Shot was used mainly in the flat-trajectory cannon. The small caliber guns fired nothing but shot, for small sizes of the other type projectiles were not effective. Shot was the prescription when the situation called for "great accuracy, at very long range," and penetration. Fired at ships, a shot was capable of breaching the planks (at 100-yard range a 24-pounder shot would penetrate 4-1/2 feet of "sound and hard" oak). With a fair aim at the waterline, a gunner could sink or seriously damage a vessel with a few rounds. On ironclad targets like the _Monitor_ and _Merrimac_, however, round shot did little more than bounce; it took the long, armor-piercing rifle projectile to force the development of the tremendously thick plate of modern times. [Illustration: Figure 41--EIGHTEENTH CENTURY PROJECTILES. (Not to scale.)] Round shot was very useful for knocking out enemy batteries. The gunner put his cannon on the flank of the hostile guns and used ricochet firing so that the ball, just clearing the defense wall, would bounce among the enemy guns, wound the crews, and break the gun carriages. In the destruction of fort walls, shot was essential. After dismounting the enemy pieces, the siege guns moved close enough to batter down the walls. The procedure was not as haphazard as it sounds. Cannon were brought as close as possible to the target, and the gunner literally cut out a low section with gunfire so that the wall above tumbled down into the moat and made a ramp right up to the breach. Firing at the upper part of the wall defeated its own purpose, for the rubble brought down only protected the foundation area, and the breach was so high that assault troops had to use ladders. The most effective bombardment of Castillo de San Marcos occurred during the 1740 siege, and shot did the most damage. The heaviest English siege cannon were 18-pounders, over 1,000 yards from the fort. Spanish Engineer Pedro Ruiz de Olano reported that the balls did not penetrate the massive main walls more than a foot and a half, but the parapets, being only 3 feet thick, suffered considerable damage. Some of the old parapets, Engineer Ruiz said, "have been demolished, and the new ones have suffered very much owing to their recent construction." (He meant that the new mortar had not sufficiently hardened.) Ruiz was not deceived about what would happen if hostile batteries were able to get closer; in such case, he thought, the enemy "will no doubt succeed in destroying the parapets and dismounting the guns."