remained uninjured.  It was recognized that the gate of Tongres was not the most assailable, but rather the strongest portion of the defences, and Alexander therefore determined to shift his batteries to the gate of Bois-le-Duc.  At the same time, the attempt upon that of Tongres was to be varied, but not abandoned.  Four thousand miners, who had passed half their lives in burrowing for coal in that anthracite region, had been furnished by the Bishop of Liege, and this force was now set to their subterranean work.  A mine having been opened at a distance, the besiegers slowly worked their way towards the Tongres gate, while at the same time the more ostensible operations were in the opposite direction.  The besieged had their miners also, for the peasants in the city had been used to work with mattock and pickaxe.  The women, too, enrolled themselves into companies, chose their officers—­or “mine-mistresses,” as they were called—­and did good service daily in the caverns of the earth.  Thus a whole army of gnomes were noiselessly at work to destroy and defend the beleaguered city.  The mine advanced towards the gate; the besieged delved deeper, and intersected it with a transverse excavation, and the contending forces met daily, in deadly encounter, within these sepulchral gangways.  Many stratagems were, mutually employed.  The citizens secretly constructed a dam across the Spanish mine, and then deluged their foe with hogsheads of boiling water.  Hundreds were thus scalded to death.  They heaped branches and light fagots in the hostile mine, set fire to the pile, and blew thick volumes of smoke along the passage with organ-bellows brought from the churches for the purpose.  Many were thus suffocated.  The discomfited besiegers abandoned the mine where they had met with such able countermining, and sunk another shaft, at midnight, in secret, at a long distance from the Tongres gate.  Still towards that point, however, they burrowed in the darkness; guiding themselves to their destination with magnet, plumbline and level, as the mariner crosses the trackless ocean with compass and chart.  They worked their way, unobstructed, till they arrived at their subterranean port, directly beneath the doomed ravelin.  Here they constructed a spacious chamber, supporting it with columns, and making all their architectural arrangements with as much precision and elegance as if their object had been purely esthetic.  Coffers full of powder, to an enormous amount, were then placed in every direction across the floor, the train was laid, and Parma informed that all was ready.  Alexander, having already arrayed the troops destined for the assault, then proceeded in person to the mouth of the shaft, and gave orders to spring the mine.  The explosion was prodigious; a part of the tower fell with the concussion, and the moat was choked with heaps of rubbish.  The assailants sprang across the passage thus afforded, and mastered the ruined portion of the fort.  They were met in the breach, however, by the unflinching defenders of the city, and, after a fierce combat of some hours, were obliged to retire; remaining masters, however, of the moat, and of the ruined portion of the ravelin.  This was upon the 3rd of April.