Concealment and The trenches and other works should be made as nearly invisible as possible, and their conceal ment should be completed by treating the front slope of the parapet so that its appearance from the front and from aircraft will corre spond to that of the surrounding ground.
Crops should he replaced where disturbed, and carefully arranged; bushes should be planted to hide the parapet, the men on watch, riflemen and observers. Sod should be preserved and placed on the parapet and parados. Dummy trenches and works should be freely employed so as to deceive the enemy as to the strength of the defender and to scatter his fire. The observation of the enemy is of first importance in position warfare. It should give complete knowledge of all elements of the hostile line and prompt information of any movement of the enemy. It is effected by observation from the ground and from aircraft. Observation from the ground is divided into three echelons: (a) In front of the firing line from small posts and listening posts. (b) On the firing line by sentinels and lookouts. (c) In rear of the fir ing line, by artillery observers, sentinels and lookouts of the shelters. Observations on the firing line are effected by lookout posts organ ized preferably at the salients where the view is more extended. They are provided with periscopes, range finders and large scale maps. The location of the lookout posts and observa tories must be determined in accordance with a complete plan for each supporting point or sector of defense. In position warfare, careful and studied attention must be given to illumina tion on the battle field, lines of information, depots for material and ammunition, the em ployment of machine guns, trench weapons, command posts, first-aid stations, kitchens, latrines, lavatories, shower baths, water supply, maintenance, protection against asphyxiating gas and protection against bombs and grenades. When circumstances render night work neces sary, special dispositions must be made.
Mining.— Underground communications are classed according to their directions as gal leries, which are horizontal or nearly so, and shafts, which are vertical or nearly so. Gal leries are classified according to their size as great or grand galleries, which are six feet high by seven feet wide; common galleries six feet by three and one-half ; half galleries, four and one-half feet by three feet; branches, three and one-half feet by two and one-half feet; and small branches, two and one-half feet by two. Shafts may be drill holes or wells, or may range in size from the smallest in which a man can work, say three feet by three feet, to any size which may be required, seldom more than 6 feet by 10 feet. If troops or guns are
to be passed through galleries, they must be made large enough for that purpose. Grand and common galleries usually meet these re quirements. The primary requisites of sub terraneous excavations are accuracy of direc tion, prevention of caving, ventilation, drainage and lighting. Picks and shovels for mining are similar in form to standard tools, but are smaller and have shorter handles. A special tool called the push pick is very convenient in soft earth. In large galleries a transit may be used, and in smaller ones a plane table or pris matic compass. The box compass cannot be sighted and read with sufficient accuracy for this work.
The transfer of the azimuth underground depends on whether the gallery starts from a shaft from a reverse slope, or, if not very deep, from a level with a descending branch. In the second and third cases, which will be the rule in military mining, the azimuth may be estab lished in the gallery by a transit or compass used in the ordinary way. In the case of a shaft, which will be the exception, the azimuth must be established across the top or mouth and transferred to the bottom by means of plumb lines. Prevention of caving is accom plished by linings. In very firm soil it is some times practicable to drive small shafts and galleries short distances without lining them. Galleries constructed during a siege are lined with wood. Wooden linings are of two general types, known as cases, and frames and sheeting.
In sinking shafts especial care must be taken to make the excavation no larger than is required for placing the lining. Partly lined shafts should only be used for small depths and when they are expected to stand for a very short time. They are a constant menace to the miners, owing to the danger of their caving in, and in a much greater degree to the probability of stones, etc., falling from the unprotected parts and seriously injuring or killing the men at the bottom. In changing direction horizon tally with frames and sheeting, if the soil will stand for a distance of one frame interval, or even less, it is only necessary to place one or more frames at an angle until the necessary change is secured. The sheeting on the outside is placed by running the forward end past the frame and then inserting the rear end behind the last bay of sheeting. When the earth shows a tendency to cave, which it will fre quently do in great galleries, the cap sill must be put in position and supported while the miner excavates the grooves for the ground sill and stanchions, for which purpose two crutches are used.