J&8o SERVICE OP ARTILLERY
mines, they should make fougasses to render all endeavoursof the garrison to regain possession of them abortive.
76. The discussion of this subject has been hitherto con-fined to shafts, galleries, branches, and chambers made infirm and solid ground; but in a loose sandy soil, the exca-vation requires the addition of frames and planks, to sup-port the earth along the whole of the subterraneous work:and as these operations must be performed as expeditioustyas possible that the besieged may have the less time to destroythem by countermining, the squads of miners should be re-lieved every six hours; since in that time each man willhave had his turn of the most laborious part of the work.Shafts are generally made square, and the dimensions ofthe smallest are 4! feet. The galleries are 6 feet high, andabout 3! feet wide. The brandies 4s feet high and 3 feetwide; a return is made at the extremity of each branch forthe chamber which is of a cubical form, and in it is placeda wooden box of the fame figure to contain the proper quan-tity of powder; the spaces between the sides of the chamberand the box are filled up with clay or dung. The faucissonis laid in a pitched wooden trough ; and the branch andadjoining shaft or gallery filled with bags of earth or sand,and the angles barred with strong oaken planks, supportedby cross beams, that the line of least resistance may be onthe side against which the mine is intended to act.
77. The quantity of powder for charging mines, in-tended only to shake the ground without making any ex-,cavation, depends on the quality and tenacity of the soil,and on the line of least resistance: but it being sometimesout of the power of the besieger to make proper experimentsfor ascertaining the tenacity ; the following table will serveas a rule for the quantity to be used, in a soil of such tenacityas to need no frames to support it, in order to shake the groundfrom the bottom of the chamber to the surface, without dis-placing it; supposing the powder to be of a strong quality andwell preserved.
Table