In some cases, a third parallel is not constructed, but the attack proceeds immediately from the second parallel to the covered way ; while, on the contrary, in many cases, not only a third, but also a fourth parallel is required, in order to cover the progress of the trenches, or the sappc, with a larger front, and to be always in readiness to support it with a sufficient force. The third, as well as the fourth parallel, are generally constructed like the approaches be tween both ; the former by the flying sappe, and the latter by the ordinary or double sappe, as described above. If a third and a fourth parallel are constructed, it is usual to fix there the dismounting-batteries ; but if the third paral lel is at the foot of the glacis of the covered way, part of them is before, and part behind.
Should it be found useful to remove the ricochetting batteries nearer to the fortress, they are placed on crotchets, or half parallels. If a fortress has lunettes, no fourth pa rallel will be wanting, but a lodgment will be necessary, or a place where the soldiers may be covered on them by a parapet. When it is necessary, however, that the sappers should be better supported, and when only a few works of the fortress al e dismounted, a fourth parallel, or a place of arms along the saliant angles of the covered way, will be required. Dismounting-batteries, too, may sometimes be too distant to reach certain parts of the fortress, and there fore the fourth parallel may be necessary to support them ; but, wherever it is possible, crotchets before the third paral lel are preferred, and the fourth parallel is omitted.
A common covered way may either be taken by an as sault, or regularly attacked by the sappe. From the third or fourth parallel, the sappe, and generally the double sappe with traverses, is conducted to about 25 to 40 yards from the covered way, and thence to the right and left pa rallel, in such a direction as completely to enfilade all the covered way. Here the cavalliers of the trenches are rais ed to a height of about 12 to 18 feet, with a traverse at the end, to prevent it from being enfiladed from the covered way.
This cavallier is generally raised by gabions, of which either four or five lines are in the lowest row, one less in the second, one less in the third, and only one in the fourth or uppermost row. One row after the other is filled with earth; but a second row is never to be begun till the first is completed. Sometimes a more simple kind of cavallier of the trench is used, having only one row of gabions, each above the other. To mount these high parapets, stairs are raised behind them by means of fascines. The attack being generally directed against the saliant angles of the polygon or front, and in most cases the heads of the sappe being equal in number to the angles of a front, viz. the two bastions and the demi-lune, it will be necessary to conduct these three attacks by the sappe towards the covered way, all on the capital lines of the saliant angles. As soon as the at
tack has arrived at the saliant angles, the cavalliers of the trench are to be constructed, to the number of at least three pair. As soon as these are finished, they are filled with soldiers, who keep up a constant fire along the covered way, so as to prevent any of the enemy from remaining there with safety.
In the meantime, the sappers are continued towards the palisadoes of the covered way, till they are not farther dis tant than from six to nine yards, when they are carried to the right and left, parallel to the covered way, till they meet in the re-entering angle before the places of arms. If the enemy be still in force in the places of arms, he is to be driven out by stone mortars and shells. But if this fire should not induce him to quit the places of arms, and if they are very strong, and not to be taken without a breach or an opening made in their rampart, then breaching bat teries must be raised against them. These are made like -the dismounting batteries, but more covered from the ene my's fire either by traverses or higher parapets.
If the place of arms can be attacked by surprise, this may be done as soon as the sappe along the palisadocs of the covered way, or lodgement on the crest of the glacis, is finished, and as soon as it is ascertained that none of the ene my are hidden behind the traverses in the covered way. The place of arms being taken, it is usual to construct a lodge ment there, or a place where the soldiers may be covered from the enemy's fire, and can oppose to him a front along all the works. Another mode of attacking the covered way is by assault. This, however, cannot be undertaken till all the guns in the fortress are silenced, and the enemy's works there can be assaulted without making a breach. But as the assault is generally attended with a considerable loss of men, a vigorous attack on the covered way is mostly pre ferred. However, when it is determined to bring the siege to a conclusion in a short time, or when the works of the enemy are very weak, and when the chance of success is probable, the assault is to be. undertaken. This attack is usually made by volunteers from different regiments, and their way is cleared by pioneers or carpenters, who hew down the palisadoes, and remove all other obstacles. When such an attack is resolved on, it is to be made at day-break : the palisadoes arc cut clown, the soldiers enter the covered way, put every one they meet to the sword, and proceed di rectly to the place of arms, which must also be taken. If this can be done by surprise, it is always more advantageous. The lodgement on the place of arms is made as soon as possible, and covered with fascines, wool, or sand-bags. Wool-sacks certainly are of the greatest advantage here, as they may be easily conveyed from one place to another, and form a good parapet for the fire of small arms.