s For a common mine c = -- l³ 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
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 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 3½ 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.