When the defence combines a number of batteries to silence one of the attack a heavy fire is concentrated upon these batteries by those from which the fire has been diverted. New batteries unmasked by the defence, or established in intermediate or other works, should receive prompt attention from the attack, with a view to silencing them if possible before they correct their ranges. It is of the first importance that the superiority of the artillery fire of the attack shall be established at the opening of fire from the first artillery position and be maintained throughout, and that the defence shall be prevented from repairing any batteries which have been silenced. To this end a few guns will keep up a slow fire upon these batteries so long as it may be necessary.
Every gun of the defence must, if possible, be kept under a heavy fire, and the fire upon the enceinte must be opened at the earliest possible date and continued day and night, as previously described.
87. Musketry fire will be opened as soon as a parallel is established at such distance as to make it effective; and this may be, for a well-regulated fire of sharpshooters, at ranges of 1200 to 1500 yards, or in some cases even greater.
88. The Advance from the First Parallel.—It is assumed that the fire from the first and second artillery positions will silence almost completely the artillery fire of the work upon the fronts attacked; but the defence will still be able to develop when necessary a strong musketry fire, aided at times by machine and rapid-fire or even some field guns. Consequently, the advance from the parallel must be under cover.
Approaches are, therefore, broken out from the parallel and pushed forward towards the work, the workmen being protected by the fire of the guards of the trenches. Usually at least three lines of approaches are constructed, concentrating upon the point of attack and following generally the lines of the capitals of the adjacent salients.
When attacking a line of detached works two or more lines of approaches may be constructed towards each work attacked. The approaches are run in zigzags, each branch so directed as to pass a short distance (30 or 40 yards) outside the most advanced work of the defence from which it could be enfiladed; at each change of direction of the zigzags a return of 10 or 20 yards is made to cover the approach in rear ([Pl. VIII], Figs. 80 and 81). The length of the branches is so regulated as not to mask too much of the front of the parallel; they consequently grow shorter as they approach the work and vary ordinarily between 200 and 50 yards, seldom exceeding 100 yards when near the work. The heads of the different approaches are advanced at about equal speed so as to afford mutual support.
89. The Second Parallel.—The second parallel is located nearer to the first parallel than to the covered way, sometimes very much nearer. It is constructed and occupied by the guard of the trenches. The principle followed is that the guards of the trenches shall always be nearer to the head of the sap than is the enemy in his most advanced place of arms; so that, in case of a sortie, the advantage will lie with the besieger. The flanks of the second parallel are refused and strengthened like those of the first, or are even carried back to the first parallel, to guard them against flank attacks.
The second parallel having been completed and occupied, serves as a base for further advance, which is conducted according to the same methods, “demi-parallels” ([Pl. VIII], Fig. 81) being run out to the right and left of the approaches when they are well advanced beyond the second parallel. These demi-parallels are sometimes joined, forming a third parallel, from which the approaches are advanced as before, with additional parallels when needed, until the foot of the glacis or exterior of the counter-mines is reached. The number of parallels is determined by the distance at which the first is established and the vigor of the defence; formerly three were considered all that were needed, and this number was used at Strasburg, 1870. At other modern sieges a larger number has frequently been required. At Belfort (1870-71) the third parallel was established at 1200 yards from the place. Five parallels were used at the siege of Fort Wagner (July-September, 1863).
The approaches are driven in zigzags by simple trench, flying or full sap, until the direct advance becomes equal to about one third of the length of trench; and from this point they are driven directly upon the work by double-traversed sap ([Pl. III], Figs. 28-35), the latter being, as a rule, used only in advancing from the foot of the glacis, or during the third period of the siege.