The general uses of the parallels are to connect the lines of approach by a covered line of communication, to allow the trenches to be kept clear of troops and free for the workmen, and, affording cover and pro tection to the guard of the trenches and a strong position for them to remain in during the day, to serve as lines of countervallation in confining the garrison of the place. As soon as the first parallel is finished, the guard of the trenches is moved into it, for the purpose of protecting the succeeding operations. While the approaches are being pushed forward, or indeed while the first parallel is being completed, the batteries c, c, are formed either in or in advance of the first parallel to keep down the fire of the place either by direct or enfilade tire as shown by the dotted lines.
Since the trenches leading from the first parallel towards the place ought net to be enfiladed from thence, it is evident that they must be formed in zig-zag or oblique directions a a a, &c., crossing and recrossing the produced capitals of the bastions or mvelins ; and that the several branches, if produced, should fall on the exterior of all the works of the place. The first oblique boyeau (branch) of the trench may be defended by the fire of the parallel; but the second may, if found necessary, be protected by the fire of troops stationed in a small trench, a b, called a parallel beyeau, at the angle between the first and second launcher' ; and a short branch for the like purpose is generally formed at the angles of all the zig-zag trenches. If the directions of the several oblique branches are not laid down on the ground, from a plan of the intended operations previously made on paper, the engineer endea voura, during 'the daylight, to observe, in the direction of the most advanced part of the glacis towards his right or left band, some object towards which the tracing-line maybe stretched and the trench carried on. These trenches are then executed by the working-parties, in the name manner as the great parallel was formed. At the siege of Iladajos, in 1912, a French corporal, in the dusk of an evening, displaced a tracing-line, which had been stretched by the British engineers, and directed it so that the trench executed aloog it might have been enfiladed by three guns on the ramparts of the castle. If tho derangement had not been discovered before darkness came on, the labour of the whole night would have been lost, and casualties might have occurred from the fire which the garrison might have directed along the trench. The workmen are usually relieved after it is dark, in order that the change may not be observed from the fortress; but the officers should be relieved earlier, that those who come on duty may have light enough to examine the actual state of the works, and to take measures for directing the operations of the men during the night.
When the heads of the trenches have arrived within 300 yards of the covered-way, which is usually about the fourth night from the time of opening the trenches, a second parallel, null, is formed, in order to facilitate the communication between the several lines of approach, and to protect the working-parties, for it is evident that having got half way between the first parallel and the place, as the boycaux advance the working-parties in them are nearer to the enemy than to their supports. This trench may be extended along the particular points attacked, and its extremities may be terminated by redouts, or con tinued till they fall into the first parallel. It is executed by flying sap fSAr], that the men may be quickly protected from the fire of musketry in the covered-way, which might now begin to take effect.* One of the principal means of accelerating the surrender of the fortress is that of enfilading the ramparts, in order to dismount the guns and drive the defenders front the parapets. But in order to be effective, the range for ricochet fire should be short ; for this reason fresh batteries, d d d, &c., are raised in the directions of the produced faces of the works, in or near the second parallel, which thus may form a secure communication between them, and are armed with guns moved up from the first parallel. [BATTERY ; ltwocuev.] From the same batteries also an oblique fire may sometimes be directed against the interior of the flanks and curtains, and a plunging fire into the ditches, in order to impede the commu nication between the place and the outworks. All the different ricochet batteries commence firing at the same time, that the attention of the defenders may be divided, and that they may be prevented from con centrating all their fire upon one battery; the guns also should be fired singly, and at intervals so regulated that there may be always some shot or shells in the air, for thus the enemy will have little time to repair the damage done to his artillery or to the parapets. The firing should be commenced during daylight, in order that the artillerymen may be able to determine by trial the charge of powder and the degree of elevation for each piece, so that the shot may just clear the parapet of the work to be enfiladed ; this being obtained, the direction of the piece may be preserved by means of timbers nailed to the platform, and thus the fire may be kept up with equal accuracy by night and by day.