Construction and design Great Siege Tunnels

view of notch seen devil s tower road.

the work carried out hand, using sledgehammers , crowbars, aided gunpowder blasts. work proceeded @ first; took thirteen men 5 weeks dig tunnel width of 8 square feet (0.74 m) , length of 82 feet (25 m). diggers hindered fumes , dust frequent blasting, decided blast horizontal shaft improve tunnels ventilation. done , found have had unexpected side benefit. colonel john drinkwater bethune, wrote account of siege in 1785, described how came about:

[t]he mine loaded unusual quantity of powder, , explosion amazingly loud, whole of enemy s camp turned out @ report : must surprise be, when observed smoke issued! – original intention of opening, communicate air workmen, before suffocated smoke remained after blowing different mines ; but, on examining aperture more closely, idea conceived of mounting gun bear on enemy s batteries, excepting fort barbara : accordingly orders given enlarge inner part [of tunnel] recoil ; , when finished, twenty-four-pounder mounted.

work progressed rapidly thereafter, though did not entirely go plan, several false starts in direction in latter part of 1782. 1 tunnel drive determined far outer face of rock , close it. consistent direction found, , end of fourth siege embrasures had been blasted overlooking spanish lines. total construction length of tunnels end of 1783 approximately 908 feet (277 m). when excavations reached notch in summer of 1783 decided instead of pursuing original idea of mounting single gun on top of it, notch hollowed out broad chamber diggers named st. george s hall , later equipped 7 guns.

by end of initial phase of tunneling, 5 galleries had been excavated: windsor gallery, king s , queen s lines, st. george s hall, , cornwallis chamber. windsor gallery first part of tunnel system, , 4 guns mounted there. st. george s hall largest of original galleries. embrasures can seen on slopes of rock when approaching gibraltar land , sea.

originally embrasures fitted mantlets or curtains of woven ropes; rails on supported can still seen. these protected guns , gunners enemy fire , prevented sparks , smoke blowing embrasures. additional safety measure, each cannon isolated wet cloth hanging above rope, prevent sparks igniting remaining gunpowder.