The most advantageous time 'for attacks is midnight, or towards morning, when the troops in the trenches are still fatigued by their labours during the night.
But no sortie should be made with two large a force, lest the enemy meanwhile make a successful attack on another part, while the largest force has been drawn farther away from the fortress. In general, the number of men employ ed in a sortie should never exceed one half of the garrison. Small garrisons should never make a sortie with more than one-fourth of their number, unless it be to open a commu nication for retreating with all the garrison. The retreat from a sortie is always to be covered by some infantry and cavalry, but particularly by riflemen in the covered way, and by ordnance mounted on the ramparts.
The defence of a common covered way can only be made by soldiers being placed there, and covered by the tra verses. They keep up a constant fire towards the enemy's works, particularly during the night, and towards the head of the sappe, in order to prevent him effecting his lodge ment along the covered way. If there is reason to suspect that a forced attack will be made on the covered way, it will be advantageous to place a greater number of soldiers to defend it, as also to Make small mines under the tra verses, and even under the glacis, where the enemy has to construct his lodgements. The hearths of these mines should be close to the work in the re-entering angle. If the enemy has driven the garrison out of the covered way, and it has retreated from one traverse to the other, and to the places of• arms, the mines are sprung gradually, and also those under the glacis. But if the enemy has entered in a disorderly manner, he is to be attacked, and if possible repulsed.
The soldiers in the places of arms, as well as some small guns placed there; will always most beneficially direct their fire upon the enemy, and be particularly useful in taking him in flank, or in fit ing at him when advancing or retreat ing. Should the covered way be attacked by the sappe, the construction of the cavaliers of the trench is particu larly obstructed by a constant fire from the covered way.
A polygon, where there are counter mines, may always be defended longer than where there are none. If the ene
my should not begin to mine, the garrison must endeavour to spring theirs under some of his works, particularly his batteries. This will retard not only his advancing from the third parallel, but also force him to commence mining. The most favourable moment for making an attack on the enemy, is after a mine has been sprung, and therefore a sortie is then to be hazarded.
I f the enemy has begun to mine and to dig a well, it will be necessary to discover the place, and immediately spring a mine under it. This is repeated as often as he can be dis covered.
The near approach of the enemy's mine may sometimes be detected by placing a drum on the ground, and scatter ing a few peas on its top. If the miner be very near, his digging will communicate a tremulous motion to the drum, and of course to the peas. When this takes place, the countermine is immediately to be sprung. If the enemy's well is discovered before the countermine is ready, the former may be filled with the poisonous smoke of fire-balls. The miners ought always to be armed with large pistols, to defend themselves, should they fall in with those of the enemy. In this case subterraneous fortifications will also be necessary.
If there be any small works in the saliant angle of the covered way, they will keep the enemy at a certain dis tance as long as they can be occupied, and therefore this is done till he is ready to assault it, and even then they are not to be left, unless it be found impossible to defend them.
An avant-fosse that is full of water, is frequently of great use, as it prevents the enemy from passing it, or construct ing a bridge, as long as there are guns to enfilade it.
A second covert way, particularly if it has a strong work in its re-entering angle, is also useful, and keeps the enemy some time longer ; but ih defending it, care is to be taken that it is not surprised, when the covert way before it is taken. In order to deceive the besieger as much as pos sible as to the number of ordnance in a fortress, the fire should be directed principally at the first parallel, at the batteries, and at the head of the sappe : in all others it may be slackened, which will induce the enemy to believe that many of the guns are dismounted.