On the evening upon which the French batteries had opened, in transferring ammunition from the grand magazine to the ramparts, a shell dropped into a tumbril that was leaving the door of the building, and igniting the powder with which it was loaded, the tumbril blew up, and most unfortunately communicating with the depôt, produced a frightful explosion. The loss of life was, of course, great--numbers both of the garrison and the inhabitants perished; half the guns were dismounted; the works shaken to their foundations; and the ammunition reduced to some fifty barrels of powder. Treachery also was at work; the Portuguese officers in a body, headed by the second in command, proceeded to the governor[107] and insisted that he should surrender; and the major of artillery, who had been sent out to propose terms, proved a traitor.[108] He acquainted the French marshal with the full extent of the misfortune occasioned by the explosion; and Massena, perceiving that Almeida was at his mercy, of course dictated what terms he pleased. On entering the ruined fortress, the French general dismissed the militia to their homes, and having paraded the troops of the line, tendered them his protection, provided they joined the invading army, and took service under Napoleon. “To the eternal disgrace of the persons thus tampered with, all, both officers and men, embraced the proposal, and all passed over, without the slightest apparent reluctance, to the ranks of the enemy.”[109] When the fall of Almeida was known, Lord Wellington, who had advanced when Massena broke ground, fell back to the position on which he had previously retired; and anxious to get into closer communication with General Hill, he retreated leisurely on Gouvea. By this movement he checked any attempt that might have been intended from Sabugal by Covilhos, and effectually secured the fortified position of Zezere from being turned. Yet the situation of the allies was truly critical. The fall of Almeida permitted Massena to advance with confidence--while in numbers, the French marshal was immensely superior;[110] and of the allied force, a great portion of the Portuguese had never been under fire. The news of Romana’s defeat by Mortier, made matters still more alarming; as the latter might come up in sufficient time to threaten the right of the allies by Alcantara or Abrantes. But Massena’s movements ended this suspense--and Wellington was about to achieve one of his most splendid victories. BUSACO. British position.--Movements.--Disposition of the Allies.--Battle of Busaco.--Casualties of both armies. It was impossible to avoid a battle. Wellington crossed the Mondego, while the French were concentrated at Viseu. The first division had been placed in observation of the Oporto road,--the light, on the road of Viseu; but the French having passed the Criz, Lord Wellington changed his position, and fell back upon the heights of Busaco.[111]