The attach- in form, or the regular attack of a fortress, begins, as in the last case, with investing, or inclosing it with troops. This investment is particularly necessary to cut off the communication between a fortress and its army. Sonic towns, however, cannot be invested completely, as, for instance, when a fortress is situated on the sea, and pos sesses a greater naval force than the besiegers. In this ease, such a place can always receive sufficient reinforcements, and this alone will very much retard the siege Those for tresses are also with difficulty invested that are situated on the bank of a large river, the conflux of two rivers, in the middle of woods and hollow ways, at a certain distance front them, or in general on ground which in any way prevents the besieging army from forming a connected circle. It is also very difficult to invest a fortress covered by a large army, or by an intrenched camp. In this case, the invest ing army has to occupy a very large space, by which its force will be more dispersed, and easily driven back by a concen trated attack faoin the garrison.
A fortress can either be invested by the army which is to carry on the siege, or by a corps sent forward before that army arrives. This will frequently be of great advan tage, unless the fortress is covered. by an army, which of course must be driven back before the siege or the invest ment can take place. It is always to he observed, however, that the investment of a fortress, or the movements and pre parations for besieging it, are to be kept most secret, that in the fortress no particular preparations may be made, either in collecting stores, or reinforcing the garrison.
Previous to the commencement of a seige, it isof the high est importance, if possible, to ascertain not only the strength of the garrison, but also the troops which it may be neces sary to oppose in the neigbourhood, in order to determine the strength of the corps necessary to invest the place. This ought never to be so numerous as to impede the ce lerity or secrecy of its movements ; but at the same time it must be sufficiently strong to encounter the force which may be opposed to it. The besciging corps must also be proportioned to the nature of the ground. if the country be open, a greater quantity of cavalry will be required; but if inclosed, the principal force must be infantry. This corps is to march without any heavy baggage, that it may move with expedition; and that no obstacles may occur from its marching, in too large a body, it will often be necessary to divide it into several columns, Which are all to rendezvous at a certain place.
As soon as this corps has been collected, it approaches suddenly to within two miles of the fortress, were every avenue and passage to the place is immediately occupied. In some cases it may be necessary also, to fortify the posi tion, to guard against a sudden attack from a reinforcement of the enemy. As soon as the corps arrives at its appointed place, small pal ties arc sent towards the fortress, to bring away or destroy all kinds of provisions and forage, that the enemy may not profit by them. The same is also to be obser
ved with regard to the cattle, and every thing else which may be of any service to the enemy. If, at the same time, re connoitring parties can be of use, they are to be sent out ; as also parties to drive back any reinforcements that may be sent to the enemy. Should a fortress be situated on the sea-shore, a fleet will also be necessary to blockade it; and, if possible, this must be some days previous to the invest ment, in order that the fortress may be inclosed every where, as soon as the troops arrive.
The corps which previously invests a fortress should be careful in preparing the best position for the large army, as well as endeavouring to gain all possible information re specting the place, in order that every thing may be prepa red by the time that the army arrives. If, however, the ar my be sent directly, it will be the business of the comman der himself to choose the most advantageous position, and to obtain as much information as can be procured.
As soon as the investment begins, the engineers arc to be employed in collecting necessary information respecting the exterior grounds surrounding the fortress, as well as itself.
All drawings which may have been obtained of the for tress and the country round it, are compared with the ground, and, if necessary, improved or corrected by actual surveys. If no plans of the fortress can be procured, the works, as well as the surrounding country, are surveyed, in which each engineer has his own work to finish. If instruments, as rulers with diopters, boussoles, sextants, theodolites, and plain tables,•can be used, the survey will be executed with greater accuracy ; but where these are not to be had, a single instrument for taking the principal angles will an swer the purpose, and the rest may be filled up by the use of the camera lucida, and by telescopes with micrometers, as will be explained in another place. Plans of almost every fortress may now be had, and these plans are correct ed by actual comparisons with the works themselves. All particulars, such as the lengths of the flanks, the faces, the curtains, the dimensions of the fosse, and all the outworks, are taken as exactly as possible ; and, if necessary, an en gineer is to go into the covered way, to examine every thing necessary in this respect. Spies may frequently be of use for this purpose, and from them, after being properly in structed, the best information may be obtained. It is also necessary to ascertain the disposition of the garrison and of its commander, as well as the quantity of its provisions and stores of every kind, and where they are placed, in or der to know to what part the fire should be principally di rected, or rendered of no use to the garrison. Workmen who have assisted either in building or repairing a fortress, can give valuable information to a besieging army, and therefore every means should be tried to discover them, as the intelligence to be gained from them cannot be obtained from any other quarter.