When the ground is homogeneous and its surface horizontal, the intersection of its surface by the crater is approximately a circle, the radius of which is called the crater radius, AB, [Pl. XI], Fig. 1.
The right line joining the centre of the charge with the nearest point of the surface toward which the explosion will take place, generally the surface of the ground, is called the line of least resistance (written generally L. L. R.), C B, [Pl. XI], Fig. 1.
A right line from the centre of the charge to the edge of the crater is called the radius of explosion, C D, [Pl. XI], Fig. 1.
The distance from the centre of the charge at which an ordinary mining gallery will be broken in by the explosion is called the radius of rupture, C L, [Pl. XI], Fig. 3. The radius of rupture varies in length with its inclination to the horizontal.
Craters whose diameters are once, twice, etc., their lines of least resistance are called one-lined, two-lined, etc., craters.
Mines in which the L. L. R. is equal to the crater radius are called common mines. (Their craters are two-lined.) Those in which the crater radius exceeds the L. L. R. are called overcharged mines or globes of compression; when it is less, they are undercharged mines; and when the charge is so small that no exterior crater is formed, they are known as camouflets.
2. In the explosion of military mines on land it may safely be assumed that the circumstances of combustion of the charge when fired are such that the energy developed is directly proportional to the charge. A portion of this energy is generally lost by the escape of the compressed gases into the air, by the heat given up to the surrounding media, and by the transmission of earth-waves or shock; the remainder and greater part, however, is expended in rupturing the case containing the charge, compressing the soil in its immediate vicinity, separating that lifted up from that forming the sides of the crater, breaking up the portion thrown out into large or small fragments, projecting them to a greater or less distance, and disintegrating the soil around the crater to a distance which varies with the soil and with the quantity and character of the explosive used.
As the proportional part of the energy expended in each of the effects above named cannot be determined in any particular case, and as each case differs in some respect from every other, it is manifestly impossible to express in any mathematical formula a rule for determining the exact amount of explosive required for any particular mine.
From the results of long experience, however, engineers have concluded that computations sufficiently exact for practical purposes can be made upon the hypothesis that for common mines and those approximating closely to them in form, the volumes of the craters are directly proportional to the charges used.
3. In order to apply this rule in practice the volumes of craters formed by known charges must be measured; but since the soil in the immediate vicinity of the crater is more or less disintegrated, and the crater itself is partly filled up by the material which falls back into it, the outlines of the original crater cannot usually be recognized or its exact geometrical figure be determined. Besides, the craters formed under circumstances seemingly identical differ more or less among themselves.