Battle techniques were dictated by the movement capability of the ship's guns. Weaponry was enhanced greatly, and because of ironclads the use of the solid shot was revived. This was done because the solid shot was better suited for piercing the tough armor of the ironclads. Solid shot was a pure iron ball that was much heavier than one with explosive powder inside of it. A shell that was used in the rifled guns was the James Shell. It was seven inches long, eighty-four pounds and when fired, the base would fill up with gases and expand, which would make it rotate. Other types of shells fired by smoothbores were grapeshot. These contained iron balls in a frame a shell with a sabot around it, which delivered a canister containing forty-eight or more small iron balls. Used very effectively, these two could be used against infantry units and did some damage to ships. Since the South had limited resources, there was some variation in the naval artillery used by the North and South. The South could not duplicate many of the North’s inventions. Many naval shells from both sides could explode up to seven seconds after being fired. This was because to the use of three timed fuses that could be set to detonate when necessary. These shells could be fired and buried into a wall of a fort and explode several seconds later to create greater damage to the enemy. Another innovation was the cannon. Together, Thomas J. Rodman of the Union Army and John A. Dahlgren of the Northern navy got together and were able to revolutionize the smoothbore cannon. They did this by developing a special iron casting technique, which followed the principle that the barrel should be thickest, where the pressure of combustion was greatest (at the breech). This made the cannons bottle shaped. Though the South was limited, they did manage to come up with some of their own creative technological advancements for use in making warfare and defense. The South bought the rifled Armstrong cannon from the British because their lack of heavy industry. The Armstrong shot a shell that was eight inches and had a thin iron cap so that on impact it could ignite a bursting charge. John M. Brooke designed many rifled guns for the South by converting smoothbores to rifled bores. Despite this however, the North had far better naval artillery than the South would ever have, and the South became mainly dependant on imports since the North took control of the few existing foundries and thus left the South with no real means of producing their own. The South also relied on mines because it was one of their most effective methods. In fact, it was Southern mines that were responsible for the destruction of thirty-one Union blockade ships in the Battle of Mobile Bay. They were placed all over the water to repel ships from the land. Various shapes and sizes were made and explosive charges were encased that would detonate with seven pounds of pressure. Overall, Southern mines damaged or sank forty-three Northern vessels. Although having less than the North, the South should be credited with their effort and intelligent use of a small amount of resources when it came to their navies.