He executed it on the night of May 2, when a dense fog chanced to favour his daring and hazardous proceedings. Having first told off some _tirailleurs_ to keep up a smart fire on the enemy’s trenches and distract his attention, he sent four sappers, each provided with a small powder-barrel, on to the bridge. The men, dressed in their grey _capotes_, crawled on hands and knees, each rolling his barrel (which was wrapped in cloth to deaden the sound) before him. They kept in the shadow, and getting close under the parapet of the bridge crept on till they reached the outermost Portuguese palisade. One after another, at long intervals, each got forward unobserved, left his barrel behind, and crawled back. The fourth sapper, starting to his feet on his return journey, was observed by the Portuguese and shot down, but Silveira’s men did not realize what he had been doing, and merely took him for some daring explorer who was endeavouring to spy out the state of the defences. After waiting for an hour, Bouchard sent out a fifth sapper, who dragged behind him a ‘sausage’ of powder thirty yards long, which he successfully connected with the four barrels. All was now ready, and a battalion of picked grenadiers from Delaborde’s division, filed silently down into the street near the bridge-head: a whole brigade came behind them. At two o’clock Bouchard fired his sausage, and the explosion followed. There were two chances of failure--one that the apparatus for firing the mine might not be disturbed by the concussion, the other that the shock might prove too strong, reach the mine, and destroy the bridge. Neither of these fatalities took place: the explosion duly broke the cord, shattered the nearest palisades, but did not affect the mine. Before the smoke had cleared away Delaborde’s grenadiers had dashed out on to the bridge, scrambled over the barricades, and driven off the guard on the further side. Regiment after regiment followed them, and charged up the mountain-side towards Silveira’s batteries and entrenchments. None of the Portuguese were under arms, save the few companies guarding the debouches from the bridge. These were swept away, and the French columns came storming into the bivouacs of the enemy before he was well awake. Hardly half a dozen cannon shots were fired on them from the batteries, and the greater part of the army of the Tras-os-Montes fled without firing a shot. Silveira escaped almost naked by the back window of the house above the bridge in which he had been sleeping.