Chap. VIII.

BRITAIN.

381

The building was carried up solid as high as there was any reason to suppose it wouldbe exposed to the heavy strode of the sea; that is, 35 feet 4 inches above its base, and27 feet above the top of the rock, or common spring tide high water mark.

At this height it was reduced to 16 feet 8 inches in diameter, the rooms here occupying12 feet 4 inches, and the walls 26 inches or 13 inches thick.

These were built with single blocks of granite or moorstone, and 16 were used ineach course, cramped together with iron, and joggled at each joint. The joggles weremade of sawn marble, 8 inches long, 4 inches broad, and 3 thick ; each end of each block,therefore, occupies 4 inches in length, 4 inches in breadth, and 1^ inch in the height ofeach joggle. To prevent any water or moisture passing through the upright joints, agroove was made in the end of each stone, into which an upright piece of stone, 6 inchesbroad and 2£ inches thick, was slid into the cavity or groove prepared for it; these weremostly of Purbeck, selected for their firmness, and were run in with mortar; they alsoserved to bond the work together. When in their places, the cramps were let in, whichwere flat bars of iron, 13 inches in length, 2 inches broad, and § inch thick, and wereturned down at each end about 3 inches in length, forming a cylinder 1£ inch in diameter.

E

Fig. 389.

FORTY-SIXTH COURSE,

The cramps being previously fitted to the stones, all that was necessary was to put eachinto a kettle of lead made red hot, and let it remain till it had acquired the samestate. About a spoonful of oil was then poured into the cramp holes, and the crampput into its place ; the ebullition of the oil caused by the heat of the iron gave an oilysurface to the whole cramp, as well as to the cavity of the stone; then the hot leadbeing poured in, the unctuous matter caused the metal to run into and occupy the mostminute cavity, and thus defend the cramp from any action that might be produced fromthe salts of the sea.

At the twenty-eighth course was introduced the vaulted floor which formed the ceilingto the upper store-room ; here was the first circular iron chain, which was lodged ina groove cut round the middle of the upper surface of this course.

The ordinary way of fixing the several courses by joggles and joint stones, and also thebonding them together by cramps, has been already described; by a reference to the section,it will be seen that each floor rests upon two courses, and that the circumference of thefloors is not supported upon the sloping abutments of an arch, in lines tending towards thecentre of the sphere, of which the underside of the floor was a portion, but upon a tripleledge carried round the two supporting courses: if each floor had been composed of a