Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/450

 436 FORTIFICATION bag loopholes are the best, as the whole loophole is not necessarily destroyed by the blow of a projectile, and they are easily moved from place to place ; two sand-bags are placed side by side with an interval between them, and a third covers this interval. Logs of wood are disposed similarly; brushwood or board loopholes are embedded in the parapets. If a town is situated near a stream or river, by which part of it may be covered by inundations, the resources this affords should never be neglected. Villages are intrenched on similar principles, and being generally surrounded by gardens with live hedges, the latter may be made use of in the lines of defence. If there should only be sufficient troops to defend part of a village or town, a part of it only should be intrenched, and should be separated from the rest by means of carts and barricades. If there are very few houses, it may be necessary to confine FIG. 50. Illustrating Defence of Towns or Villages. the defence to the church or churchyard. Villages are often in one long street ; then only the rear portion is in trenched, clearing in front of this portion a space sufficient for the effective action of musketry. The Destruction of Bridges. Nothing is of greater con sequence to a retreating army than to destroy the bridges in its rear, in order to retard the advance of the enemy. Its existence may depend upon the success with which this operation is performed. To destroy a stone bridge with gunpowder a trench in the form of a cross is made in the crown of the arch, the branches of which are about 10 feet in length, and sunk to the top of the arch-stones. For an arch 3 feet thick, 1GO K) of powder are placed in each trench ; strong planks are then laid over the powder, and covered with rubbish. The fire is communicated by means of powder-hose or Bickford s fuze. Stone bridges are de stroyed also by simply cutting a trench about 1 8 inches deep across the crown of the arch, and placing in it 345 Bb of powder covered. in the manner just described. This quantity has been found sufficient to destroy semicircular arches of 25 feet span, and of 3 feet in thickness at the key. Wooden bridges may be destroyed in various ways ; they may be pulled to pieces, burned, or blown up. When there is time to take them to pieces, they are unspiked, and the timbers so separated that they may be speedily removed. The best method of burning such bridges is to tar them, and then to cover and surround them with fascines or tarred brushwood. Wooden bridges may be blown up by large charges of powder suspended under the superstructure, and fired in the manner above described. But these and other hasty demolitions may be more quickly and more certainly effected by the use of Compressed Gun-cotton, which is, weight for weight, four times as strong as gunpowder, and requires no tamping. Compressed gun-cotton is stored damp, and in this state it is perfectly safe for transport and handling, as it can only be detonated by the agency of a small quantity of dry gun-cotton, called a primer, and a de tonator. The detonator is fired in the ordinary way or by electricity. Compressed gun-cotton is made in many con venient forms and sizes, such as 1 oz. and oz. discs, 1 ft) or 2 Ib slabs, pellets, and grains. Generally it should be evenly distributed over the surface of stone bridges if heaped, it forces an opening clean through, Supports of timber bridges may be destroyed by a girdle of half-ounce discs tied round them ; or, if time avails, by charges placed in auger holes bored in them ;  oz. of gun-cotton is sufficient for each square inch of the sectional area of the support. Iron bridges are destroyed by attacking the bottoms of the piers, or, in the case of girder bridges, the junction of 2 girders ; 2 ft&amp;gt; of compressed gun-cotton per foot lineal laid against an 18-inch brick wall will de stroy it. A ford is rendered impassable by throwing large stones into it, by sinking boards with spikes standing upright in them, or by placing crows feet or harrows in it. A low rubble wall may be formed across it, so as not to be per ceptible above^the water; strong stakes may be driven into the bottom, and trees fastened to them ; waggons loaded with stones, with the wheels removed, may also be employed ; and a number of other things which may often be found at hand will answer the purpose equally well. Rendering a ford impassable is only second in importance to the destruction of a bridge, when the enemy, whose progress it is desired to retard, either has no pontoon-train, or has outstripped it by the rapidity of his advance. By such expedients to retard the pursuit much valuable time may be gained by the retreating army. The essential characteristic of all intrenchments which are formed of earth is that the musketry fire, on which the defence must mainly depend, being discharged over the crest of the parapet, the line of fire, will be nearly in the plane of the superior slope of the parapet, and perpendicular to the line of its crest ; and hence opposite a salient angle, as in the Redan (fig. 51), there will be a large space of ground FIG. 51. Redan. in this case extending over 180 - 60 = 120, undefended by the fire of the work itself ; this space is called a &quot; dead angle.&quot; Opposite re-entering angles, on the other hand, the defect is of a different kind, as the plane of the supe rior slope or plane of fire passing high above the foot of the escarp necessarily leaves it unseen and unprotected, not withstanding that the two lines theoretically flank each