1604
THEORY AND PRACTICE OF ENGINEERING, Book II,
of the walls is filled in with rubble, and the shafts of the columns are of marble, from thePurbeck quarries. At the intersection of the nave with the great transept rises a noblestone tower and octagonal spire, the total height of which is 400 feet; the stone of thespire is in thickness about 2 feet to the height of 20 feet above the tower, after which it isonly 9 inches in thickness to the summit: this spire, though braced and strengthenedthroughout by timbers and ironwork, has declined from the perpendicular 22-t inches;but since 1681, when the observation was made, there has been no further declination.
The walls, after they were carried up to the floor of the triforium, appear to have beenincreased by corbelling, as if it had been doubted whether, as originally set out, there wouldbe sufficient strength to carry the cross springers of the vaulted nave; the total widthis exactly 100 feet. The clear width of the nave, as measured on a level with the triforium,is 33 feet 3 inches, and that of each side aisle half that dimension, or 16 feet 9 inches;had this last been 16 feet 7^ inches only, the proportions shown by a section wouldhave been exactly one-third for walls and two-thirds for voids ; after appropriating thethird of the 100 feet to the walls, half the remainder is given to one side, and halfto the other; we also find that each of these dimensions of 16 feet 8 inches is dividedinto three, two parts of which are given to the outer wall and buttress, and the other tothe main pillar that divides the nave and side aisles, or nearly so.
The inclination of the arched buttresses is not such as to resist the spreading of thevault at its base, the knowledge of their use not having then been attained. The heightof the vaulting of the nave from the pavement is 81 feet.
Wells Cathedral has some peculiarities in its construction, particularly in the application ofits arched buttresses : they pitch against a stone corbel inserted below the springing of the
Fig- 3003. PLAN OF THIPORl'
Fig. 3002. PLAN OF OLBRE8TORY.
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middle vault, and a tangent drawn at the back of the vault and elongated determines theinclination of the top of the flying buttress: here some improvement is shown upon those