Each battery was clad on its fighting side with three successive layers of squared timber, three feet in thickness; within this wall ran a body of wet sand, and within that again was a line of cork soaked in water and calculated to prevent the effects of splinters, the whole being bound together by strong wooden bolts. To protect the crews from shells or dropping shot, a hanging roof was contrived, composed of strong rope-work netting, covered with wet hides, and shelving sufficiently to prevent the shot from lodging. Not the least remarkable part of these vessels was a plan for the prevention of combustion from red-hot shot. A reservoir was placed beneath the roof from which numerous pipes, like the veins of the human body, circulated through the sides of the ship, giving a constant supply of water to every part, and keeping the wood continually saturated. To form these powerful batteries, ten ships, from six hundred to fourteen hundred tons burden, were cut down to the proper proportions, and upward of two hundred thousand cubic feet of timber were used in their construction. Each battery was armed with from eight to twenty heavy brass cannon of new manufacture, with a reserve of spare pieces. The crews varied in number from seven hundred sixty to two hundred fifty men. One large sail propelled each ship. Besides this tremendous armament which was to annihilate the line of defence from the sea, preparations of no less magnitude were being made for the attack on the northern front. Not fewer than twelve hundred pieces of heavy ordnance were ready for use in the artillery park, enormous quantities of ammunition and warlike stores were in the magazines, and the reserve of gunpowder alone was reported at eighty-three thousand barrels. Immense works were being hurried forward on the isthmus of a grandeur which eclipsed anything that had been previously constructed. In twenty-four hours a flying sap was thrown out with a rapidity of execution unequalled. The parallel extended to a length of two hundred thirty _toises_, with a _boyau_ of six hundred thirty toises from the place where it joined the principal barrier of the lines. The construction of this boyau required one million six hundred thousand bags of sand, and thousands of casks were used in forming the parallel. In a single night this enormous work was raised to the height of twelve feet with eighteen feet of thickness, and it was supposed that during the seven hours in which it was erected ten thousand men were at labor. To assist in the assault by sea, the combined fleets of France and Spain, amounting to fifty sail of the line, with forty gunboats, numerous frigates, and fifty mortar-vessels, were to act in support. Three hundred boats, fitted with hinged platforms at their prows, were to accompany the expedition, and at the proper moment to land the troops.