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.