The building had hitherto been carried up solid as high as there was any reason to suppose it exposed to the heavy stroke of the sea, _i. e._ to thirty-five feet four inches above its base, and twenty-seven feet above the top of the rock, on the common spring-tide high-water mark. At this height it was reduced to sixteen feet eight inches diameter; and it was necessary to make the best use of this space, and make all the room and convenience therein that was possible, consistent with the still necessary strength. The rooms being made twelve feet four inches in diameter would leave twenty-six inches for the thickness of the walls. These walls were made of single blocks in the thickness, so shaped that sixteen pieces formed a complete circle, and from their figure composed a stout wall. These pieces were cramped together with iron, and also secured to the lower courses by marble plugs as before. To hinder the passage of wet through the upright joints, flat stones were introduced into each joint so as partly to be lodged in one stone, partly in another, thus making it tolerably certain that the rooms would be kept comfortable and dry in all weathers. On the 30th of September the twenty-eighth course was completely set. This and the next course received the vaulted floor, which made the ceiling of the store-room and floor of the upper store-room. For further security, therefore, there was a groove cut round the upper surface of this course, in which was placed a circular chain of great strength. Upon this chain, in the groove, was poured melted lead, until the cavity was filled up. The next course was then laid on, and this was also secured by a chain in like manner, it being considered that the courses on which the floors rested demanded every possible security. The formation of the floors, and the care taken to avoid the danger of lateral pressure on the walls, is worthy of notice. Each floor rested upon two courses; being firmly supported by a triple ledge going circularly round the two supporting courses. ‘Had each floor,’ says the architect, ‘been composed of a single stone, this lying upon the horizontal bearings furnished by these ledges, would, while it remained entire, have no lateral pressure or tendency to thrust out the sides of the encompassing walls; and that in effect the several pieces of which the floors were really composed might have the same property as whole stones, the centre stone was made large enough to admit of an opening, from floor to floor, to be made through it; and being furnished with dove-tails on its four sides, like those of the entire solid, it became the means by which all the stones in each floor were connected together; and consequently, the whole would lie upon the ledges like a single stone, without any tendency to spread the walls. But if by the accident of a heavy body falling, or otherwise, any of those stones should be broken, though this might not destroy its use as a floor; yet the