WATER-SUPPLY.
116. The problem of obtaining a sufficient and wholesome water-supply for a besieging army is usually one difficult of solution. The precautions which are necessary in ordinary camps (Art. of War, 352 and 358) become of still greater importance in this case, owing to the choice of the source of supply being limited to those which are not controlled by the besieged, and to the constantly increasing danger of pollution of all ground waters by the bodies of the dead men and animals and the refuse and filth of the camps. The evils arising from these sources may be largely or entirely removed by boiling the drinking-water, and the disagreeable tastes and smells may be removed by filtering through good filters. It is very difficult, however, to compel the men to boil the water, or to drink it after it is boiled, unless it is properly aerated and filtered. All available measures should, therefore, be adopted to supply them with wholesome water.
The results of the most recent researches show that properly conducted intermittent filtration with sand-filters will render a polluted water almost if not entirely safe. (See reports of Massachusetts State Board of Health on Purification of Sewage and Water, 1890.) And the analysis of water sterilized by a steam-jet at the Columbian Exposition in Chicago, 1893, gives reason to believe that this process may be very effective in removing disease germs. (See report of Allen Hazen, Chemist, Department of Water-supply, published in Engineering News, March 29, 1894.) In camps of some permanence one or both of these processes may be well worth applying.
Small filters for limited amounts of water may be bought in the market, or may be improvised and set up for officers or company messes. Figs. 108-110, [Pl. X], are given as suggestions; they serve as strainers in any case. If used intermittently they may have a high sanitary value, and if made up partly of either animal or wood charcoal they remove more or less completely any offensive taste or odor which water may have. Security, however, requires doubtful water to be boiled.
In all cases arrangements should be made to protect the water from surface pollution, for convenient access for the men, and for watering horses.
(See the following books, treating on Military Hygiene in Camp and Garrison: Parker’s Practical Hygiene; Traité d’Hygiène Militaire, Morache; Manuel d’Hygiène militaire, Viry; Military Hygiene, Woodhull; the Soldier’s Pocket-book, Wolseley; etc., etc.)
Part II.
MILITARY MINING, BLASTING, AND DEMOLITION.
CHAPTER I.
NOMENCLATURE AND THEORY.
1. Military Mining includes all the operations necessary for placing charges of explosive underground and exploding them at the time desired, for the purpose of destroying the men, materials, or works in their vicinity, or for breaking up the surface of the ground either to advance or retard the operations of a siege.
The excavation for receiving the charge is called the chamber. The approaches leading to the chamber when horizontal or somewhat inclined are called galleries, and when vertical are known as shafts. When very steep they are sometimes called slopes. The charge, chamber, and approaches taken together constitute a mine.
The pit formed by the explosion is called the crater.
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.
For convenience in computation, however, several simple geometrical figures have been assumed as giving with sufficient accuracy the form of the crater of a common mine. See [Pl. XI], Fig. 1. Among these Vauban assumed a cone, ACD, with its vertex at the centre of the charge; Valière a paraboloid of revolution, AHD, with its focus at the centre of the charge; Müller truncated this paraboloid by a horizontal plane through its focus; while Gumpertz and Lebrun adopted the form in common use at their time, and which has been generally accepted since, viz., a frustum of a cone, AEFD, the smaller base of which passes through the centre of the charge and has a radius, EC, equal to one-half the crater radius, AB (or one-half L. L. R., CB).
The volumes of these figures are as follows:
| Vauban’s cone | 1.05 (L. L. R.)3, |
| Valière’s paraboloid | 1.90 (L. L. R.)3, |
| Müller’s truncated paraboloid | 1.84 (L. L. R.)3, |
| The frustum of a cone | 1.83 (L. L. R.)3 = nearly (11/6)(L. L R.)3. |
The cone of Vauban (lately assumed also by Höfer) was abandoned as unsatisfactory, because it did not conform to the craters produced, and, as treated by Höfer, because the charges computed by its use were found to be too small (an error in the wrong direction). The paraboloid of Valière or Müller would seem to conform more nearly to the actual shape assumed by the crater; but it will be observed that the volume of the latter is sensibly the same as that of the truncated cone, and as the volume of earth thrown out is the quantity to be considered, the truncated cone will be assumed as the measure for it.
4. The principle that “the volumes of the craters are proportional to the charges used” is the general statement of the miner’s rule. Assume C and C´ to represent the charges of two mines whose volumes are V and V´, lines of least resistance l and l´, and crater radii r and r´. Assume also that the craters are frustums of cones, the radii of whose larger bases are twice those of the smaller. Then
| C : C´ :: V : V´ :: (11/6)(lr2) : (11/6)(l´r´2), |
or
| C´ = C(V´/V) = C[(11/6)(l´r´2)/(11/6)(lr2)] = C[(l´r´2)/(lr2)] (1) |
Equation (1) is applicable to mines in which r does not differ materially from l or r´ from l´.
From an experimental mine giving a crater of this general type the relations between C, l, and r may be determined, and assuming any two of the quantities C´, l´, and r´ for a mine with a crater nearly similar in form, the other may be found from eq. (1).
When l = r and l´ = r´, we have
| C : C´ :: (11/6)l : (11/6)l´3, |
and
| C´ = C[(11/6)(l´3)/(11/6)(l3)] = C[(l´3)/(l3)] (2) |
Equation (2) is applicable to common mines, and shows that in common mines the charge varies as the cube of the line of least resistance.
Assuming C´ as the charge which will produce a crater with a volume of unity, equations (1) and (2) become, by omitting the primes from l and r,
| C = C´(11/6)lr2, (3) |
and
| C = C´(11/6)l3 (4) |
Equation (4) gives the rule for determining the charge for common mines whose L. L. R. is given, viz.: Multiply 11/6, the cube of the line of least resistance in yards, by the quantity of explosive required to throw out one cubic yard.
The latter quantity is determined by experiment. A similar rule may be written out from eq. (3) for mines differing but little from common mines.
5. The quantity of gunpowder required to throw out a cubic yard of material has been calculated from a great number of mines fired in different kinds of soil. The following table gives the quantities required according to Lebrun and Macaulay, respectively the French and English authorities on the subject:[8]
| Number. | Description of Earth, Rock, or Masonry. | Weight per cubic foot. | Charge, Gumpertz and Lebrun.v | Charge, Macaulay. | Proportional value of charge. |
|---|---|---|---|---|---|
| lbs. | lb. oz. | lb. oz. | |||
| 1 | Light sandy earth (common earth, Lebrun) | 85 | 1.8 | 1.13 | 1.12 |
| 2 | Hard sand | 111 | 1.10¾ | 2.0 | 1.25 |
| 3 | Fat earth mixed with sand and gravel (common earth, Macaulay) | 116 | 1.5⅓ | 1.10 | 1.00 |
| 4 | Wet sand | 118 | 1.12 | 2.2 | 1.30 |
| 5 | Earth mixed with stones | 118 | 1.14 | 2.4 | 1.40 |
| 6 | Clay mixed with tufa | 124 | 2.1 | 2.8 | 1.55 |
| 7 | Fat earth mixed with pebbles | 143 | 2.4 | 2.12 | 1.69 |
| 8 | Rock | 143 | 3.0 | 3.10 | 2.25 |
| 9 | New or old moist brickwork or masonry | 2.2 | 1.30 | ||
| 10 | Inferior brickwork or masonry | 2.11 | 1.66 | ||
| 11 | Good, new ditto | 3.10 | 2.25 | ||
| 12 | Good, old ditto | 4.1 | 2.50 | ||
| 13 | Roman ditto, or other equally good in warm climates | 4.11 | 2.90 |
6. For common mines in ordinary earth a convenient rule, very generally used, and which gives results nearly the same as those deduced from the table, is:
The charge of gunpowder in pounds is equal to one tenth the cube of the line ne of least resistance in feet, or
| C lbs. = (1/10)l3 ft. (5) |