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) l3.
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 Shafts and galleries. 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.