket, and a six-foot measuring-rod. The officer stations the first sapper at the initial point, and, taking one end of his tracing-tape, moves along the line of the parallel, accompanied by the rest of the sappers; the first sapper places his picket between his feet, and the N. C. officer drives it into the ground far enough to make it secure. The sapper drops the ball of tape on the ground and lets it run out through his hands until nearly out, when he checks it, and when it is all out places the loop over his picket and lies down to await the arrival of the working party. The N. C. officer, as soon as he has driven the first picket, follows on to the second, etc.

The officer, having run out the tape of the first sapper, halts the second, takes the end of his tape, and proceeds as before until the parallel is traced, and a sapper is left at each 50 yards of its length. Each sapper is told the designation, by number and section, of the point he occupies.

15. Tracing Approaches.—The approaches are traced in the same manner as the parallels, but at each turning point of the zigzags a picket is driven, around which the tape is carried. After tracing the branch in front, the tape is cut at five yards in rear of the picket, and the end carried out to the rear in prolongation of the branch in front to indicate “the return,” which is then prolonged to the proper length (from 10 to 20 yards) by a short piece of tracing-tape.

POSTING THE WORKING PARTIES.

16. The working parties are commanded by their own officers, under the guidance of the engineers. They carry their arms and ammunition. Each battalion (or other unit) is marched in column to the depots, where the tools are laid out in lines, so that each man can take up his pick and shovel when drawn up in front of them. But when time does not allow of this arrangement, they are piled, the picks in one pile, the shovels in another, and the men pass the piles in single file to the right of the picks and the left of the shovels, each man receiving a pick and shovel as he passes the pile. If gabions are to be carried, they are distributed in a similar way. When two are carried, the shovel is secured inside one and the pick inside the other, and the gabions are then carried by the picket inserted for that purpose. When one only is carried, the pick is usually secured inside it, the gabion carried on the shoulder, and the shovel in the hand.

The working party, in column, provided with its tools, etc., is then led by the engineer officer to the parallel, and forms on right (or left) into line along it in single rank at five-foot intervals, beginning at the initial point of each section.

The N. C. officer of engineers assists in this formation, and each sapper points out to the men of the working party the five-foot intervals marked upon the tracing-tape of his 50 yards, verifies their positions along it, and subsequently superintends their work. Each man when properly placed drives his pick into the ground at the left of his task, places his shovel beside it, and lies down until the command “Commence work” is given.

When gabions are used the working party is posted in a manner entirely similar, except that the column is of necessity formed in single rank when marching to the initial point. The men form on right (or left) into line and place their gabions in front of the tape and touching each other; each man then takes out his tools, places them behind the gabions, lies down, and awaits the command to commence work. The sapper sees that the gabions of his 50 yards are properly aligned and touch each other throughout.

Both the sappers and working parties are divided into reliefs, usually of eight hours. The sappers of the tracing parties superintend the work of the first relief of the working party, but are relieved long enough before them (½ hour to 1 hour) for the second relief to become acquainted with the details of its sections before the second relief of the working party arrives. A similar arrangement is made for the third relief.

EXECUTION OF PARALLELS AND APPROACHES.

17. Simple Trench.—A trench made by excavating the earth and forming a parapet without revetment of any kind is known as a “simple trench,” or as “simple trench-work.”

Flying sap or flying trench-work.—When, in order to obtain cover more quickly, gabions are used to hold the earth first excavated, and subsequently to serve as a revetment to the interior slope of the parapet, the trench is known as a “flying sap” or “flying trench-work.”

18. Construction by Simple Trench.—The first parallel and the distant approaches are usually constructed by the use of the simple trench, as follows, viz. ([Pl. I], Figs. 9 and 10): The men having been posted along the tracing-tape at five-foot intervals, as previously described, and their positions verified by the engineer officers, the command “Commence work” is given. Each man marks the left and front of his task by a line dug with his pick, and, commencing at the left of his task, at once excavates a trench 3 feet long, 1½ feet deep, and 6½ feet wide, throwing the earth to the front, and making a parapet 1½ feet high, leaving a berm of 1½ feet. Then, commencing at 1½ feet from the front of his trench, he deepens it to 4 feet, making the parapet 3 feet high. When the task of a party is finished each man cleans off his pick and shovel, places them at the rear of the trench, and leaves them there for the use of the second relief.

By excavating in this way, partial cover while at work and a defensible parapet are rapidly obtained, and, at the completion of the task, the parapet admits of a strong defence, and affords cover sufficient to allow the first relief to be withdrawn and the second to be posted without exposure. Special care must be taken during the work to make the men face toward the parapet while digging, in order to avoid striking their neighbors with the pick when raising it for a blow.

The second relief widens the trench 4 feet; forms a bottom step 18 inches wide with such materials as are available; heightens the parapet to 4½ feet, and throws the rest of the earth to the front to thicken it ([Pl. I], Fig. 8).

The third relief widens the trench 2½ feet at the bottom and slopes off the reverse as steep as the earth will stand. The earth is used to thicken the parapet, additional shovels and shovellers being provided if found necessary.

The approaches ([Pl. VIII], Fig. 82) are extended in a similar manner; the tasks of the reliefs are marked on the sections.

Variations from these sections are made when rendered necessary by the presence of rock or water in the soil ([Pl. I], Fig. 11); when a wider trench is required for a tramway or for free communication; or, in special cases, when a narrower trench will answer the purpose and save work. Should a specially heavy fire make additional cover necessary, it may be obtained by deepening the ditch and thickening the parapet, leaving its crest at the same height. The sections above given have been found best for ordinary cases.

19. Construction by Flying Sap.—The construction of the first parallel having indicated to the defence the front of attack, further operations will usually be subject to a more destructive small-arm and machine-gun fire. This will, as the siege advances, render the losses experienced in constructing a simple trench too extravagant, and a quicker method of obtaining cover must be used. This method is found in the flying sap ([Pl. I], Figs. 8 and 13), which is executed as follows, viz.:

The men are posted and the gabions placed as previously described. The engineer officer having marked the lines, the order “Commence work” is given. Each man marks with his pick the front and left of his task (which in this case is 4 × 6½ feet, leaving a berm of 1½ feet), and proceeds at once to dig on its left, filling first the left gabion, next the right, and then throwing the earth over and in front of the gabions. Each gabion, when it is half filled, is tipped outward until it has a slope of about 4 on 1. The filling is then completed.

As each man of the first relief occupies only 4 feet of front (2 gabions), his task is four-fifths as great as it is in executing the simple trench.

The second and third reliefs have the same tasks as in the simple trench. When the first relief finishes its task, every fifth workman (indicated by the sapper of the section) retains his pick and shovel and returns them to the depot when he marches past it.

The others leave their tools for the use of the second and third reliefs. In good soil the gabions may be filled in from 7 to 15 minutes.

The English sap-shield is designed for use when the fire is so severe that the flying sap with gabions becomes impracticable. Owing to its weight (80 lbs.) a man can carry but one; hence a carrying party equal to the working party assists in placing the shields and then retires. This gives to each workman a task of 3½ × 6½ feet.

The shields are placed as shown in [Pl. I], Fig. 6; the trench is executed as previously described, the earth being thrown over and beyond the shield. The shields are removed after the task of the first relief is finished.

The sap-shield is designed to be used in special cases for covering the head of a full sap (described further on), in which case it is placed as shown on [Pl. I], Fig. 4; and also as a body cover for a man moving for a short distance in the face of a heavy fire, as is necessary at times in sapping and mining operations. It has not yet stood the test of service in a siege.

SPLINTER-PROOF COVER.

20. Splinter-proofs for the guards of the trenches, for field-hospitals, latrines, etc., should be provided as soon as possible after the parallels are finished. They may be placed in the returns of the approaches, or in rear of the parallels, and be connected with them by trenches. [Pl. II], Figs. 14-16, show their construction when in rear of the parallel and revetted with logs, fascines, or sawn lumber. The trench is 9 feet wide by 4½ feet deep. Its front edge should be 25 or 30 feet in rear of the reverse slope of the parallel. This width of trench will allow 2 to 4 men per yard of its length to work advantageously. They should finish it in 8 hours.

In digging the trench the earth is thrown out on both sides, leaving a berm of about 1½ feet on each side to allow the woodwork to be properly placed. When this is completed the earth in rear is thrown forward to complete the cover, as shown in the plate.

Steps for egress and openings for light and ventilation may be placed at intervals along the rear, and, when desirable, bunks may be built, as shown in the figures.

When the splinter-proofs are built in the returns of the approaches, the overhead cover may extend entirely across, steps and openings being provided as in the previous case; or posts and longitudinal beams may be set in the trench to hold up the rear end of the cross-beams, leaving the rear of the splinter-proof open. Portions of this may be closed, if desired, by leaning short posts or fascines against the longitudinal beam and banking earth against them.

The splinter-proofs may generally be drained into the parallels or approaches. When this cannot be done drainage-pits must be used. Limited portions of the splinter-proofs may be protected against leakage through the earth cover by first filling over the covering beams with earth, packing it to a smooth surface with a gentle slope, placing upon it raw hides, roofing felt or other waterproof material, and then completing the cover by adding the necessary thickness of earth, giving finally to its top surface a slope to carry off the rain.

BOMB-PROOFS.

21. When, in the close attack of the work, the besiegers are subject to vertical fire from small mortars, better overhead cover must be obtained by bomb-proofs, constructed by deepening the trench, using stronger beams, and a greater thickness of earth. Twelve-inch timbers laid touching each other, with spans of 5 feet and 5 feet of earth cover, have been considered sufficient; but with the improvement of high-angle fire and the use of high-explosive shells greater protection will be needed in the future. Experimental data for fixing the amount is not now available; an approximate thickness of earth cover may be computed as indicated below.

The mining effect of dynamite in common earth is something less than twice that of an equal weight of gunpowder. (See Military Mining, [Arts. 13] and [14.])

An explosive enclosed in a strong case, however, expends a part of the energy due to explosion in rupturing this case. The stronger the explosive the less will be the percentage of the total energy required for breaking the case, and the greater the percentage remaining for performing other work. For this reason equal weights of high explosives and of gunpowder enclosed in strong shells will not produce the same relative effects in forming craters, &c., that they would if contained in paper cases. The effect of the high explosive is relatively much greater when contained in a strong shell. Experiments made at Fort Hamilton, 1890-91, with 8-inch shells loaded with explosive gelatine, showed this explosive to have between 4 and 5 times the effect of gunpowder, while in paper cases the relative effects were as 1.7 to 1.0. (See Report of Board of Ordnance and Fortification, Ex. Doc. No. 12, 52d Congress, 1st Session, January 5th, 1892.)

Since the mining effects of the charges contained in shells are, however, less than when packed in thin cases, the thickness of cover determined by the use of the usual mining formulas should err on the side of safety.

Knowing the charge contained in shells to be fired against a bomb-proof, and their probable penetration, the formulas given in [Arts. 7] to 12, Military Mining, may be used for finding equivalent common mines and radii of rupture for dynamite and explosive gelatine by substituting in them ¹/₁₇ for ¹/₁₀.

The values given in [Art. 11] will probably be sufficiently accurate for the radii of rupture. The cover given in the direction of the fire must be greater than the sum of the penetration and the radius of rupture.

When the penetration is equal to or greater than twice the L. L. R. of an equivalent common mine a camouflet will probably be formed, whose radius of rupture, from the formulas, will be equal in all directions and may be assumed as ⅕ of the L. L. R. of the equivalent common mine.

In this connection, see par. 61, p. 58.

SAPPING.

22. When the trenches have been carried so near the work that the simple or flying trench cannot be used without undue loss, recourse must be had to the sap.

A sap is a narrow trench (subsequently widened), which is continually prolonged in the desired direction by digging away the earth at its head and throwing it to the front and exposed flank as a cover for the working party.

When the sap is subject to an oblique front-fire, exposing one flank only, the parapet is constructed on that flank and at the head. This is known as a single or full sap. When both flanks and the head of the sap are exposed to fire two full saps are driven parallel and very near to each other, each with its parapet on the outer flank. The tongue of earth left between them is removed to widen the narrow trenches, thus making a single trench with a parapet on each side. This is called a “double sap.”

The trench is sometimes deepened and given a splinter-proof roof or cover. This is known as a “blinded sap.” A sap gaining ground to the right and front is called a “right-handed sap;” its parapet is on its left flank. A “left-handed sap” has its parapet on its right flank.

To expedite the work in sapping several reliefs should be employed, and task-work should be adopted to induce the men to work rapidly.

In all sapping operations the use of the simple trench and flying sap will be resumed when circumstances admit without involving too great losses.

23. The full sap ([Pl. II], Figs. 17-21) requires a detachment, or “brigade,” of 1 non-commissioned officer and 8 men, provided with the following tools, viz.:

For the rest of the detachment, a 6' measuring-rod, knee-caps for 4 men, 2 shovels and 1 pick (in reserve), and, when necessary, a crowbar, axe, and bill-hook. From 100 to 150 sand-bags are supplied to each detachment.

24. Organization and Duties of the Detachment.—The sappers are numbered 1, 2, 3, and 4 in each rank; the front rank extends the sap 1 yard and is then relieved by the rear rank, and so alternately. The sappers change places at each relief; those who serve as Nos. 1 and 2 during their first task becoming Nos. 3 and 4 during the second, and so on throughout their tour.

If a detachment is reduced below 8 in number by casualties it nevertheless keeps 4 men at work driving the sap, and reduces its reserve until new men are supplied.

25. Driving the Sap.—The sap is driven as follows, viz.: Nos. 1 and 2 dig a ditch 4' 6" deep, 1' 6" wide at bottom, and 3' or more at top; the berm side has a slope of 3/1, and the reverse is vertical, or as nearly so as the earth will stand. They leave no berm, as they need all the cover they can get. Nos. 3 and 4 widen this trench 2 feet and form a berm of 1' 6" by digging away the foot of the parapet and throwing the earth upon its top and exterior slope. The head of their work is kept at 9' in rear of the head of the sap.

The side parapet made by Nos. 1 and 2 is about 2' 6" high and bullet-proof (about 2' 6" to 3' thick) at 18 inches above the ground. The head parapet is made up of about 60 sand-bags, from ½ to 2⅔ filled. It joins the side parapet and extends across the head of the sap. It is about 2' 6" high.

As the sap is driven forward the head parapet is advanced by throwing the rear sand-bags over to the front by hand or by the use of the sand-bag fork. In excavating the trench No. 1 kneels down, undermines, and digs down enough earth to advance his trench about 9 inches. He is replaced by No. 2, who shovels this earth upon the side parapet toward the head of the sap. No. 1 then resumes his place and throws the sand-bags at the head of the trench over the parapet until he has uncovered about a foot in advance. He uses a sand-bag fork when necessary. The trench is advanced 9 inches more in the same manner. No. 2, besides throwing out the earth dug by No. 1, trims up the slopes and gives the trench its proper width and depth.

Nos. 1 and 2 change places when they have advanced the head of the sap 1' 6", and are relieved as before stated when 3 feet is gained.

Nos. 3 and 4 work together upon their task. In shovelling the earth upon the parapet they throw it somewhat toward the front and regulate its height with the scraper. The rate of advance is usually from 2 to 4 feet per hour.

Widening the Sap.—The sap is widened by working parties, usually of infantry, who work at about, but not less than, 25 feet in rear of the head of the sap. In an approach their task is equal in volume to that of the sappers, and can be finished in one relief. In a parallel when steps are to be provided, a second relief makes the steps, drainage ditches, drainage pits, etc.

A change of direction in a full sap ([Pl. II], Fig. 20) is made by No. 1 turning in the new direction and working through the old side parapet; No 2 throwing the earth over the old head parapet. The sand-bags of the old are gradually removed and used for a new head parapet, 20 or 30 additional sand-bags being ready for use if needed before the others can be safely removed. Nos. 3 and 4 follow on as before.

A return can be driven back when desired by another detachment of sappers. No head parapet will be needed, but the side parapet will be kept a little in advance of the head of the sap.

26. Breaking Out a Sap from a Parallel.—The head parapet of a sap is about 2' 6" high. The parapet of a parallel is about 4' 6" high. A sap of the usual form driven through the parapet of a parallel will expose the latter to fire through the opening formed. To reduce the danger from this exposure, the sap is broken out at night, and to cover the opening in the shortest time two or three men may creep over the parapet of the parallel and cover themselves by rapidly digging a hole, from which they may work back and join the sappers, who are working outward. The sap being driven obliquely to the front, the trench widened, and the parapet made full size ([Pl. II], Fig. 21), the opening will be covered; or a few men may in some cases construct in front of the parallel a short section of flying sap, under cover of which the full sap may be broken out ([Pl. III], Fig. 28).

Preliminary preparations for breaking out should be made during the day, but should be so conducted as not to indicate the selected point to the enemy.

27. Circular places of arms ([Pl. IX], Fig. 83) may be formed in front of a parallel by breaking out two single saps from points 80 to 100 yards apart and so directing them as to meet at 25 or 30 yards in front of the parallel. They may be used by the guards of the trenches or as depots for trench material, etc.

28. Shallow Sap.—When the presence of water in the soil or of rock near the surface prevents driving the full sap 4' 6" deep, a shallow or modified sap ([Pl. II], Figs. 22 and 23) may be used, provided a trench 3' deep can be driven forward. In this case Nos. 1 and 2 must both work kneeling, and Nos. 3 and 4 must throw the earth well to the front and keep the parapet as high as possible, leaving the construction of the berm to the widening party, who will give to the trench the necessary width, and will then obtain earth for strengthening the parapet by deepening the trench when practicable and widening it when necessary, making, however, no irregularities which will injure it as a communication and no depressions which will collect water. This sap advances about as rapidly as the full sap.

29. Overground Approaches.—When the water or rock comes to the surface of the ground, approaches can, under favorable circumstances, be driven for short distances by carrying forward earth in sand-bags, forming with them head and side parapets, and moving forward by continually building up the latter and advancing the former as before described.

The expenditure of time, labor, and sand-bags is so great in driving approaches in this way that the minimum height of parapet (possibly 5') should be made with sand-bags. This may be subsequently heightened and strengthened with earth brought forward in barrows or hand-carts and thrown upon the top and exterior slope.

30. Double Sap.—([Pl. III], Figs. 24, 25, and 26).—The double sap consists of two parallel single saps driven side by side, the cutting lines of the berms, usually 10 feet apart, making the bottom of the completed trench 7 feet wide. It is used when the zigzags, to avoid enfilade, make such a slow advance as to be no longer profitable, i.e., when the amount gained in advance does not exceed ⅓ the length driven. The double sap is directed toward the work, and is exposed to an enfilading and also to a slant fire from both directions. It must therefore have a parapet in front and on both flanks.

31. Execution of the Double Sap.—The double sap is driven by two detachments, each organized and equipped exactly as for a single sap, except that a greater number of sand-bags should be supplied when practicable. The sappers work as in driving a single sap, with the following modifications only: The Nos. 1 prolong their head parapets until they meet, and in advancing their heads of sap leave undisturbed the 4 feet of head parapet intervening between their trenches, but, by continually throwing sand-bags or earth obliquely to the front, keep the head parapet continuous and nearly straight.

This leaves between the trenches made by the Nos. 1 and 2 of the two detachments a tongue of earth 4 feet thick, surmounted with a parapet about 2' or 2' 6" high, which serves as a parados and protects these sappers from reverse fire.

Nos. 3 and 4 of both detachments, in completing their tasks, remove this tongue and pass forward the sand-bags forming its parapet for use by Nos. 1 and 2; otherwise the tasks are as in driving a single sap. When sufficient sand-bags are not available the middle part of the head parapet must be made of loose earth, giving much less protection to Nos. 1 and 2.

This sap, from its method of construction, is completed by the sappers without the assistance of the infantry working parties. Its rate of advance is about the same as that of the single sap.

32. Changing Direction of a Double Sap.—When a change of direction is to be made No. 1 of the first detachment marks on the berm the width of the top of the trench (10'), Nos. 1 and 2 of the wheeling flank come around the tongue and the leading sappers of the two detachments start their sap-heads in the new direction. Nos. 3 and 4 of both detachments remove the tongue of earth and complete the parapets of the original trench and then follow up their Nos. 1 and 2 as before.

33. Breaking Out a Double Sap from a Parallel.—A double sap is broken out by methods entirely similar to those already described for the single sap ([Pl. III], Figs. 27 and 28). The figures explain themselves.

34. Traversed Sap.—A sap may be traversed to protect it against enfilade by frequent changes of direction, generally rectangular ([Pl. III], Figs. 29, 30, and 31), or by making hollow traverses by blinding the sap at points separated by limited distances ([Pl. III], Figs. 32-35).

In traversing a double sap by change of direction, a single sap is broken out to the right or left (or one in each direction) and pushed forward to the desired length. From the flank of this the double sap is again broken out and driven to the front until another traverse is required. When the saps are driven to both right and left a double sap is driven to the front from the extremity of each, and at the next change of direction the single saps are driven towards each other until they meet, and the double sap is driven in the prolongation of its original direction. This forms what is called a cube traverse, and gives additional room in the communications. The single sap is used in making traverses, since by throwing all the earth on one side better cover is given. When the sap is so far advanced that it becomes subject to a reverse fire the double sap will have to be used in making the traverses.

Length of Traverses.—Traverses should extend at least 12 feet beyond the trench in their rear, which will give them a length of from 25 to 30 feet on the berm. The salient angles of the sides of the trenches should be rounded as much as practicable to allow the easy passage of guns, and those of the parapets should, when necessary to screen the trench, be held as nearly vertical as practicable by sand-bags. Ramps leading to the surface of the ground may be made in rear of the traverses when needed. For guns they are 8' wide, with slope not greater than ¼. The work in making traverses being considerable, they should be spaced as far apart as practicable.

Spacing Traverses.—In driving the sap to the front the low head parapet of the sap will defilade a less length in the rear than would the finished parapet of the traverse, which is from 2 to 3 feet higher. The sap is, nevertheless, pushed forward to as great a length as will be defiladed by the traverse when finished, the sappers meanwhile passing the partly protected portion by stooping or creeping when necessary.

35. Traverse by Blinded Sap.—In traversing a sap by blinding a part of its length ([Pl. III], Figs. 32-35) the sap is first deepened 2 feet over this part; mine-cases, frames similar to mining-frames, or regular blindage-frames (see Military Mining, [Arts. 53]-55) are then put in position, the side slopes are held up by sheeting, when necessary, and the top is covered with sheeting, fascines, rails, or other material; earth is then thrown upon the top to bring it up to the desired height for a traverse, which will usually give at least 3 feet of earth covering. When a considerable thickness of earth is to be used the frames must be made correspondingly strong. For a clear opening, 6 feet at bottom, 8 feet at top, and 6' 6" high the English engineers recommend frames at 3-foot intervals, with 6-inch square posts, 2-inch thick sills, and 9 inch × 6 inch caps 12 feet long. The end frames should be braced against outward thrust by 6" × 6" struts.

These traverses are usually made at least 20 feet in length. They can be used only when good drainage can be secured.

36. Crowning the Covered Way ([Pl. IV], Fig. 36).—The traversed sap is used for “crowning the covered way,” which consists in constructing a battery or infantry parapet along the crest of the covered way, from which a fire can be brought upon the ditch and the scarp-wall of the work. To accomplish this the sap is run parallel to the crest, with its nearer cutting line 18 or 20 feet from it. For an infantry trench the traverses may be of the dimensions already given.

To cover a battery they should be about 33' long. It will generally be necessary to use the double sap altogether in their construction, but usually the earth excavated by Nos. 3 and 4 of both brigades will be thrown on the parapet next the work, the parapet on the reverse side formed by Nos. 1 and 2 affording sufficient cover for constructing the sap and traverses. The parapet is prepared for infantry fire as described for the parallels. The emplacements for guns, the service magazines, etc., etc., are prepared and the embrasures are cut, or the parapet prepared for the overbank carriages at the last moment, under cover of the small-arm and machine-gun fire from the parallels and places of arms, and the artillery fire from batteries, which do not endanger the working parties.

37. Trench Cavalier.—In order to obtain a greater plunge upon the covered way and ditch, short lengths of double sap are sometimes run at right angles to the direction of the crests of the covered way at about 30 yards outside its salient. The parapet on the side of the sap towards the work is thrown forward and built up to the desired height with gabions and sand-bags, and provided with steps and sand-bag loopholes, giving a short length of parapet with considerable command, firing directly along the covered way at short range.

This construction is called a Trench cavalier. It will be seldom, if ever, used in the future.

38. Former Methods of Sapping.—Before the general introduction of machine and rapid-fire guns and of small arms of extreme accuracy and penetration saps were constructed by No. 1 sapper driving a trench 18" × 18", which was enlarged successively by Nos. 2, 3, and 4. Cover for the sappers was obtained by the use of a sap-roller (a large gabion, 7' 6" long and 4' in diameter, stuffed with fascines and rods) as a movable head parapet, and the construction of the side parapet was expedited by the use of gabions, sap-fagots, etc. This method cannot be used against an enemy well equipped with modern weapons. It is referred to only as a suggestion that a readily improvised modification of it might be used to capture, with the least possible loss, a party of rioters, criminals, or other badly-armed men occupying an isolated house or other cover.

PASSAGE OF THE DITCH.

39. When breaches which are practicable are made in both counter-scarp and scarp of a dry ditch an assault may sometimes be made successfully; but when the scarp-wall is not breached, or when, for other reasons, an assault from the crowning of the covered way is not considered advisable, the ditch must be crossed and the breach, when made, must be crowned by regular approaches. This is accomplished by the use of a sap, single or double, depending upon whether it is exposed to fire upon one or both flanks. Owing to the plunging fire of the defence it may be necessary to make the sap deeper than 4' 6", or in some cases to blind it for a part or the whole of its length. It is generally impracticable to drive the sap down the slope of a breach in the counter-scarp; therefore a blinded descent (Military Mining, pars. 53 and 54) is used. It is so directed that when the counter-scarp is reached the floor of the gallery will be at the required depth below the bottom of the ditch; i.e., at the depth fixed for the bottom of the sap. When the small-arm fire of the defence is so severe as to necessitate blinding the sap from the counter-scarp across the ditch, it will usually be imperative to provide a shield, under cover of which the sappers may start the blinded sap. This may be made of boards covered with bullet-proof iron-plates, and of such width and length that it may be carried through the gallery, thrust out into the ditch, and then turned, placed in position, and blocked up at such angle and to such height as may be wished, by men who move on their hands and knees and support the shield on their backs. Under cover of this shield the head parapet of the blinded sap may be thrown up and the sap then driven in the usual way.

For method of breaching by mines, see Military Mining, pars. 91 and 92.

After breaching the scarp, if an assault is to be made, the counter-scarp, for a length equal to or greater than the length of the breach, should be blown down, to give the assaulting party access to the breach. If the breach is to be crowned, and approaches are to be driven against interior retrenchments, a gallery of descent should be driven to the counter-scarp at one side of the breach before the assault is made. From this a trench should be driven to the crowning of the breach (usually by flying sap), by means of which communication is maintained between the crowning of the breach and the exterior.

40. A wet ditch without current may be crossed by building a causeway, upon one or each side of which a parapet is constructed ([Pl. IV], Figs. 37, 38, and 41).

The floor of the gallery of descent should strike the ditch at about one foot above the level of the water. The counter-scarp wall having been broken through, a shield similar to that described in the preceding paragraph may be used to cover the sappers working at the outlet of the gallery in the first stages of the succeeding work. The causeway is built by throwing into the ditch short fascines or brushwood mats, having bound up with them stones enough to sink them, broken-stone gravel or other available material, until the causeway is 8 or 10 feet long, about one foot above the level of the water and wide enough for the roadway and parapets. The head and side parapets are then constructed with sand-bags, which are passed out under the shield and piled at its head and sides. The causeway is continued by throwing material over the head parapet, and the approach is driven forward somewhat like a sap. So soon as the head and side parapets are made the shield may be raised up and supported upon two or three cross-balks resting upon the side parapets. It may then be progressively moved forward and the approach in its rear be blinded by sappers working under its protection. Unless the plunge of the fire upon the approach is equal to or greater than 45° this shield will, however, when vertical, cover a greater length of trench than when in a horizontal position. It may, therefore, if desired, be turned into a vertical position and be supported by a frame built for that purpose, which can be moved forward as the approach advances ([Pl. IV], Fig. 41).

When the head of the approach is subjected to the fire of rifle-bullets only, it may be practicable to dispense with the head parapet, replacing it with a bullet-proof screen covering the head of the approach and with wings extending back and overlapping the side parapets ([Pl. IV], Fig. 40); the shield floating on a raft of light logs, or other material which cannot be sunk by rifle fire. This shield would, of course, be erected and launched under cover of the one already referred to. To save sand-bags, etc., the interior slope of the side parapets may be revetted with gabions about 4' 6" long, resting on two short fascines and crowned with three others, giving a height of about 6' 3". Upon these, when necessary, cross-beams are laid and the blinding of the approach is finished with sand-bags thrown on top.

41. A wet ditch with current, or one in which the water level may be varied by the defence, presents greater difficulties. The method of crossing which seems most promising is by a causeway made of materials which will allow the water to pass freely through it. For this purpose it is usually recommended to use casks with their heads knocked out, or strong gabions lashed to balks, so as to form continuous tubes, which are loaded with stones and sunk with their axes parallel to the counter-scarp by sappers working under cover of a shield. When the top of the causeway is about a foot above the high-water mark it is levelled off with fascines and the approach driven forward as previously described. When available, iron or terra-cotta pipes of large diameter may, perhaps, be advantageously substituted for casks or gabions. Cormontaigne, at Philipsburg, in 1734, successfully used floating bridges of fascines with parapets of gabions and fascines covered with raw hides. Two bridges were made. They were 128 feet long, 48 feet wide, and 6 feet thick. The water was about 15 feet deep. To construct and hold in place, in a strong or varying current, a floating bridge of sufficient width and depth to support, without sinking or capsizing, the parapets necessary for protection against modern small arms and machine guns is a task presenting such great difficulties that it will hardly be undertaken, except as a last resort, and then with a very uncertain issue.

When, however, the fire of the work is nearly or entirely silenced, a floating bridge of pontoons, casks, spars, or other materials, with a slightly masked roadway, may furnish a sufficiently good crossing and may be constructed with little difficulty and loss.[1]

CHAPTER IV.
BATTERIES, OBSERVATORIES, AND MAGAZINES.

42. Batteries in siege operations are for field-guns, siege-guns, howitzers, and mortars.

When the gun-platform is on or above the level of the ground they are known as “elevated batteries,” when it is below the surface as “sunken batteries.”

When they are concealed from the view of the enemy by natural or artificial screens they are called “screened” or “masked batteries,” and when on sites which can be seen by the enemy “exposed batteries.”

GENERAL REQUIREMENTS OF SIEGE BATTERIES.

43. 1st. A good platform for and sufficient space to work each gun. The platform must be suited to the gun used. The space required is about 15 feet front by 20 to 25 feet depth.

2d. A parapet which cannot be penetrated by the projectiles which will be fired against it, and which is high enough to afford cover to the gun and its detachment against curved fire. A thickness of 30 feet of earth will usually be enough for the most exposed batteries. A less thickness may be used when the conditions justify it. The height of the interior crest above the terre-plein should not be less than 7½ feet when this is attainable and may sometimes be greater.

3d. Traverses. Each gun is usually separated from the next by a traverse, whose thickness when subject to enfilade fire is the same as that of the parapet (30 feet); under other circumstances the thickness may be reduced if deemed advisable, but should, when practicable, be such that a shell bursting at any point within it will blow out at the top or on one side only.

4th. Bomb and splinter proofs sufficient to cover the gun detachment and reserves against vertical fire. The thickness of cover for these is to be regulated according to the principles laid down in par. 21.

5th. Magazines which will hold at least 24 hours' supply of ammunition, besides recesses near the guns for shells and a few cartridges.

6th. Easy and direct communications for bringing up the guns and placing them in position; including tramways, ramps, etc., etc.

7th. Look-outs or observatories from which the effect of the fire can be seen. These when possible will be placed in high sheltered places well on the flanks of the battery and preferably in advance of it. They may be connected with it by signals, telegraph, or telephone, when necessary.

8th. Screens.—Earthen screens should when possible be thrown up in front of all exposed batteries.

CONSTRUCTION OF BATTERIES.

44. Batteries for Field Guns.—When the place is invested, the field artillery is placed in positions considered most advantageous for repelling attacks from the garrison upon the investing force. Gun-pits (described in “Field Fortifications”) are usually made at once for cover for the guns and their detachments. When any of these sites are occupied during the siege the gun-pits may be connected and converted into a battery as indicated by [Pl. IV], Figs. 42-45.

A similar construction may sometimes be used during the siege when the artillery fire of the place is weakened, and it is desirable to place a field battery in position for reaching some point in the work. As a rule, however, batteries for field guns will during the siege be constructed in the same way as are those for siege guns and howitzers.

45. Batteries for Siege Guns and Howitzers.—These may be screened or exposed, sunken, or elevated. As a rule each battery has a magazine on each flank. The amount of powder necessary to serve two guns for 24 hours (150 to 200 rounds per gun = 2500 to 6000 lbs.) is as much as it is advisable to have in one magazine, in order to limit so far as possible the disastrous effects of an explosion. For this reason the number of guns in a battery is usually restricted to four. This number may be increased when necessary, or when howitzers firing small charges render it unobjectionable.

Elevated batteries require much more labor for their construction and for obtaining cover for the men and material than the sunken batteries. They are therefore used only when the target has to be seen and the gun has to be raised for this purpose, or when owing to the presence of rock or water in the soil, or the liability of the site to be flooded it is impracticable to sink the platforms below the surface. As a rule they can be constructed only when covered by a screen either natural or artificial, and then with earth carried in wheelbarrows, sand-bags, etc., etc.

Sunken Batteries.—When constructed under cover of a screen the depth of the terre-plein of a sunken battery may be limited by the presence of rock or water in the soil, the character of the guns and carriages, and the time available for the work. In a hasty construction the depth of the terre-plein is usually limited to from 3 to 4 feet, which can be dug out in a short time. When more time is available the gun platforms may be put at 5 to 6 feet below the surface and the other parts of the terre-plein may be sunk still lower. This gives but little height of parapet, and the extra earth may be used for giving additional thickness of cover to the splinter-proofs under the traverses and flanks, and also to the magazines.

A great variety of plans and profiles may be adopted for batteries of this class, the details of which need not be given, since they will be modifications of those described in Field Fortifications and Permanent Fortifications, and of the exposed battery to be next described. As they are built under cover of screens and are not subject to fire during construction, work upon them may be continuous and by day as well as by night.

46. Screens.—The natural screens used for cover are elevations, woods, hedges, existing buildings, walls, etc., etc.

Artificial screens may be made by setting out bushes to imitate hedges or adopting similar devices, which, however, will usually fail to deceive an active enemy. A trench with the earth thrown to the front, forming a glacis-shaped parapet, will, however, generally be effective. It must be made of such length that the enemy cannot know the exact position of the battery, and of such height and thickness that he cannot afford to expend enough ammunition to breach it.

This affords not only concealment during construction, but also a remarkably efficient cover to the battery against hostile fire.

Screens, natural or artificial, should be from 50 to 100 yards in front of the batteries, so that the enemy’s aim may not be corrected by seeing the points struck by his shells.

Unless the screen is of material which will break up into injurious splinters under hostile fire, only enough should be removed before opening fire to unmask the target of each gun, leaving the remainder for concealing the points struck by shells, even if it affords no cover against their penetration.

47. Exposed Sunken Battery.—Before describing the construction of this battery it is necessary to state that upon a site fully exposed to the accurate concentrated fire of a work, directed at night by light balls or electric lights, it will in general be practicable to construct batteries only by sapping, and even then with considerable losses. But these conditions seldom exist, since in the distant attack it is usually possible to construct and arm the battery before it is discovered by the defence, and in the close attack the fire of the defence is generally so much reduced that some exposure is justifiable. While the battery to be described is classed as an “exposed battery,” it is understood that it may also be constructed under cover of a parallel or other trench, and that in all cases when practicable a natural or artificial mask is used to conceal the first night’s work from the enemy. It is assumed from the results obtained in practice that, with the material conveniently stored, the battery can be traced, a central trench and splinter-proof covers be made during the first night, and the battery finished and armed during the second.

The general design and details of this battery are due to the Royal (British) Engineers.

48. Tracing the Battery.—The battery is traced under the direction of an engineer officer by one or two tracing parties, each composed as follows: 1 non-commissioned officer with a 6-foot measuring-rod and tracing-lantern, and 4 sappers, one carrying a measuring-tape and bundles of pickets, one a field-level, one several tracing-tapes, and one a mallet or hand-axe; about 75 pickets and 1200 feet of tracing-tape should be provided. The line of fire of the first gun of the battery (xy, [Pl. V], Fig. 46) is accurately laid out and marked by daylight. At dusk one party drives a picket at I, where the directrix crosses the projection of the base of the interior slope, and from this as an origin lays out the cutting lines of the central trench, I, II, III, IV, V, I, making the trench 5 feet wide and of the length required for the number of guns (= No. of guns × 45'—10'); commencing then at a point A, 7' 6" to the left of I and in the rear cutting line, this party lays out the line a, b, c, d, e, etc., ... m, n, o, as indicated, the direction n, o, leading to the parallel.

The second party, beginning at A, lays out A, B, C, communicating with the parallel, and then the inner cutting line of the ditch D, E, F, G, H, I, allowing for a thickness of parapet of 30 feet and an ultimate width of ditch of 12 feet (D, E, and H, I).

Two parties should trace the battery in 25 minutes, one party in 45 minutes.

49. Constructing the Central Passage and Splinter Proofs.—The first relief of working party for the central passage is posted and commences work at once ([Pl. V], Figs. 47-48). Each man’s task is 5 feet in length and 4 feet in depth (giving 100 cubic feet). It may be completed in 4 hours, and should be in 6 at most.

The second relief ([Pl. V], Figs. 49-52) excavates the cartridge recesses, trims up the work done by the first relief, lowers any earth that stands too high, revets the slopes of the gun portions, puts in frames and sheeting when needed in the splinter-proofs, places the bearing planks and balks of the latter, which should be at least 9 inches thick and 9 feet long, except over the cartridge recesses, where they are 12 feet, and when possible deepens the central passage under the splinter-proofs to 5' 6" for a width of 2 feet to form a seat for the men. It also places one or two planks along the passage to serve as a bench for shells. The latter part of this work can be done by daylight. The parapet formed by this excavation is about 2 feet high. This is so masked or so inconspicuous as not to draw upon itself the artillery fire of the defence. The construction of the battery will be continued usually on the following night.

50. Construction of the Battery (Pls. V and VI, Figs. 53, 55, 60, 67).—Two reliefs are required for this. The first relief receives its tools and arrives upon the ground at dusk. It is divided into four parties, one for the front ditch, one for the gun portions, one for the rear trench, and a reserve of ten per cent for substitutes and casualties. They are posted and supervised by the engineer officer, n. c. o^s., and sappers as described in paragraph 16, ante.

The Front-ditch Party.—Each digger is assigned a task 5' wide, 6' long, and 3' 6" deep. He throws the earth as far into the parapet as he can. The shovellers, one to each two diggers, are posted 12 feet from the cutting line of the ditch. They pass the earth back toward the interior crest and the traverse, keeping the top surface nearly level.

The gun-portion party is divided up equally among the gun portions, each digger is allotted a task 4' wide, 7' 6" long, and 3' 6" deep. The gabions around the gun portion are placed by the shovellers under the direction of the engineer soldiers, a short one being placed at the throat of the embrasure. The shovellers spread and level the earth thrown out by the gun-portion parties and the rear-trench party. They work in connection with the other shovellers to give to the traverses and parapet near the interior crest the proper shape.

The Rear-trench Party.—This party excavates to a width of 7' 6" the rear trench and the communications with the parallel or approach. Each digger has a task 4' wide, 7' 6" long, and 3' 6" deep. The two directly in rear of each gun portion throw the earth to the rear, the others throw it to the front, leaving a berm of 4' 6" at the rear of the traverse. The men of the reserve who are not otherwise occupied fill sand-bags from the earth thrown to the rear, and cut a ramp 8 feet wide and not steeper than ¼, in rear of each gun-portion, when needed. It is essential that the excavation of the gun-portion be finished by the first relief, so that the platforms may be laid by the second relief in time to allow the guns to be placed before daylight.

The first relief leaves in the battery the tools required by the second and carries the rest back to the depot.

The second relief is divided into three parties and a strong reserve of one quarter or one fifth of its strength. The first, or front-ditch party, works in the front ditch, widening it 6 feet and throwing the earth back to form the front of the parapet. The shovellers, one to each two diggers, spread and level it. The task of a digger is 5' wide, 6' long, and 3' 6" deep.

The second, or platform party, places the platforms and gives way to the gun detachments.

The third, or rear-trench party, widens the trench 3' towards the front by cutting off the rear of the traverses.

The reserve completes any work left unfinished by the first relief, fills sand-bags and places them around the gun portions, digs ditches and drainage-pits when needed, and does any other work necessary for the completion and arming of the battery.

When a tramway is laid in the trench for bringing up the guns and carriages, the ramps in rear need not be cut.

51. Alternative Construction in Position Very Much Exposed.—When the earth thrown up in making the splinter-proofs cannot be concealed, it may attract such a severe fire from the defence as to make the above-described construction impossible. In this case the battery is traced as above described, the balks for covering the splinter-proofs are placed in position resting on bearing-planks, and the construction of the front ditch, gun portions, and rear trench are commenced at once; and the battery is as nearly finished as time allows, and armed if possible. The splinter-proofs are subsequently mined out and the remaining necessary details finished before opening fire.

52. Splinter-proofs, in addition to those in the central trench, are usually constructed under the rear of the traverses ([Pl. VI], Figs. 65-67). These may be made during the construction of the battery or after its completion. They are about 5 feet wide, 6 feet deep, and 10 feet shorter than the width of the traverse. Their floor is at 6 feet below the surface. The earth is held up by frames and sheeting, and the roof is supported by cross-balks resting on posts and running back into the traverse. The roof consists of railroad iron or heavy timbers covered with earth, and access is given by steps from the rear trench; the space not occupied by the steps may be shielded with inclined posts or other covering if thought necessary. These splinter-proofs differ in no essential from those described in Field Fortifications. The finished battery is shown in [Pl. VI], Figs. 62-64.

53. Sunken Battery in a Parallel ([Pl. VII], Figs. 68 and 69).—A battery similar to the one above described is sometimes constructed in a parallel. In this case the traverses have to be built up, and therefore do not usually exceed 20 feet in thickness. Pickets are driven at intervals of 35 feet along the banquette of the parallel to mark the centres of the gun spaces, and the rest of the battery is traced in the usual way. The steps of the parallel are cut away and the slope revetted for the gun spaces and the central trench. Gabions are placed along the back of the central trench and the sides of the traverses. A rear trench 7' 6" wide is cut from the parallel at an easy curve, so that its front cutting-line shall be 25 feet from the foot of the interior slope; this, as before, is widened 3 feet by the second relief cutting away the rear of the traverses. The reverse slope of the parallel in rear of the gun portions is cut back to the rear trench. A trench 2 feet wide and 2 feet deep is cut between the front of the traverses and the foot of the interior slope, and the cartridge recesses are excavated. The gabions of the traverses are filled, balks placed over the central trench, and the tops of the traverses and splinter-proofs are raised to the height of the parapet of the parallel. A ditch in the front of the parallel 12' wide and 3' 6" deep, traced at dusk, and excavated during the night, supplies earth to make the parapet 30' thick and 4' 6" high. The work done in and behind the parallel is not seen from the front, hence a great part of it may be done by day, undetected by the enemy. The upper part of the traverses is made by night, and the front ditch and front of the parapet are made the same night or subsequently, depending upon the number of workmen available.

Since the gabions of the traverses seriously obstruct the parallel, they should not be placed in position until all arrangements are made to open the rear trench.

In the special case of a battery on the crowning of the covered way, the traverses have been already constructed in running the sap. The splinter-proofs may be constructed by blinding portions of the sap, or by mining them under the traverses. Owing to the height of the parapet, embrasures of some depth will have to be cut through it. This is done by a shallow sap started by one man, who is subsequently assisted by a second, if the splay requires it. The cheeks are revetted with sand-bags, covered with hides. The mouth of the embrasure is left closed with the head parapet of the sap until fire is to be opened, when the earth is dug away or blown away by the gun.

54. Battery Behind the Crest of a Hill ([Pl. V], Figs. 57-59).—In a battery behind the crest of a hill the front ditch may be omitted, the gun-portions may be entirely in excavation, and the platforms given such a reference as to require a shallow groove to be cut through the crest to allow the gun to fire. When the ground falls away very rapidly to the rear it may be stepped under the traverses to prevent their sliding, and the rear of the gun emplacement may be raised when necessary to give the platform the proper slope. The central trench is cut deep enough to give 5 feet of cover over the splinter-proofs.

55. Batteries on Sloping Ground ([Pl. VII], Figs. 70-72).—When the ground to be occupied by the battery slopes towards or from the place or falls off on either side, the battery is constructed essentially as upon level ground. The central passage is driven, following the surface of the ground, the gun emplacements, front and rear trench are excavated as before described, the additional excavation or filling required in each gun emplacement to make the platform horizontal is regulated for the particular site, any excess of earth being used to give greater cover on the more exposed side, and any deficiency being supplied from the front or rear trench, as may be most convenient. Where the extra work imposed by the slope is considerable, a third relief may be required to finish the battery, and its arming may be necessarily postponed until the next night.

56. Embrasures.—Modern siege guns are generally mounted either on “overbank” or “disappearing” carriages, firing over parapets of sufficient height to give cover to the men. (The axis of the trunnions of the U. S. 5" siege gun is 6" above the platform.) Embrasures when used are generally shallow grooves cut in the top of the parapet. In this case the bottom of these grooves must cut the surface of the top of the parapet at or in rear of the highest line visible to the enemy, so that no indentations which can be seen by him will indicate the position of the guns. To effect this, the exterior crest will usually be as high as and sometimes higher than the interior crest, and the top of the parapet (“superior slope”) will be level or will slope to the rear. In rare instances, however, deeper embrasures with revetted cheeks must be made. The only serviceable revetment for use with high-power guns is one of sand-bags wrapped in raw hides. This may be made by laying down a hide, piling a number of sand-bags upon it, and folding the free end back over them; placing another hide on top of this with more sand-bags and so on. Or large packages may be made by wrapping up a number of sand-bags in each hide and these packages may be used for making the revetment.

The embrasure should be bottle-shaped in plan, shaped like a segment of an ellipsoid immediately in front of the muzzle of the gun, then drawn in like the neck of a bottle and narrowed to as small a mouth as possible, so as to diminish the effect of the blast and give the least possible exposure to the gun. When the battery is exposed to slant or enfilading fire, instead of embrasures, bonnets of sand-bags may be built upon the parapets to protect the guns.

57. Observatories.—Observatories or look-outs, as previously stated, should as a rule be placed on high points well on the flanks of the battery.[2] When this is impracticable, they may be made by building up at the rear of the traverses, on the flanks, or even in the gun portions, glacis-shaped covers pierced with a sight-hole in all respects similar to a loop-hole for musketry, and with just sufficient splay to include the desired field of view. A number of these should be provided for each battery, so that the enemy may not know which one is in use at any time. If subject to close and accurate fire, the crest-line in their vicinity must be of the same level as the tops of the look-outs, and provision must be made to prevent the light showing through them.

58. Drainage.—After the completion and arming of the battery, gutters should be cut on each side of the gun-portion leading into one running along the reverse of the rear trench which carries the water to low ground on the exterior, or which is provided with dry wells or drainage-pits for collecting the water so that it may soak into the ground or be pumped out with hand-pumps.

59. Mortar Batteries.—The introduction of rifled mortars of all calibres, with the corresponding increase in accuracy of fire, together with the destructive effects of shells charged with high explosives, will doubtless lead to the extensive use of mortars in future sieges.

In a distant attack the requirements of a mortar battery are very simple, consisting principally of a stable platform, magazines for ammunition, and bomb-proof covers for the gunners; since the battery as a rule will be concealed from the view of the work by intervening obstacles, and will in consequence not be subject to direct fire. When the soil is favorable, cover against plunging fire will be most easily obtained by sinking pits for the mortars to such depth as may be necessary to furnish earth for a splinter-proof parapet surrounding the pit, and for cover for the bomb-proof shelters for the men and the magazines.

When ample space exists which is well concealed, and in which the soil is good, a separate emplacement should be made for each mortar. When necessary, however, two or more mortars may be placed in each pit. The magazines, splinter and bomb proofs are similar to those elsewhere described. When no natural mask exists, the battery may be constructed behind an artificial screen, and be made of the general type of the “exposed siege battery,” the gun portions being made with front enough to accommodate one or two mortars as may be preferred, and of such length only as is needed for working the mortar employed. The terre-plein may be placed at any convenient depth below the surface of the ground, and the revetment of the interior slope, if any be used, will not ordinarily be carried higher than the muzzle of the mortar. As the traverses are not subject to gun-fire, the splinter-proofs afforded by the central passage may be added to by building others along both sides of the traverse; and by deepening the mortar emplacement sufficiently, they may be given enough cover to make them true bomb-proofs.

A mortar battery fulfilling these conditions can hardly be silenced by hostile fire.

The conditions under which the batteries may be constructed are, however, so varied that detailed dimensions will not be given. No difficulty will exist in making the battery of a size suitable for the pieces to be employed.

The U. S. rifled siege mortar is of 7-inch calibre, about 5 feet long, weighs 1715 lbs., and is designed to throw a 125-lb. shell with a charge of 5½ lbs. of powder, giving an initial velocity of 685 f.s. and a range of about 4000 yards. With reduced charges the range may be reduced to about 650 yards without undue sacrifice of accuracy.

In the closer attack upon the work, batteries for the smaller siege and field mortars may be readily constructed in front or rear of the parallels, or in the parallels or approaches themselves; splinter-proofs and temporary magazines being constructed by methods previously indicated. In many cases, however, the lighter mortars, field and Coehorn, which do not require fixed platforms, may be placed behind any part of the trenches affording cover, and fire be opened and continued until the fire of the enemy becomes too annoying, when the mortars may be removed to some other locality.

MAGAZINES.

60. Magazines should be provided, at least two to each battery, not only to localize the injury due to an explosion, but also to prevent the battery being disabled by the explosion of a single one.

As previously stated (par. 43), they should contain 24 hours' supply (from 150 to 200 rounds) for each gun which they are designed to serve,[3] which may require a capacity in a single magazine of as much as 6,000 lbs. of powder.

This amount should be reduced when possible by increasing the number of magazines. The cartridges should be made up and packed in boxes at the depots or parks, and the powder chambers in the magazines should be of such size as to store these boxes with only such vacant space as is necessary for ease in handling them.[4]

61. Cover.—The chamber should be covered with strong balks or rails and enough earth to form a sloping roof; over this raw hides or tarpaulin should be spread, and the remainder of the earth filling be spread upon this and rammed solidly. The amount of earth cover required for security must be determined from the principles given in par. 21. The English engineers recommend as sufficient protection against ordinary fire for a magazine 5 feet wide, two layers of 9" × 9" fir laid crossing each other, or one layer of 12" × 12" oak, covered with 5 feet of earth.

In experiments at Lydd in 1883, however, an 8-inch howitzer shell falling at an angle of about 30° penetrated through a covering of 7 feet of soft clay and burst upon the timber roof of a magazine, cutting it through. This shows that complete protection is not always possible, and that the chances of hitting must be reduced by making the horizontal area of the magazine chamber as small as possible, and placing its smaller dimension in the line of the hostile fire. The clear height of the magazine should be 4' 6" to 5' minimum, when practicable, and the top of the covering balks should be at or below the level of the ground.

62. Location.—A magazine should be located at such distance from the battery that its explosion will not disable the guns, injure the parapets or traverses, or seriously endanger the cannoneers;[5] but, on the other hand, it should be near enough to allow the ammunition boxes to be conveniently carried to the cartridge recesses; and the communications for this purpose should be well covered from hostile fire. The entrance to the magazine should be so protected that splinters cannot enter the chamber. Any natural hollows, banks, etc., in the immediate vicinity of the battery should be taken advantage of to facilitate the construction of and give better cover to the magazines. When nothing of this kind exists the magazines may be placed on the flanks or in rear of the battery, and should be masked and screened by the parapet of the parallel, approach, communications, or battery, or by special glacis-shaped screens made, for the purpose; which should be much longer than the width of the magazines that they cover, so that the discovery of the location of the latter by the enemy may be made more difficult. The magazines should not be located in rear of the centre of the screens nor symmetrically with reference to the battery, nor, when it can be avoided, directly in rear of a gun. The passages leading to them should enter the battery in rear of a flank or a traverse, and should be so directed as to escape enfilade. They should be so graded that the surface water will run away from the door of the magazine and be discharged upon lower ground or received in drainage-pits placed at the lowest points.

63. Construction of a Magazine Subject to Direct Fire Only ([Pl. VII], Figs. 73-77.)—The method of tracing the magazine and its approaches is too evident to need description. In this example an earth cover of 5 feet against vertical and 20 feet against horizontal fire is given. Should more or less be desired, a corresponding change may be made in the plan, depth of excavation, and depth and width of approaches; and the earth for additional cover may be obtained from a ditch or pit in rear of the magazine. The excavation for the chamber is given a width of 6', a depth of 5' 6", and a length of 12', the entrance a width of 3', a depth of 5' 6", and a length of 6'.

The sides of the chamber and entrance are held up by frames 4' 9" high and 2' 11" wide, outside measurement. The caps are 6" × 5", stanchions 4" × 5", and the ground sills 3" × 5"; sheeting 1" thick is inserted between the frames and the earth. The covering balks are 9" × 6" and 10 feet long; their tops are flush with the surface of the ground; cleats nailed on their under side keep the tops of the side frames from being pushed in by the pressure of the earth. The earth cover is 5 feet high at the centre and 4 feet at the crest of the outer slope. The passages are 5' 6" deep, 3' wide at bottom, and 5' at the top. The entrance is blinded by placing balks across the passage for such part of its length as may be thought necessary, and extending the earth covering over them, as shown in the section (Fig. 75). A door, swinging outside, is hung on the outside frame. Heavy railroad iron may be substituted for the timber balks with advantage. When thicker balks are used, or when a second layer is added, the chamber and passage should be correspondingly deepened.

64. Manner of Executing the Work.—The powder chamber and passage are excavated and the frames and balks placed during the first night, while the central passage of the battery is being constructed. The excavated earth is thrown out far enough to allow the balks to be put in position, and is so spread as not to be seen by the defence. This may be done by one relief of 8 hours, or two reliefs of 4 hours each. If the work is not completed during the night, the sheeting, frames, and balks may be placed by day under cover of the earth thrown out and the existing screens.

The passages are excavated and the earth cover completed on the second night by two 4-hour reliefs, the first excavating to a depth of 3' 6", and the second to 5' 6", trimming up the slopes and completing the work. When necessary, the sides of the passage will be revetted by the second and a third relief.

When the necessity for great haste exists, the excavation of the powder-chamber, entrance, and passages may be carried on at the same time, the excavated earth being thrown in front and on the sides of the powder-chamber until the balks are in position, and then thrown back upon them, levelled and rammed.

65. Mined Magazine.—When the soil, by absence of rock and water, admits of mining, greater cover against vertical fire can be obtained with less work by mining out the powder-chamber and passages ([Pl. VIII], Figs. 78, 79). The figures illustrate one of minimum dimensions, which is constructed as follows: The entrance 10' × 5' by 5' 6" deep is first excavated, revetted with frames and sheeting and covered with balks and earth as indicated. At 1 foot from the end a shaft 2' × 5' is sunk to a depth of 12'. From the front of this a gallery 2' × 5' 6" is driven for about 6'; at the end of this galleries 5' 6" × 2' are broken out on each side and driven so far as may be necessary to store the requisite number of ammunition-boxes. (For method of sinking shafts, driving galleries, etc., see Military Mining, [Arts. 25], [33], and [44]-48.) The excavated earth is spread on top of the magazine to increase the thickness of the cover already given by that excavated from the passages. A ring-bolt is placed in the balk directly over the shaft, for attaching a hoisting tackle for removing the earth during construction and for hoisting and lowering ammunition-boxes afterward.

A door opening outward may be hung at the entrance, and the passage may be blinded as previously described, if it is thought necessary. The communications are arranged in essentially the same way as for the magazine previously described.

The dimensions given are the least which will allow moderately free access and good cover. The magazine should be constructed, by good miners, in two nights and the intervening day, and will store about 4000 pounds of cartridges in boxes. When time and the character of the ground admit, and larger capacity is desired, the shaft may be made wider and deeper, the gallery wider and longer, and the powder-chamber deeper, longer, and wider, if desired.

The excavation for the entrance and approaches, the placing of balks, and the levelling and ramming of the earth-cover, should be done by night; the mining work can be carried on both night and day.

66. Elevated Magazines.—When the presence of rock or water in the soil prevents sinking the magazines to the full depth above given, they must be sunk so far as practicable and given the least possible clear height of powder chamber, with the best attainable overhead cover. This should be strengthened by the use of railroad iron or rolled iron beams, when available. The cover against direct fire should be increased up to 30 feet, and the front slope be made gentle, like a glacis. A screen made of an earthen bank with a glacis slope should also be used if possible. These precautions having been taken, the depth of the powder-chamber in the direction of the hostile fire should be reduced to a minimum, and the storage of large quantities of powder be avoided, so far as possible, by constructing a number of small magazines at the most convenient places in the vicinity of the battery.