Chapter XII: Part 12

It has been necessary to dwell at some length on the siegecraft of Vauban and his time, not merely for its historical interest, but because the system he introduced was practically unaltered until the end of the 19th century. The sieges of the Peninsular War were conducted on his lines; so was that of Antwerp in 1830; and as far as the disposition of siege trenches was concerned, the same system remained in the Crimea, the Franco-German War and the Russo-Turkish War. The sieges in the Napoleonic wars were few, except in the Iberian peninsula. These last differed from those of the Vauban period and the 18th century in this, that instead of being deliberately undertaken with ample means, against fortresses that answered to the requirements of the time, they were attempted with inadequate forces and materials, against out-of-date works. The fortresses that Wellington besieged in Spain had rudimentary outworks, and escarps that could be seen and breached from a distance. At that time, though the power of small arms had increased very slightly since the last century, there had been a distinct improvement in artillery, so that it was possible to breach a visible revetment at ranges from 500 to 1000 yds. Wellington was very badly off for engineers, siege artillery and material. Trench works could only be carried out on a small scale and slowly. Time being usually of great importance, as in the first two sieges of Badajoz, his technical advisers endeavoured to shorten sieges by breaching the escarp from a distance--a new departure--and launching assaults from trenches that had not reached the covered way. Under these circumstances the direct attacks on breaches failed several times, with great loss of life. Wellington in one or two earlier despatches reflected on his engineers for not establishing their batteries on the crest of the glacis. The failures are, however, clearly due to attempts to push sieges to a conclusion without proper preparation.

So much has been written of late years in criticism of the
fortification to what may be called the Vauban period that it is
important to note what were the preparations considered necessary for
a siege at that time (_Journals of Sieges in Spain, 1811 to 1814_).
Sir John Jones summarizes his own experience in Spain and the data
accumulated by practical engineers in former sieges from the time of
Vauban onwards, in the following conclusions: The actual work of
entrenching, sapping, &c., on the front attacked was much the same
whether the fortress contained 5000 or 10,000 men. On the other hand
the guard of the trenches was proportionate to the fighting men inside
the fortress. (The total number of men had of course to be sufficient
to allow three or four complete shifts or "reliefs" for all work and
duties.) Adding a proportion of men for camp and other duties, he
calculates, for the vigorous siege of an ordinary place situated in
open country and containing 5000 men, a corps of 32,080 effectives,
and remarks further that this force would be greatly exhausted after a
month's service. The same place held by 10,000 would call for a
besieging army of 50,830 men (guards and duties increasing, but not
working parties). Thus the besieger should if possible have a
superiority of 7 to 1 if the garrison numbered 5000, 6 to 1 if 10,000
and 5 to 1 if 15,000 and so on. As regards artillery, he should have
as many, and if possible twice as many, guns as those of the defender
on the front of attack, as well as howitzers for sweeping every line
subject to enfilade and mortars for destroying traverses, &c. Later in
the siege, more howitzers and mortars to clear the covered way and
places of arms, and finally, after the covering of the covered way,
fifty additional battering guns would be required. It is apparent from
this that the practical engineers of the day looked upon a siege as a
serious matter, and did not find permanent fortifications wanting in
defensive strength.

Crimea.

During the long peace that followed the Napoleonic wars, one advance was made in siegecraft. In England in 1824 successful experiments were carried out in breaching an unseen wall by curved or indirect fire from howitzers. At Antwerp in 1830 the increasing power and range of artillery, and especially of howitzers, were used for bombarding purposes, the breaches there being mostly made by mines. Then came one of the world's great sieges; that of Sevastopol in 1854-1855 (see CRIMEAN WAR). The outstanding lesson of Sevastopol is the value of an active defence; of going out to meet the besieger, with countertrench and countermine. This lesson has increased in value for us in proportion to the increased power of the rifle.

In comparing the resistance made behind the earthworks of Sevastopol
with the recorded defences of permanent works, it is essential to
remember that the conditions there were quite abnormal. Sir John Jones
has told us what the relative forces of besiegers and besieged should
be, and the necessary preponderance of artillery for the attack. The
following quotations may be added:

"The siege corps should be sufficiently strong--(1) To invest the
fortress completely, and maintain the investment against all the
efforts of the garrison. (2) If a regular siege is contemplated, to
execute and guard all the siege works required for it. Complete
investment may sometimes be impossible, but experience has repeatedly
shown that the difficulties of a siege are enormously increased if the
garrison are able to draw fresh troops and supplies from outside, and
to rid themselves of their sick and wounded." (Lewis). Again as
regards artillery: "In a regular attack, where every point is gained
inch by inch, it is impossible to succeed without overpowering the
defensive artillery"; and "it is useless to attempt to sap near a
place till its artillery fire is subdued..." (Jones).

These conditions were so far from being fulfilled at Sevastopol that
(a) there was no investment--in fact the Russians came nearer to
investing the Allies; (b) the Russians had the preponderance in guns
almost throughout; (c) the Russian force in and about Sevastopol was
numerically superior to that of the Allies. We must add to this that
Todleben had been able to get rid of most of his civilian population,
and those who remained were chiefly dockyard workmen, able to give
most valuable assistance on the defence works. The circumstances were
therefore exceptionally favourable to an active defence. The weak
point about the extemporized earthworks, which eventually led to the
fall of the place, was the want of good bomb-proof cover near the
parapets.

Franco-German War.

The Franco-German War of 1870 produced no great novelty. The Germans were not anxious to undertake siege operations when it could be avoided. In several cases minor fortresses surrendered after a slight bombardment. In others, after the bombardment failed, the Germans contented themselves with establishing a blockade or detaching a small observing force. By far the most interesting siege was that of Belfort (q.v.). Here Colonel Denfert-Rochereau employed the active defence so successfully by extemporizing detached redoubts and fortifying outlying villages, that he obliged the besiegers (who, however, were a small force at first) to take up an investing line 25 m. long; and succeeded in holding the village of Danjoutin, 2000 yds. in advance of the enceinte, for two months after the siege began. He also used indirect fire, withdrawing guns from the ramparts and placing them in the ditches, in the open spaces of the town, &c. At Paris the French found great advantage in placing batteries in inconspicuous positions outside the forts. Their direct fire guns were at a disadvantage in being fired through embrasures. These had served their purpose when artillery fire was very inaccurate, but had now for a long time been recognized by the best engineers as out of date. The Germans since the siege of Duppel in 1864 had mounted their siege guns on "overbank" carriages; that is, high carriages which made it possible to fire the guns over the parapet of the battery without embrasures. The guns in the Paris forts which were further handicapped by conspicuous parapets and the bad shooting of the gunners were easily silenced.

At Strassburg indirect fire against escarps was used. The escarp of Lunette 53 was successfully breached by this method. The breaching battery was 870 yds. distant, and the shot struck the face of the wall at an angle (horizontally) of 55 deg., the effect being observed and reported from the counterscarp. 1000 rounds from 60-pounder guns sufficed to make a breach 30 yds. wide.

Fig. 71 is a good example of the attack in the late stages. It will be
observed that batteries for mortars and field guns are established in
the captured lunettes. The narrow wet ditch of Lunette 53 was crossed
by a dam of earth and fascines, the headway protected by a parapet or
screen of sandbags.

"Lunette 52 was unrevetted, and its ditch was more than 60 yds. wide,
and 6 to 9 ft. deep.... It was determined to effect the passage by a
cask bridge, for which the casks were furnished by breweries near at
hand.... The formation of the bridge was begun at nightfall. A pioneer
swam across, hauled over a cable, and made it fast to the hedge on the
berm. Four men were stationed in the water, close to the covered way,
the casks were rolled down to them one after the other, and fitted
with saddles, so as to form piers ... these piers were successively
boomed out along the line of the cable.... In two hours the bridge was
finished, and the lunette was entered.... The work had not been
discovered by the besieged, and the formation of lodgments inside the
lunette was already begun, when the noise made by some troops in
passing the bridge attracted attention, and drew a fire which cost the
besiegers about 50 men. A dam was afterwards substituted for the
bridge, as it was repeatedly struck by shells." (_R.E. Professional
Papers_, vol. xix.)

It is curious to realize that this happened at so recent a time. Such
operations would be impossible now, as long as any defending guns
remained in action.

Modern siege warfare.

On the whole it may be said that siegecraft gained practically nothing from the Franco-German War. The Russo-Turkish war taught less, Plevna (q.v.) having been defended by field works and attacked by the old-fashioned methods. For the last ten years of the 19th century military opinion was quite at a loss as to how the sieges of the future would work out. As guns and projectiles continued to improve the "attaque brusquee" proposed by von Sauer had many adherents. It was thought that a heavy bombardment would paralyse resistance and open the way for an attack, to be delivered by great numbers and with special appliances for crossing obstacles. Others thought that the strength of the defence, as manifested by the Plevna field works, would be greater than ever when the field works were backed by permanent works, good communications and the resources of a fortress. One thing was obvious--namely, that as long as the artillery of the place, of even the smallest calibres, remained unsubdued, the difficulty of trenchwork and sapping would be enormously increased, and no one seemed to have formed a clear conception of how that difficulty was to be met. A lecture delivered in Germany about 1895 is worth quoting as a fair example of the vagueness of idea then prevailing: "For the attack, the following is the actual procedure: Accumulation and preparation of material for attack before the fortress: advance of attacking artillery, covered by infantry. Artillery duel. Throwing forward of infantry: destruction of the capability for defence of the position attacked; when possible by long-range artillery fire, otherwise by the aid of the engineers. Occupation of the defensive position. Assault on the inner lines of the fortress." That seemed quite a simple prescription, but the necessary drugs were wanting. And even since Port Arthur great uncertainty as to the future of the attack remains.

FIG. 71.--Strassburg, Lunettes 52 and 53, 1870.]

Modern artillery has much simplified the construction of siege
batteries. Formerly siege batteries and rampart batteries opposed each
other with direct fire at ranges not too long for the unaided human
eye, and the shells, travelling with low velocity, bit into the
parapets, and, exploding, produced their full effect. Accordingly the
task of the gunners was, by accurate fire, to destroy the parapets and
embrasures, and to dismount the guns. The parapets of siege batteries
were therefore made from 18 to 30 ft. thick, and the construction of
such batteries, with traverses, &c., involved much work. The height of
parapet necessary for proper protection being 7 ft. 6 in. to 8 ft., a
great deal of labour could be saved by sinking the gun-platforms about
4 ft. below the surface level, but of course this was only possible
where rock or water were not near the surface.

The effect of modern projectiles was to reduce the thickness of earth
necessary for parapets. High velocity projectiles are very easily
deflected upwards by even a slight bank of earth. This is especially
the case with sand. Loose earth is better than compacted earth, and
clay offers the least resistance to penetration. These facts were
taken note of in England more than on the Continent in the design of
instructional siege batteries.

The construction of batteries is moreover vastly simplified by the
long ranges at which artillery will fight in future. It will as a rule
be possible to place howitzer batteries in such positions that even
from balloons it will be difficult to locate them; and even direct
fire batteries can easily be screened from view. This renders parapets
unnecessary, and probably no more protection will be used than light
splinter-proof screens to stop shrapnel bullets or fragments of common
shell. Moreover batteries can be constructed at leisure and by
daylight.

The most important point about the modern battery is the gun platform
for the larger natures of guns and howitzers. These require very solid
construction to resist the heavy shock of discharge. Not long ago it
was thought that the defence would have larger ordnance than the
attack, as anything heavier than an 8 in. howitzer required a concrete
bed, which could not be made at short notice. The Japanese, however,
at Port Arthur made concrete platforms for 11 in. howitzers. It may be
remarked that difficulties which loom largely in peace are often
overcome easily enough under the stress of war.

Another gain to the attack is in connexion with magazines. The old
powder magazines were particularly dangerous adjuncts to batteries,
and had to be very carefully bomb-proofed. Such propellants as
cordite, however, are comparatively harmless in the open. They are
very difficult to detonate, and if set on fire do not explode like
gunpowder. It is therefore unnecessary to provide bomb-proof magazines
for them in connexion with the batteries.

In future sieges the question of supply will be more important than it
has ever been. Leaving out of the question the bringing up of supplies
from the base of operations, the task of distribution at the front is
a very large one. The Paris siege manoeuvres of 1894 furnish some
instructive data on this point. The main siege park was at Meaux, 10
m. from the 1st artillery position, and the average distance from the
1st artillery position to the principal fort attacked was 5000 yds.
The front of attack on Fort Vaujours and its collateral batteries
covered 10,000 yds. There were 24 batteries in the 1st artillery
position; say 100 guns, spread over a front of 4000 yds. To connect
Meaux with the front, the French laid some 30 m. of narrow gauge
railway largely along existing roads. The line was single, with
numerous branches and sidings. They ran 11 regular trains to the front
daily and half-a-dozen supplementary. The amount of artillery material
sent up was over 5000 tons, without any projectiles; but it can easily
be imagined that large demands were also made on transport for other
purposes. For instance, one complete bakery train was sent up daily.
The amount of ammunition sent up would be limited only by the power of
transporting it. A siege train of 100 pieces could probably dispose of
from 500 to 1000 tons of ammunition a day, at the maximum rate of
firing.

But the most important question affecting the sieges of the future
(putting aside accidental circumstances) will be the configuration of
the ground. Assuming that local conditions do not specially favour the
artillery of either side, it is highly probable that the artillery
duel will result in a deadlock. If the besiegers' guns do not succeed
in silencing those of the defence from the 1st or distant artillery
position (which, whether they are in cupolas or in concealed
positions, will in any case be an extremely difficult task), it will
be necessary for the infantry to press in; to feel for weak points,
and to fight for those that offer better positions for fire and
observation. In doing this they will have to face the defenders'
infantry, entrenched, backed by their unsilenced guns, and having
secure places of assembly from which to deliver counter-attacks. The
distance to which they can work forward and establish themselves under
these conditions will depend on the ground. It will then be for the
engineers to cross the remaining space by sap. This, under present
conditions, will be a tedious process, and may even take long enough
to cause the failure of the siege.

As to the manner of the sap, it will certainly be "deep," as long as
the defence retains any artillery power. When the 4 ft. 6 in. sap
already described was first introduced, it was known as a "deep sap";
but the sieges of the future will probably necessitate a true deep
sap, that is one in which the whole of the necessary cover is got
below the surface of the earth.

Such a sap may consist of an open trench, about 6 ft. deep, the whole
of the excavated earth being carried away through the trench to the
rear; or a blinded trench, covered in as it progresses by
splinter-proof timbers and earth; or a tunnelled trench, leaving a
foot or so of surface earth undisturbed. In either case nothing should
be visible from the front to attract artillery fire. As the sap is
completed, it will sometimes be necessary to add a slight parapet in
places, to give command over the foreground for the rifles of the
guard of the trenches.

The sap will have to be pushed up quite close to the defenders'
trenches and obstacles. After that further progress must either be
made by mining, or as seems very probable, by getting the better of
the defenders in a contest with shells from short-range mortars.

Just as in the feudal ages a castle was built on some solitary
eminence which lent itself to the defensive methods of the time, so in
the future the detached forts and supporting points in the girdle of
a fortress will be sited where smooth and gentle slopes of ground give
the utmost opportunity to the defenders' fire, and the least chance of
concealment to the enemy. There will be considerable latitude of
choice in the defensive positions; though not, of course, the same
latitude as when the existence of a precipitous hill was the _raison
d'etre_ of the castle. In some places, as at Port Arthur, the whole
country-side may by reason of its steep and broken slopes be
unfavourable to the defence, though even then genius will turn the
difficulties to account. But wherever it is possible the defender will
provide for a space of 1000 yds. or so, swept by fire and illuminated
by searchlights, in front of his lines. That space will have to be
crossed by sap, and it needs little imagination to realize how great
the task will be for the besieger.

There are other modern methods of siege warfare to be noticed, the use
of which is common to besiegers and besieged. Much is expected of
balloons; but the use of these in war is unlikely to correspond to
peace expectations. They must be kept at a considerable distance from
the enemy's guns, a distance which will increase as the means of
range-finding improve; and as the height from which they can observe
usefully is limited, so is the observers' power to search out hidden
objects behind vertical screens. Thus, suppose a captive balloon at a
height of 2000 ft., and distant 4000 yds. from an enemy's howitzer
battery: and suppose the battery placed behind a steep hill-side or a
grove of trees, at such a distance that a shell fired with 30 deg.
elevation can just clear this screen. The line of sight from the
observer to the battery is inclined to the horizontal at {2000/3 x
4000}, that is 1/6, or roughly 10 deg. It is obvious, therefore, that
the observer cannot see the battery.

Balloon observers are expected to assist the batteries by marking the
effects of their fire. For this to be done on any practical scale a
balloon would be required for each battery: that is, for only 100
guns, some 20 or 25 balloons. These would require an equal number of
highly skilled observers (of whom there are not too many in
existence), besides the other balloon personnel and accessories, and
the means of making gas, which is too much to expect, even if an enemy
were obliging enough to give notice of his intentions.

Telephones and all other means of transmitting intelligence rapidly
are now of the utmost importance to both attack and defence. Maps
marked with numbered squares are necessary for directing artillery
fire, especially from cupolas. Organization in every branch will give
better results than ever before, and the question of communication and
transport from the base of supplies right up to the front needs
detailed study, in view of the great weight of ammunition and supplies
that will have to be handled.

The use of light mortars for the trenches, introduced by Coehoorn and
revived with extemporized means at Port Arthur, needs great attention.
It may be prophesied that the issue of important sieges in the future,
when skilfully conducted on both sides with sufficient resources, will
depend mainly on the energy of the defenders in trench work, on mining
and countermining in connexion with the trenches, and on the use of
light mortars made to throw large charges of high explosive for short
distances with great accuracy.

For a brief narrative of the siege of Port Arthur in 1904, one of the
greatest sieges of history, both as regards its epic interest and its
military importance, the reader is referred to the article
RUSSO-JAPANESE WAR.

DEFINITIONS.--The following definitions may be useful, but have no
place in the evolution of the attack, to which this section is mainly
devoted.

_Investment._--This most necessary, almost indispensable operation of
every siege consists in surrounding the fortress about to be besieged,
so as to cut off its communications with the outside world.
_Preliminary investment_ which is carried out by cavalry and light
troops before the arrival of the besieging force, consists in closing
the roads so as to shut out supplies and reinforcements. _Close
investment_ should be of such a character as to prevent any sort of
communication, even by single messengers or spies. The term
"_blockade_" is sometimes loosely used instead of investment.

_Lines of Circumvallation and Contravallation._--These now obsolete
terms were in great use until the 19th century. The _circumvallation_
was a line of parapet which the besieger made outside the investing
position of his own force, to protect it when there was a chance of
attack by a relieving army. The line of _contravallation_ was the line
of parapet and trench sometimes made by the besieger all round the
town he was attacking, to check the sorties of the garrison.

_Observing Force._--When circumstances make the reduction of a
particular fortress in the theatre of operations unnecessary a force
is often detached to "observe" it. The duty of this force will be to
watch the garrison and prevent any hostile action such as raids on the
lines of communications.

_Bombardment._--This operation, common to all ages, consists in a
general (sometimes an indiscriminate) fire against either the whole
target offered by the fortress or a particular section of that target.
In ancient and medieval times the effect of a bombardment--whether of
ordinary missiles, of incendiary projectiles, or of poisonous matters
tending to breed pestilence--upon a population closely crowded within
its walls was very powerful. In the present day little military
importance is attached to bombardment, since under modern conditions
it cannot do much real harm.

IV. MILITARY MINING

It has been noted already that mining is one of the most ancient resources of siege warfare. The use of gunpowder in mining operations dates from the end of the 15th century. When Shakespeare makes Fluellen say, at Henry V.'s siege of Harfleur, "th'athversary is digt himself four yards under the countermines; I think 'a will plow up all, if there is not better directions," he is anticipating the development of siegecraft by nearly 100 years. Pedro di Navarro, a Spanish officer, is credited with the first practical use of explosive mines. He employed them with great success at the siege of Naples in 1503; and afterwards, when rebuilding the Castello Nuovo after the siege, was probably the first to make permanent provision for their use in countermines. Countermining had been a measure of defence against the earlier methods of attack-mining; the object being to break into the besiegers' galleries and fight hand to hand for the possession of them. When the explosive mine was introduced, it became the object of the defenders to establish their countermines near the besiegers' galleries and destroy them by the effect of the explosion. In the 400 years or so that have passed this branch of warfare has changed less than any other. Methods of mining have not advanced much, and the increased power of high explosives as compared with gunpowder has its least advantage in moving masses of earth.

When a besieger has arrived by means of trenches within a certain distance of the enemy's works without having subdued their fire, he may find that the advance by sap becomes too slow and too dangerous. He can then advance underground by means of mine galleries, and by exploding large charges at the heads of these galleries can make a series of craters. These craters are then occupied by infantry, and are connected with each other and with the parallel in rear by trenches, thus forming a new parallel. If not interfered with by the defenders the besieger can advance in this way until he reaches the counterscarp. His mines will now be turned to a new purpose, viz. to breach the counterscarp and afterwards the escarp. This is done by placing suitable charges at intervals behind the scarps at such a height above the foundations that the pressure of the earth above the mine will more than counterbalance the resistance of the masonry.

Mines and countermines.

But if the defenders are active, they will countermine. There is as a general rule this broad difference between the mines of the defence and those of the attack, that the defenders do not wish the surface of the ground broken, lest increased opportunities of getting cover should be offered to the besiegers. The object of the defence, therefore, is to destroy the besiegers' galleries without forming craters, and for this purpose they generally endeavour to get underneath the attack galleries. The defenders may, however, wish, if the opportunity is allowed them, to explode mines under the attack parallels, in which case there is of course no objection to disturbing the surface.

"At the commencement of the subterranean war the main object of the
defence is to force the besieger to take to mining operations as early
as possible, as it is a tedious operation and will prolong the siege.
Every endeavour must be made to push forward countermines so as to
meet and check the attack. On the approach of the opponents to each
other careful listening for the enemy must be resorted to. To this end
it is necessary at _irregular_ intervals to suspend all work for some
minutes at a time, closing doors of communication and employing
experienced listeners at the heads of the countermines. This matter is
a most important one, as a premature explosion of the defender's mines
is a double loss to the defender, a loss of a mine and an advantage to
the enemy in more than one way. As soon as the overcharged mines of
the besieger have been fired, a heavy fire should be brought to bear
on the craters, and if possible sorties should be made to prevent the
enemy occupying them. At the same time every effort should be made
underground to surround with galleries, and as it were isolate, the
craters so as to prevent the besieger making a new advance from them.
The efforts of the attack at this stage will probably be directed to
the formation of what are called "Boule shafts" (i.e. shafts partially
lined in which charges are hastily fired with little or no tamping),
and to meet these in time the defender may resort to the use of boring
tools, and so place charges somewhere in advance of the heads of the
countermines. His great object must be to prevent as long as possible
the besieger from getting underground again; and these occasions, when
the power of resistance is temporarily equal to, if not greater than,
that of the attack, should be made the most of by the defence."
(Lewis, _Text-book on Fortification, &c._, 1893.)

The defence has the advantage, in the case of fortresses, of being able to establish beforehand a system of countermine galleries in masonry. Many systems have been worked out for this purpose. A good typical arrangement is that of General Marescot, published in 1799, shown in fig. 72.

FIG. 72.]

The main galleries (those running out in a straight line from the counterscarp gallery _e_ to three of the points _a_) fall gently to the front to a depth of 30 or 40 ft. below the surface--the deeper they are the less they will suffer from the enemy's mines. Branch galleries (marked _c b + d c_) run obliquely upward from them to right and to left, leading to the mines, which are placed at various depths, according to circumstances.

Two main points must be observed in any system of countermines: the branch galleries must run obliquely forward, so as not to present their sides to the action of the enemy's mines; and the distance between the ends of the branches from adjacent main galleries should be such that the enemy cannot pass between them unheard. This distance will vary with the nature of the soil, but may be taken roughly as 20 yds. A convenient size for main galleries is 6 ft. high by 3 ft. wide: branch galleries may be 5 ft. by 3 ft. When the enemy is approaching, other branch galleries, called _listeners_, will be pushed out from main and branch galleries. The section to fig. 1 of fig. 72 shows openings left for the purpose.

Another use of mines in defence is in connexion with breaches. A permanent arrangement for this purpose, by General Dufour, is shown in fig. 72. Yet another use, on which much ingenuity was expended in the 18th century, is to extemporize retrenchments.

Different kinds of mines.

The charges of mines depend of course upon the effect which is desired. When the charge is strong enough to produce a crater, the radius of the circular opening on the surface of the ground is called the _radius of the crater_. The line drawn from the centre of the charge to the nearest surface, which is expressed in feet, is called the _line of least resistance_ (L.L.R.). When a mine produces a crater the diameter of which is equal to the line of least resistance, it is called a one-lined crater; when the diameter is double the L.L.R., a _two-lined crater_ and so on. _Common mines_ are those which produce a two-lined crater. _Over-charged mines_ produce craters greater than two-lined, and _undercharged mines_ less. A _camouflet_ does not produce a crater; it is used when the object is to destroy an enemy's gallery without breaking the surface. Fig. 73 shows sections of the different kinds of mines, with their craters and the effect they will produce downwards and horizontally in ordinary earth.

Probable spheroids of rupture for overcharged Mines

From _Instructions in Military Engineering_, by permission of the Controller of H.M. Stationery Office.

FIG. 73.--Mines.]

Consideration of this figure will show that it is possible to place a long charge at such a depth below the surface that it will destroy all galleries of the enemy within a considerable radius, without much disturbing the surface of the ground.

Bored mines, which have been alluded to above, are a comparatively
recent innovation. When the enemy is heard at work in one of his
galleries and his position approximately determined by the sound, it
is necessary to drive a branch gallery with all speed in that
direction, and when it has advanced as far as appears necessary, to
load, tamp and discharge a mine before the enemy can fire his own
mine. This is one of the most delicate and dangerous operations of
war, and success will fall to those who are at the same time most
skilful and most determined. The work can be hastened and made less
dangerous as follows: Instead of driving a branch gallery, a hole
several inches in diameter is bored in the required direction. With
suitable tools there is no difficulty in driving a straight bore hole
20 or 30 ft. long. A small charge of high explosives is then pushed up
to the end of the borehole and fired. This forms a small camouflet
chamber by compressing the earth around it. Into this chamber the
charge for the mine is passed up the bore-hole. No tamping of course
is required.

Mine warfare is slow, dangerous and uncertain in its results. It will certainly delay the besiegers' advance very much and may do so indefinitely. One point is distinctly in favour of the defence, namely that when ground has been much mined it becomes charged with poisonous gases. Some explosives are less noxious than others in this way, and it will be advantageous for the attack, but not necessarily for the defence, to make use of these.

_Calculation of Charges._--The quantity of powder required for a
charge is expressed in lbs. in terms of L.L.R.^3, and the following
formulae are used:

l = L.L.R. in feet, r = radius of crater in feet, c = powder charge in
pounds, s = a variable dependent on the nature of the soil.

s
For a common mine c = -- l^3
10
s
For an overcharged mine c = -- {l + .9(r - l)}^3.
10

s
For an undercharged mine c = -- {l - .9(l - r)}^3.
10

The values to be given to s are:

Nature of Soil. Value of s.
Very light earth 0.80
Common earth 1.00
Hard sand 1.25
Earth mixed with stones 1.40
Clay mixed with loam 1.55
Inferior brickwork 1.66
Rock or good new brickwork 2.25
Very good old brickwork 2.50

Military mining is carried on by means of vertical _shafts_ and horizontal or inclined _galleries_. When the soil is very stiff, very little or even no lining is required for shafts and galleries; but usually they have to be lined either with cases or frames.

Cases make a complete lining of 2 in. planking. Frames are used at
intervals of 4 or 5 ft. to support a partial lining of planks. Cases
are of course preferable in other respects; but in ordinary soil they
take up more timber.

Shafts and galleries.

There are two kinds of gallery in ordinary use in the British service,
namely the _common gallery_ whose interior dimensions with cases are 5
ft. 6 in. X 2 ft., and the _branch gallery_ which is 4 ft. X 2 ft. The
_shaft_ has about the same dimensions as a branch gallery. Formerly it
was sometimes necessary in the systematic attack of a fortress to get
guns down into the ditch. For this purpose a "great gallery" was used,
6 ft. 6 in. in height and 6 ft. 8 in. wide, internal dimensions.

_Miners' Tools._--These are few and simple. The pick and shovel differ
from the ordinary types in having rather shorter helves suitable for
the confined space in which they are used. There is also a
_push-pick_, an implement with a straight helve and a pointed shovel
head 6 in. long and 3-1/2 in. wide. The _miner's truck_, used for
drawing the earth from the end of the gallery to the bottom of the
shaft, is a small wooden truck holding about 2 cub. ft. of earth.
Formerly the noise of the wheels of the truck passing over the uneven
wooden floor of the gallery was very liable to be heard by the enemy.
To obviate this they now have leather tyres and should run on battens
nailed to the floor. The _miner's bucket_ is a small canvas bucket
with a couple of ropes attached, by which the earth can be drawn up
the shaft. Nowadays, however, the truck itself has chains attached to
it, by which it is drawn up, with the aid of a windlass, to the
surface. By this method more earth can be taken up in one lift, and
time and labour are not wasted in transferring the contents of the
truck to the bucket.

_Ventilation_ is an important point. The breath of the miners and the
burning of their candles (when electric light is not available)
vitiates the air in the galleries; so that even in clean ground a
gallery should not be driven more than 60 ft. without providing some
means of renewing the air. This is usually done by forcing fresh air,
by means of a pump or bellows, through a flexible hose to the head of
the gallery. Where mines have been fired close by, there is great
danger from poisonous gases filtering through the soil into the
gallery. This difficulty is nowadays met by the use of special
apparatus, such as helmets into which fresh air is pumped, so that the
wearers need not breathe the air of the gallery at all. Ventilation
can also be assisted by boring holes vertically to the surface of the
ground.

Where a point has been reached at which it is proposed to fire a mine,
a chamber just large enough to hold the charge is cut in the side of
the gallery. The object of this is to keep the charge out of the
direct line of the gallery and thus increase the force of the
explosion. The charge may be placed in canvas bags, barrels or boxes,
precautions being taken against damp.

Charging mines.

The operation of loading is of the first importance, for if the mine
is not exploded with success, not only is valuable time lost, which
may give the enemy his opportunity, but it will probably be necessary
to untamp the mine in order to renew the fuze; an operation attended
by considerable danger. The loading of the mine should therefore be
done by the officer in charge with his own hands. He has to work in a
very cramped position and practically in the dark (unless with
electric light) as of course no naked lights can be allowed near
powder. Everything should therefore be prepared beforehand to
facilitate the loading of the mine and placing of the fuze. At Chatham
a 1000 lb. mine, at the end of a gallery 136 ft. long, has been loaded
in 30 minutes. The powder was passed up the gallery by hand in
sandbags, and emptied into a box of the required size.

Whatever method of firing (see below) is employed, the officer who
loads the mine must be careful to see that it is so arranged as to
make firing certain, and that the leads passing out of the gallery are
not liable to damage in the process of tamping.

_Tamping._--This operation consists in filling up the head of the
gallery solidly, for such a distance that there shall be no
possibility of the charge wasting its force along the gallery. The
distance depends on the charge and on the solidity of the tamping. For
a common mine it should extend to about 3/2 L.L.R. from the charge,
when the tamping is of earth in sandbags; for a 3-lined crater, to
about 2 L.L.R. Tamping can be improved by jamming pieces of timber
across the shaft or gallery among the other filling.

_Firing._--This may be done electrically, or by means of _safety_ or
_instantaneous fuze_ or _powder hose_.

Electric firing is the safest and best, and allows of the charge being
exploded at any given moment. For this purpose _electric fuzes_ (for
powder) or _electric detonators_ (for guncotton or other high
explosive) are employed. The current that fires them is passed through
copper wire leads.

The safety fuze used in the British service burns at the rate of about
3 ft. a minute. Instantaneous fuze burns at the rate of a mile a
minute. Both can be fired under water. They are often used in
conjunction, a considerable length of instantaneous fuze, leading from
the charge, being connected to a short length of safety fuze.

Powder hose, an old-time expedient, can be extemporized by making a
tube of strong linen, say 1 in. in diameter, and filling it with
powder. It burns at the rate of 10 to 20 ft. per second.

_Explosives._--The old-fashioned gunpowder of the grained black
variety is still the best for most kinds of military mines. Pebble and
prism powders do not give as good results, presumably because their
action is so slow that some of the gases of explosion can escape
through the pores of the earth. High explosives, with their quick
shattering and rending effect, are little more effective than
gunpowder in actually moving large quantities of earth. Most of them
give off much more poisonous fumes than gunpowder. Some recent high
explosives, however, have been specially designed to be comparatively
innocuous in this respect.

Effects of mines.

Some formulae have been given above for the calculation of charges. It will, however, simplify matters for the reader to record some actual instances of charges fired both in peace and war.

In the matter of scientific experiment we find Vauban as usual leading
the way, and the following results among others were obtained by him
at Tournay in 1686 and 1689: A charge of 162 lb. placed 13 ft. below
the surface produced a crater of 13 ft. radius (a two-lined crater, or
"common mine"). Galleries were destroyed at distances equal to the
L.L.R. in both horizontal and vertical directions. Double the charge,
placed at double the depth, i.e. 324 lb. with an L.L.R. of 27 ft. made
no crater, but like the first destroyed galleries below it and on each
side at distances equal to the L.L.R. A charge of 3828 lb. with L.L.R.
of 37 ft. made a two-lined crater and destroyed a gallery distant 61
ft. horizontally.

Bernard Forest de Belidor, a French engineer, made many experiments at
La Fere about 1732, and 20 years later, as a general officer and
inspector of miners, continued them on a larger scale. His experiments
were directed towards destroying an enemy's galleries at greater
distances than had hitherto been supposed possible, by means of very
large charges (in proportion to the L.L.R.) which he called "globes of
compression." In one of them a charge of 4320 lb. of powder placed
only 15 ft. 9 in. below the surface damaged or "compressed" a gallery
distant 65 ft. horizontally. The radius of the crater was 34 ft. 8 in.

At Frederick the Great's siege of Schweidnitz in 1762 some very large
charges were exploded. One of them, of 5400 lb. with an L.L.R. of 16
ft. 3 in., made a crater of 42 ft. 3 in. radius. Readers of Carlyle's
_Frederick the Great_ may recall his description of the contest of the
rival engineers on this occasion.

At Graudenz in 1862 (experiments) a charge of 1031 lb. of powder
placed 10 ft. deep, untamped, in a vertical shaft, made a crater of 15
ft. 6 in. radius. A charge of 412 lb. of guncotton, calculated as
being equivalent to the above charge of powder and placed under the
same conditions, made a crater of 14 ft. radius. The absence of
tamping in both cases of course placed the gunpowder at a
disadvantage.

The Petersburg Mine, 1864.

Perhaps the most interesting mine ever fired was that at the siege of
Petersburg in the American Civil War, in June 1864. The circumstances
were all abnormal, and the untechnical account of it in _Battles and
Leaders of the Civil War_ (vol. iv.) is well worth perusal. No mining
tools or materials and no military miners were available; and no one
had any confidence in the success of the attempt except its
originator, Lieut.-Colonel Pleasants, a mining engineer by profession,
his regiment which was recruited from a mining population, and General
Burnside the corps commander. The opposing entrenchments were 130 yds.
apart. The mine gallery was started behind the Federal lines and
driven a distance of 510 ft. till it came under a field redoubt in the
Confederate lines. There two branches were made right and left, each
about 38 ft. long, and in them eight mines aggregating 8000 lb. of
powder were placed. The first attempt to fire them failed, and an
officer and a sergeant volunteered to enter the gallery to seek the
cause of the failure. A defective splice in two lengths of fuze was
thus discovered and repaired. At the second attempt all the mines were
fired simultaneously with success, and made a gigantic crater 170 ft.
long by 60 ft. wide and 30 ft. deep. The occupants of the redoubt, at
least several hundred men (they have been stated at 1000), were blown
up and mostly killed. The assault which followed, however, failed
completely, for want of organization. The infantry was drawn up in
readiness to advance, but no outlets had been provided from the
parallel, and this and other causes delayed the occupation of the
crater and gave the defending artillery a moment's respite. Thus the
assailants gained the crater but could not advance beyond it in face
of the defenders' fire, nor could they establish themselves within it,
on its steep clay sides, for want of entrenching tools. A good many
troops were sent forwards in support, but being in many cases of
inferior quality, they could not be induced to go forward, and huddled
in disorder in the already overcrowded crater. Over 1000 of these were
captured when the Confederates retook the crater by a counter-attack
and the total loss of the Federals in the attack was nearly 4000.

The wars of the last generation have done little or nothing to advance the science of military mining, but a good deal has been done in peace to improve the means. Electric lighting and electric firing of mines will be a great help; modern drilling machines may be used to go through rock; ventilating arrangements are much improved; and the use of bored mines is sure to have great developments. The Russo-Japanese War taught nothing new in mine-warfare, or as to the effects of mines, but the siege of Port Arthur had this moral among others; just as in future, in the frontal attack of positions, trench must oppose trench, so in fortress warfare mines will be more necessary than ever. It appears that they will be essential to destroy both the ditch-flanking arrangements of forts and the escarp or other permanent obstacle beyond the ditch.

V. FIELD FORTIFICATION

_Field Fortifications_, now more often spoken of as field defences, are those which are constructed at short notice, with the means locally available, usually when the enemy is near at hand. Subject to the question of time, a very high degree of strength can be given to them, if the military situation makes it worth while to expend sufficient labour. A century or more ago, the dividing line between permanent and field fortification was very rigidly drawn, since in those days a high masonry escarp surmounted by a rampart was essential to a permanent fortress, and these could naturally not be extemporized. Works without masonry, in other ways made as strong as possible with deep ditches and heavy timbers,--such as would require about six weeks for their construction--were known as _semi-permanent_, and were used for the defence of places which acquired strategic importance in the course of a war, but were not immediately threatened. The term _field_ fortification was reserved for works constructed of lighter materials, with parapets and ditches of only moderate development. Redoubts of this class required a fortnight at most for their construction.

In modern fortification if cupolas and deep revetted ditches were essential to permanent defences, the dividing line would be equally clear. But as has been shown, this is not universally admitted, and where the resources exist, the use of our present means of construction, such as steel joists, railway rails, reinforced concrete and wire, in conjunction with the defensive power of modern firearms, makes it possible to extemporize in a very short time works having much of the resisting power of a permanent fortress. Further, such works can be expanded from the smallest beginnings; and, if the site is not too exposed, in the presence of the enemy.

Field fortification offers, as regards the actual constructions, a very limited scope to the engineer; and a little consideration will show that its defensive possibilities were not greatly affected by the change from machine-thrown projectiles to those fired by rude smooth-bore guns. There is therefore nothing in the history of this branch of the subject that is worth tracing, from the earliest ages to about the end of the 18th century. One or two points may be noticed. The use of obstacles is probably one of the earliest measures of defence. Long before missile weapons had acquired such an importance as to make it worth while to seek shelter from them, it would obviously have been found desirable to have some means of checking the onrush of an enemy physically or numerically superior. Hence the use by savage tribes, to this day, of pits, pointed stakes hidden in the grass, entanglements and similar obstacles. In this direction the ages have made no change, and the most highly civilized nations still use the same obstacles on occasion.

Another use of field defences common to all ages is the protection of camps at night, where small forces are operating against an enemy more numerous but inferior in arms and discipline. In daylight such an enemy is not feared, but at night his numbers might be dangerous. Hence the Roman practice of making each foot-soldier carry a couple of stakes for palisades; and the simple defence of a thorn zariba used by the British for their camps in the Sudan.

Palisades and trenches, abatis and sharpened stakes have always been used. Except wire, there is practically no new material. As to methods, the laagers of the Boers are preceded by the wagon-forts of the Hussites, and those no doubt by similar arrangements of British or Assyrian war chariots; and so in almost every direction it will be found that the expedient of to-day has had its forerunners in those of the countless yesterdays. The only really marked change in the arrangements of field defences has been caused not by gunpowder but by quick-firing rifled weapons. For that reason it is worth while to consider briefly what were the principles of field fortification at the end of the 18th century. That period has been chosen because it gives us the result of a couple of centuries of constant fighting between disciplined troops with fairly effective firearms. The field defences of the 19th century are transitional in character. Based mainly on the old methods, they show only faint attempts at adaptation to new conditions, and it was not till quite the end of the century that the methods now accepted began to take shape.

The essential elements of fieldworks up to the time of the Peninsular War were _command_ and _obstacle_; now they are _protection_ and _concealment_.

Old type of field defences.