The great development of the line which must be occupied by the besieger, owing to modern methods of fortification and the range of rifled cannon, prohibits the construction of complete lines of circum- and countervallation. The besieger constructs in their stead one or more lines of detached works upon advantageous points, and covers the intervening ground more or less thoroughly by patrols, outposts, etc. He then so disposes his main force as to be able to concentrate enough to meet any sorties of the defence; and, if necessary, detaches a force, called an “_army of observation_,” sufficiently large to meet any relieving army and defeat it; or hold it in check until he can concentrate the besieging force with the army of observation, and meet the relieving army in a favorable position. As a rule, this position will be one well outside the besieger’s cordon of works; since the latter by its extent will necessarily be weak to resist a determined attack (Art of War, par. 258), and by its proximity to the work will render possible the co-operation of the garrison and the relieving army. This, under the circumstances assumed, would seriously endanger the besieging army. In opposing sorties from the work, however, the conditions which fix the point of conflict are reversed, and place it as near the work as practicable. The shortening and strengthening of the line of investment by closing it in upon the work make it imperative to hold all ground gained; and this is generally best accomplished by intrenching the line of outposts with continuous shelter trenches, strengthened at intervals by batteries of field guns, and supported by field works of considerable strength, placed within accurate cannon range of each other, but not exposed to the direct fire of the guns of the place. Behind the shelter trenches the outposts, supports, and reserves, strengthened when necessary by troops from other points of the line, should be able to hold their own against all ordinary sorties. The main line of field works serves to resist a general attack made by the mass of the garrison. Placing the first intrenchments further back exposes the outposts to the confusion resulting from falling back, frequently at night or in a fog, and also enables the besieged to seize upon ground from which it may be very difficult to dislodge them. To allow the different parts of the line to be rapidly reinforced, good roads protected from the fire of the work, and well marked with sign-posts, etc., must be opened between the adjacent divisions of the besieging force, and all streams must be provided with bridges secure against floods, ice, etc.