When the parallels and approaches have to be constructed upon ground sloping downward towards the work, in soil containing large stones, or in which the rock is close to the surface, in marshy ground or that containing much water or liable to be flooded, the difficulty of their construction and defilade are evident. The point of attack considered most favorable to the besieger is one which, fulfilling the first essential condition, is more or less salient, so that it can be partially surrounded, and which admits of the approaches being driven toward it in favorable soil, over ground sloping gently from the work, or gently rolling with the crests and valleys of sufficient difference of level to afford cover, and running generally in the direction of the parallels.
A favorable location for parks, etc., with free, safe, and short communications between them, also has great weight in selecting the point of attack.
SECOND PERIOD.
76. First Artillery Position.—Every siege begins with a bombardment, which is designed, as previously stated, to drive in the outlying posts of the defence, to silence, so far as possible, the artillery annoy and wear out the garrisons of the works to be attacked, to interrupt the communications between them, break up bomb and splinter proofs, destroy magazines and depots, and, if the enceinte can be reached by the artillery, to bring a fire upon the population which will lead to or hasten the surrender of the place.
The considerations which determine the location of the batteries for the general bombardment have already been given (par. 7), as well as the construction of the batteries and screens used (Chap. IV.). For the systematic attack, however, the necessity of dismounting or silencing the guns bearing upon the proposed approaches introduces the additional condition that the batteries should be so located that besides their general effect each shall fulfil, so far as practicable, its special design by bringing an enfilading or reverse fire upon certain fronts; or, in connection with other batteries, shall keep down the fire of certain fronts by a preponderance of direct fire. Many batteries which fulfil these last conditions occupy their original positions during the entire siege. The requisite concentration of fire upon the point of attack and its careful regulation for the special object in view will frequently restrict the arc occupied by the batteries below that desirable for a general bombardment only; and will necessitate a closer grouping of the batteries for their easier control by the artillery commanders. This line of batteries first established is known as the “first artillery position” ([Pl. VIII], Figs. 80, 81). As the batteries must be secure against the attacks of the defence, they must of necessity be outside the besiegers defensive line. Their distance in yards will result from the character of the defence and may vary from 2000 or 2500 yards for a weak defence, to 3500 or 4500 yards for an active one. On account of their long range and the object to be obtained by their fire, they are armed with the heaviest rifles and howitzers available, supplemented with rifled mortars of as large calibre as can be obtained, firing, if practicable, torpedo shells charged with high explosives.
Batteries of field guns which have already been favorably located for the defence of the heavier batteries against attack, or for firing upon the more advanced works, may, by modification of their gun pits into finished batteries (par. 44), be used in conjunction with the heavy batteries of the first artillery position.
The total number of guns employed should be such as to give to the attack a marked superiority over the defence at the opening of the bombardment.
77. Opening Fire.—The batteries having been completed and armed, the magazines finished and supplied, and the parks, depots and communications put in such order that the batteries can be kept fully supplied with ammunition; the fire of the batteries is commenced simultaneously, the signal being given by a gun from some selected battery. The fire once opened is continued day and night during the siege, unless stopped by the commanding officer or from inability to keep it up. It usually begins at daylight, in order to enable the ranges to be corrected by the first shots, before the defence has accurately located the batteries unmasked during the preceding night. To open fire from a few batteries before the others are ready is inexcusable, as it enables the defence to concentrate its fire upon them and destroy them in succession. The targets of each battery and gun and the rate of fire are prescribed before the fire is opened, and these are changed only by subsequent orders or from sudden emergencies. The fire is as a rule deliberate, seldom exceeding an average of 4 shots per hour for each gun by day, and 2 per hour at night. This rate may be increased or diminished by the commanding officer for special reasons and for a limited time.
The fire of the batteries is directed upon all the works of the place within range, but with greater vigor upon the more important, and especially upon those near the point of attack. The fire against powder-magazines and storehouses should be uninterrupted, to prevent the removal of powder and munitions. If the artillery of a part of the work is silenced, the fire upon it may be slackened, but some fire, especially vertical, should be kept up.
At night the fire is directed against the larger targets, such as communications and covers, rather than upon the guns; but the fire against the interior of the place (especially a city) is kept at about the same rate day and night.