British land-based batteries and pound them into submission by numbers and weight of shots fired, before a storming party attacked from the siege works on the Isthmus and further troops were put ashore from the waiting Spanish fleet. The French engineer Jean Le Michaud d'Arçon invented and designed the floating batteries—'unsinkable' and 'unburnable'—intended to attack from the sea in tandem with other batteries bombarding the British from land. The floating batteries would have strong, thick wooden armour— timbers packed with layers of wet sand, with water pumped over them to avoid fire breaking out. In addition old cables would also deaden the fall of British shot and, as ballast, would counterbalance the guns' weight. Guns were to be fired from one side only; the starboard battery was removed completely and the port battery heavily augmented with timber and sand infill. The ten floating batteries would be supported by ships of the line and bomb ships, which would try to draw away and split up the British fire. Five batteries each with two rows of guns, together with five smaller batteries each with a single row, would provide a total of 150 guns. The Spanish enthusiastically received the proposal. D'Arçon sailed close to shore under enemy fire in a skiff to get more accurate intelligence. On 13 September 1782 the Bourbon allies launched their great attack: 5,260 fighting men, both French and Spanish, aboard ten of the newly engineered 'floating batteries' with 138 to 212 heavy guns under the command of Don Buenaventura Moreno. Also in support were the combined Spanish and French fleet, which consisted of 49 ships of the line, 40 Spanish gunboats and 20 bomb-vessels, manned by a total of 30,000 sailors and marines under the command of Spanish Admiral Luis de Córdova. They were supported by 86 land guns and 35,000 Spanish and 7,000–8,000 French troops on land, intending to assault the fortifications once they had been demolished. An 'army' of over 80,000 spectators thronged the adjacent hills on the Spanish side, expecting to see the fortress beaten to powder and 'the British flag trailed in the dust.' Among them were the highest families in the land, including the Comte D'Artois. The batteries slowly moved forward along the bay and one by one the 138 guns opened fire, but soon events did not go according to plan. The alignments were not correct: the two lead ships Pastora