=39.= When breaches which are practicable are made in both counter-scarp and scarp of a dry ditch an assault may sometimes be made successfully; but when the scarp-wall is not breached, or when, for other reasons, an assault from the crowning of the covered way is not considered advisable, the ditch must be crossed and the breach, when made, must be crowned by regular approaches. This is accomplished by the use of a _sap_, _single_ or _double_, depending upon whether it is exposed to fire upon one or both flanks. Owing to the plunging fire of the defence it may be necessary to make the sap deeper than 4' 6", or in some cases to _blind_ it for a part or the whole of its length. It is generally impracticable to drive the sap down the slope of a breach in the counter-scarp; therefore a _blinded descent_ (Military Mining, pars. 53 and 54) is used. It is so directed that when the counter-scarp is reached the floor of the gallery will be at the required depth below the bottom of the ditch; i.e., at the depth fixed for the bottom of the sap. When the small-arm fire of the defence is so severe as to necessitate blinding the sap from the counter-scarp across the ditch, it will usually be imperative to provide a _shield_, under cover of which the sappers may start the blinded sap. This may be made of boards covered with bullet-proof iron-plates, and of such width and length that it may be carried through the gallery, thrust out into the ditch, and then turned, placed in position, and blocked up at such angle and to such height as may be wished, by men who move on their hands and knees and support the shield on their backs. Under cover of this shield the head parapet of the blinded sap may be thrown up and the sap then driven in the usual way. For method of breaching by mines, see Military Mining, pars. 91 and 92. After breaching the scarp, if an assault is to be made, the counter-scarp, for a length equal to or greater than the length of the breach, should be blown down, to give the assaulting party access to the breach. If the breach is to be crowned, and approaches are to be driven against interior retrenchments, a gallery of descent should be driven to the counter-scarp at one side of the breach before the assault is made. From this a trench should be driven to the crowning of the breach (usually by flying sap), by means of which communication is maintained between the crowning of the breach and the exterior. =40.= =A wet ditch without current= may be crossed by building a causeway, upon one or each side of which a parapet is constructed (Pl. IV., Figs. 37, 38, and 41).