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.
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 Different kinds of mines. 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.
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| 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.
For a common mine c = (s/10) l 3.
For an overcharged mine c = (s/10) {l + .9 (r − l)}3.
For an undercharged mine c = (s/10) {l − .9 (l − r)}3.
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.
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. × 2 ft., and the branch gallery which is 4 ft. × 2 ft. The shaft has about the same dimensions as a branch gallery. Formerly it was Shafts and galleries.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 312 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
