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 1/4, 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.

=67.= =Precautions against Dampness in Magazines for Siege Batteries.=--Underground magazines of the character above described are, of necessity, sometimes damp. The only ventilation usually possible is obtained by leaving the door open, the air being changed more or less by the men going in and out.

The passage leading to the powder-chamber should enter it at the middle, and in the service of the guns one half of the chamber should be emptied on one day and the other half on the next. This will usually limit the time which a cartridge is exposed to the dampness of the magazine to a maximum of one or two days.