_Second period._--As soon as the engineers have completed their reconnaissances and determined on the front of attack, and all the other preparations are made, the general will direct the opening of the trenches. The ground being previously marked out, battalions of light troops, termed _guards of the trenches_, as soon as it is dark, are placed about thirty yards in front of the first parallel, (A. Fig. 54,) with smaller sections, and sentinels about the same distance further in advance. These guards lie down, or otherwise conceal themselves from the fire of the work. The engineer troops and detachments of workmen being first marched to the depots and supplied with all the necessary tools for carrying on the work, now commence their labors under the protection of these guards. By daybreak the construction of the first parallel, and the trenches connecting it with the depots, will be sufficiently advanced to cover the men from the fire of the place; the guards will therefore be withdrawn, and the workmen continue their labors during the day to give the trenches the proper size and form. The _parallels_ are the long lines of trench which envelop the besieged work, and serve both as covered ways for the circulation of the besiegers, and as means of defence against sorties from the garrison; they are therefore arranged with banquettes for musketry fire. The boyaux are trenches run in a zigzag direction along the capitals of the front of attack, and are intended exclusively for the circulation of the troops; they have no banquettes. The first parallel is about six hundred yards from the place, and consequently beyond the reach of grape. It is constructed by the _simple sap_. After the first night, the guards, instead of advancing in front of the work, are placed in the trenches. The second parallel (B) is made some three hundred or three hundred and fifty yards from the place, and being much exposed to grape, the _flying-sap_ is employed in its construction. Batteries (H) are established between the first and second parallels to silence the fire of the demi-lunes of the collateral bastions, and others (I) near the second parallel, to enfilade the faces of the front of attack. These are armed in part with mortars and in part with heavy siege-pieces. The works are now gradually pushed forward to the third parallel, (C), which is constructed about sixty yards from the salients of the place. As the operations of the besiegers are here greatly exposed to musketry fire, the trenches are constructed by the _full-sap_. The third parallel, having to contain the guards of the trenches, and being of less development than the two preceding, is made much wider. The second parallel now contains the reserve, and the first parallel becomes the depot of materials. _Demi-parallels_ (G) are frequently established between the second and third, to be occupied by detachments of guards.