=56.= =Embrasures.=--Modern siege guns are generally mounted either on “overbank” or “disappearing” carriages, firing over parapets of sufficient height to give cover to the men. (The axis of the trunnions of the U. S. 5" siege gun is 6" above the platform.) Embrasures when used are generally shallow grooves cut in the top of the parapet. In this case the bottom of these grooves must cut the surface of the top of the parapet at or in rear of the highest line visible to the enemy, so that no indentations which can be seen by him will indicate the position of the guns. To effect this, the exterior crest will usually be as high as and sometimes higher than the interior crest, and the top of the parapet (“superior slope”) will be level or will slope to the rear. In rare instances, however, deeper embrasures with revetted cheeks must be made. The only serviceable revetment for use with high-power guns is one of sand-bags wrapped in raw hides. This may be made by laying down a hide, piling a number of sand-bags upon it, and folding the free end back over them; placing another hide on top of this with more sand-bags and so on. Or large packages may be made by wrapping up a number of sand-bags in each hide and these packages may be used for making the revetment. The embrasure should be bottle-shaped in plan, shaped like a segment of an ellipsoid immediately in front of the muzzle of the gun, then drawn in like the neck of a bottle and narrowed to as small a mouth as possible, so as to diminish the effect of the blast and give the least possible exposure to the gun. When the battery is exposed to slant or enfilading fire, instead of embrasures, bonnets of sand-bags may be built upon the parapets to protect the guns. =57.= =Observatories.=--Observatories or look-outs, as previously stated, should as a rule be placed on high points well on the flanks of the battery.[2] When this is impracticable, they may be made by building up at the rear of the traverses, on the flanks, or even in the gun portions, glacis-shaped covers pierced with a sight-hole in all respects similar to a loop-hole for musketry, and with just sufficient splay to include the desired field of view. A number of these should be provided for each battery, so that the enemy may not know which one is in use at any time. If subject to close and accurate fire, the crest-line in their vicinity must be of the same level as the tops of the look-outs, and provision must be made to prevent the light showing through them. =58.= =Drainage.=--After the completion and arming of the battery, gutters should be cut on each side of the gun-portion leading into one running along the reverse of the rear trench which carries the water to low ground on the exterior, or which is provided with dry wells or drainage-pits for collecting the water so that it may soak into the ground or be pumped out with hand-pumps.