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. THIRD PERIOD. =90.= =The Third Period of the Siege= frequently called the “_close attack_,” includes all the steps between establishing the last parallel and the surrender of the place. These are the capture and crowning of the covered way, breaching the scarps and counter-scarps, passing the ditch, capturing and crowning the breaches of the outworks and main works in succession, and the final reduction of the interior retrenchments, or keep. All these operations are carried on within close and deadly range of small arms and shells of Coehorn mortars, and many of them within range of hand grenades and upon ground honeycombed with mines and countermines, or liable to be flooded or inundated. They are slow in progress, uncertain in results, and require an extravagant expenditure of men and material. They can be pushed to a successful issue only when the artillery fire of the place is silenced and its small-arm fire is almost completely kept down by the fire of the attack. The conditions of modern warfare are such, however, that by the time the attack has reached the foot of the glacis the losses and exhaustion of the garrison are frequently so great as to preclude an obstinate, close defence; and, in the majority of cases, the place is compelled to surrender before the close attack is commenced.