At daylight the trenches will be far enough advanced to protect the covering force which will occupy them. This force is from this time known as the “_guard of the trenches_,” and is relieved usually every 24 hours, the time of relief being so chosen as not to interfere with the working parties. The working parties are, as previously indicated, divided into reliefs of 4 or 8 hours. For continuous work the besieging force should be large enough to allow each man, after being one day in the guard of the trenches and one day in the working party, to have one day in camp. =81.= =The Second Artillery Position.=--By the second artillery position previously referred to is meant the position occupied by the guns of the attack, placed in batteries, accurately located for breaching, enfilading, counter-battering or other specific duty. These batteries are usually of the class described under the head of “exposed sunken batteries” (Plates V, VI, VII), and are constructed behind or in the parallels, as explained in pars. 48 to 55. When behind a parallel they should be, if possible, at least 150 yards from it in order that the blast of the guns shall not interfere too much with the occupants of the parallel. =82.= =Counter-batteries=, designed to dismount guns or destroy embrasures of earth or masonry at ranges from 700 to 1000 yards by direct fire may well be armed with 4½ or 5 inch rifles, since their projectiles have sufficient energy for the desired result, and the guns admit of a more rapid and long-continued fire than do those of greater calibre. The batteries must be so placed as to look through the embrasure attacked, and the number of guns pitted against any battery must considerably exceed that in the battery. Counter-batteries designed to silence by direct fire guns in turrets or behind shields must be armed with guns of large calibre, mounted with the best available cover, and must be aided by rapid fire guns of moderate calibre, designed to disable the turret guns either by embrasure shots or by oblique shots penetrating the parts which project from the turret. =83.= =Enfilading batteries= act in conjunction with counter-batteries or independently; they are designed to take the faces in flank or slightly in reverse, but are of necessity at times limited to a slant fire. They are located as nearly as possible in the prolongation of the terre-pleins. When the salients are obtuse these prolongations lie near the adjacent faces for some distance, and consequently the only possible emplacements of enfilading batteries will give ranges which may vary from 1000 to 4000 yards. They are armed with cannon of sufficiently large calibre to make their projectiles efficient even at moderate velocities, and, when the faces enfiladed are well provided with traverses, the charges are reduced so as to give to the projectiles a large angle of fall. When the batteries are on commanding heights higher velocities may be used.