The charge should be estimated by the rules already given, and increased by 20 to 30 per cent, so as not only to throw down the walls, but also to break up the earth and form a practicable breach. =92.= The galleries for placing the chambers behind the counterscarp are branches from the gallery of descent into the ditch; those behind the scarp may branch out from a gallery driven under the ditch, when water or rock do not forbid, or from a gallery driven through the scarp wall after crossing the ditch by sap or by a bridge. =93.= To start the gallery through the scarp wall, a miner is “_attached_” to the wall by protecting him from fire along the ditch, from sorties, and from loaded shells, etc., rolling down upon him from the parapet by suitable traverses and splinter-proof. This operation is of course very dangerous, and is generally impossible unless the fire of the defence along the ditch is previously silenced. To expedite the work of the miner a gun is sometimes brought down through the gallery, and the face of the wall is shattered by its fire before the miner is “=attached=.” CHAPTER IV. BLASTING AND DEMOLITIONS. BLASTING. =94. Blasts= are small mines used generally for breaking up rocks, or masonry in demolitions. =95.= Holes for placing the charges are drilled usually with _drill-bars_ or _churn-drills_, known also as _jumpers_. These are steel bars sharpened to a chisel edge. The _drill-bar_ is usually held by one man and struck with a hammer by another; it is turned slightly after each blow in order to make a round hole. For small holes the driller holds the drill in one hand and strikes with the other. The _churn-drill_ is a longer bar, generally sharpened at both ends and enlarged in the middle. It is used for drilling vertical holes by raising and dropping it in the holes, turning it slightly after each blow. =96.= The _charge_ may be gunpowder or high explosive. If the former, it must be thoroughly tamped. If the latter, tamping will greatly increase its effect; but it is in some cases preferable to obtain the desired effect by increasing the charge and saving the time taken in tamping. =97.= Blasts are fired by electric fuses, Bickford fuse, firing-tubes, needles, etc. The fuses have been already described. The _firing-tube_ is a very small iron pipe, which is inserted in the powder charge and the tamping rammed around it. After the tamping is finished the tube may be filled with fine powder poured in it if the hole is vertical or inclined downward, or straws filled with powder may be inserted if it inclines upward or is horizontal. A “squib” of wet powder is also sometimes placed in the tube and ignited, when it passes down the tube like a rocket and fires the charge.