Within the Line-Wall, immediately fronting the bay, are the casemates and barracks for the artillery regiments that are to serve the guns. The casemates are designed to be absolutely bomb-proof, the walls being of such thickness as to resist the impact of shot weighing hundreds of pounds, while the enormous arches overhead are made to withstand the weight and the explosion of the heaviest shells. Such at least was the design of the military engineers who constructed them: though, with the new inventions in war, the monster guns and the new explosives, it is hard to put any limit to man's power of destruction. This Line-Wall is armed with guns of the largest calibre, some of which are mounted on the parapet above, but the greater part are in the casemates below, and therefore nearer the level of the sea, so that they can be fired but a few feet above the water, and thus strike ships in the most vital part. The latest pets of Gibraltar are a pair of twins--two guns, each of which weighs a hundred tons! These are guarded with great care from the too close inspection of strangers. No description can give a clear impression of their enormous size. In the early history of artillery, the Turks cast some of the largest pieces in the world. Those who have visited the East, may remember the huge cannon-balls of stone, that may still be seen lying under the walls of the Round Towers on the Bosphorus. But those were pebbles compared with shot that can only be lifted to the mouth of the guns by machinery. The bore of these monsters would delight the soul of the Grand Turk, for, (as a man could easily crawl into one of them,) if the barbarous punishment of the old days were still reserved for great offenders, a Pasha who had displeased the Sultan might easily be put in along with the cartridge, and be rammed down and fired off!