1446

THEORY AND PRACTICE OF ENGINEERING.

Book II.

faces or surfaces by which stones are circumscribed at their junction edges or arrises, themethod of projecting right lines in every possible position must be thoroughly understood ;and no better study can be suggested for the mason than laying down upon paper therepresentation of the solids in various positions by which he will acquire facility in com-prehending this most difficult part of his art.

Hexagonal groined Vaults , with pointed Arches or Hibs. —These are composed of parts ofcircles less than 90°,so arranged as to form different compartments; the intervals are filledin with small stones bedded in plaster or mortar. To cut the ribs two moulds only arerequired, one taken from the plan, which gives the upper and lower surfaces, and the otheron the section for the profile of the height comprised between the intrados and extrados.The key is a truncated reversed pyramid, whose base is a hexagon inscribed in a circle, as inthe right hand figure : we commence by cutting a surface, on which is set out thepolygon, or base of the pyramid; its inclined sides are set out by taking the bevel; oneach of these surfaces are drawn parallel lines, the first marking the contour of the joint atthe extrados, and the other that of the intrados. The first stone of the ribs is shown at thebottom of the figure, and its section above : A is the keystone, and its plan is shown at B.

Fig. 2556. HEXAGONAL VAULTING WITH BIBS.

A great improvement was effected by the masons of the middle ages in the constructionof polygonal vaults: the groins are always covered, as well as supported by ornamentalmouldings, and the ribs so formed carry the vault above them; we may imagine the ribs,after they were built up, to constitute a centre or support, upon which the covering was tobe laid, and a beautiful result is produced from the expression given to the construction,differing so materially from the former practice of concealing the ribs, or covering them upin the mass of concrete which constituted the vault.

We have several examples of light stone or chalk filling up the spaces between the ribs,sometimes laid to constitute an arch from one to the other, forming a groin in thecentre of each compartment, or pitching against a smaller rib in the middle, producing