So soon as the supporting points for the investing force are secured, a line of outposts is pushed forward towards the work and sentinels, pickets, etc., are established (Art of War, pars. 167-194). The lines of sentinels, pickets, and supports are placed as near the work as practicable, and the line of resistance is advanced at every favorable opportunity. The usual rules for posting and relieving the outposts, establishing day and night cordons, the use of patrols, etc., are applied, with such modifications as circumstances render advantageous. Any advanced points affording marked advantage to the attack which have been seized are strengthened and held when possible, even at considerable cost in men or with some delay in completing the investment. =71.= =Bringing up and Posting the Besieging Force.=--The main besieging force, consisting principally of infantry, artillery, and engineers, with the siege train, follows closely after the investing force, and, upon arrival is encamped upon sites previously selected, sending out at once, however, such reinforcements and supports as are needed by the line of investment. Engineer and artillery parks are established outside the zone of fire of the works and in proximity to the main routes of communication. Branch railways and tram-roads running through the parks, storehouses, repair shops, etc. etc., are located and constructed. Sites for storage magazines for ammunition are carefully selected at the most secure places, and isolated when possible from the camps and parks by intervening elevations of ground. The cover of these magazines, so far as possible, is made up of wood and sand or earth free from stones large enough to be dangerous projectiles in case of explosion. Rooms for loaded shells and cartridges, and laboratories for making up ammunition are constructed upon similar principles. Carefully studied arrangements for the health and comfort of the men are made. Some of these are outlined in Chapter VII. =72.= =Fortifying the Camps, Parks, etc., etc.=--In former sieges it was customary to completely surround the ground occupied by the besieger with a continuous line of works of simple trace and light profile called the “_line of circumvallation_;” and to construct between the camps and the work another line, either continuous or with intervals, called the “_line of countervallation_.” These lines were placed respectively at about 200 yards in rear and in front of the camps. The principal object of the first was to prevent by a small force the entry of small reinforcing detachments and supplies; that of the second was to resist vigorous sorties by the defence, or sudden attacks from the outside by strong reinforcing parties. For this purpose the detached works of the line of countervallation were so disposed as to cover the main depots, parks, roads, etc., and to be in defensive relations with each other.