=47.= =Exposed Sunken Battery.=--Before describing the construction of this battery it is necessary to state that upon a site fully exposed to the accurate concentrated fire of a work, directed at night by light balls or electric lights, it will in general be practicable to construct batteries only by sapping, and even then with considerable losses. But these conditions seldom exist, since in the distant attack it is usually possible to construct and arm the battery before it is discovered by the defence, and in the close attack the fire of the defence is generally so much reduced that some exposure is justifiable. While the battery to be described is classed as an “exposed battery,” it is understood that it may also be constructed under cover of a parallel or other trench, and that in all cases when practicable a natural or artificial mask is used to conceal the first night’s work from the enemy. It is assumed from the results obtained in practice that, with the material conveniently stored, the battery can be traced, a central trench and splinter-proof covers be made during the first night, and the battery finished and armed during the second. The general design and details of this battery are due to the Royal (British) Engineers. =48.= =Tracing the Battery.=--The battery is traced under the direction of an engineer officer by one or two tracing parties, each composed as follows: 1 non-commissioned officer with a 6-foot measuring-rod and tracing-lantern, and 4 sappers, one carrying a measuring-tape and bundles of pickets, one a field-level, one several tracing-tapes, and one a mallet or hand-axe; about 75 pickets and 1200 feet of tracing-tape should be provided. The line of fire of the first gun of the battery (_xy_, Pl. V, Fig. 46) is accurately laid out and marked by daylight. At dusk one party drives a picket at _I_, where the directrix crosses the projection of the base of the interior slope, and from this as an origin lays out the cutting lines of the central trench, _I_, _II_, _III_, _IV_, _V_, _I_, making the trench 5 feet wide and of the length required for the number of guns (= No. of guns × 45'--10'); commencing then at a point _A_, 7' 6" to the left of _I_ and in the rear cutting line, this party lays out the line _a_, _b_, _c_, _d_, _e_, etc., ... _m_, _n_, _o_, as indicated, the direction _n_, _o_, leading to the parallel. The second party, beginning at _A_, lays out _A_, _B_, _C_, communicating with the parallel, and then the inner cutting line of the ditch _D_, _E_, _F_, _G_, _H_, _I_, allowing for a thickness of parapet of 30 feet and an ultimate width of ditch of 12 feet (_D_, _E_, and _H_, _I_). Two parties should trace the battery in 25 minutes, one party in 45 minutes.