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.