A fortress is said to be countermined when subterranean galleries are funned under the ramparts of the bastions and ravelins; under the covered-way, and under the ground at the foot of the glad., with galleries of communication from one of these to another. And as the defenders can form chambers and place powder in or mew any con venient parts of these galleries to destroy the works of the besiegers aboveground, the besiegers find themselves under the necessity of Finking shafts and forming galleries for the purpose of finding out and destroying those of the defenders, or of blowing up any of their ad vanced works. [Jlimas, 3ItterAnv.]1 A sieve conducted according to the rules of art will be attended with comparatively small loss to the besiegers or besieged, the troops of both parties being but little exposed to each other's fire except at the times when the assaults; are made on the ravelins or bastions. And if circumstances, such as the prospect of the place being relieved, did not compel the besiegers to expedite the surrender, the assaults by main force might be avoided ; for after a breach has been formed, and the parapets of the place have been in a great measure ruined by the artillery of the besiegers, a sapper might be sent across the ditch by night with instructions to commence a trench under cover of one ex tremity of the broken wall ; then if ho succeed in getting cover for himself, others may follow, and gradually there may be formed on the breach a lodgment sufficiently large to contaiu troops, whose fire would protect the succeeding operations : it being understood that a firing party in the batteries on the glacis force the defenders to retire as often as they endeavour to disturb the sappers while at work.
The want of time and means to carry on the approaches as far as the covered-ways was the cause of the great losses sustained in getting possession of the fortresses garrisoned by the French in Spain during the Peninsular war. The breaches in the walla of 13adajoa (1812) were made by guns in batteries at the distance of 500 yards; and when the assailing troops had descended into the ditch, being ignorant of the positions of the breaches, and confused by the darkness of the night, which was relieved only by the appalling and destructive blaze of live shells and other combustibles thrown upon them from the parapets, they took a wrong direction, or remained patiently to be slaughtered till the order was given to retire. The effort would have entirely failed, but for the success of Major-General Picton in escalading the walls of the castle, and of General Walker in escalading the bastion of S. Vincente.