This was the crater. It was filled with seven thousand of gunpowder, of a kind superior to anything known, and prepared by Gianibelli himself. It was covered with a roof, six feet in thickness, formed of blue tombstones, placed edgewise. Over this crater, rose a hollow cone, or pyramid, made of heavy marble slabs, and filled with mill-stones, cannon balls, blocks of marble, chain-shot, iron hooks, plough-coulters, and every dangerous missile that could be imagined. The spaces between the mine and the sides of each ship were likewise filled with paving stones, iron-bound stakes, harpoons, and other projectiles. The whole fabric was then covered by a smooth light flooring of planks and brick-work, upon which was a pile of wood: This was to be lighted at the proper time, in order that the two vessels might present the appearance of simple fire- ships, intended only to excite a conflagration of the bridge. On the 'Fortune' a slow match, very carefully prepared, communicated with the submerged mine, which was to explode at a nicely-calculated moment. The eruption of the other floating volcano was to be regulated by an ingenious piece of clock-work, by which, at the appointed time, fire, struck from a flint, was to inflame the hidden mass of gunpowder below. In addition to these two infernal machines, or "hell-burners," as they were called, a fleet of thirty-two smaller vessels was prepared. Covered with tar, turpentine, rosin, and filled with inflammable and combustible materials, these barks were to be sent from Antwerp down the river in detachments of eight every half hour with the ebb tide. The object was to clear the way, if possible, of the raft, and to occupy the attention of the Spaniards, until the 'Fortune' and the `Hope' should come down upon the bridge.