Gothic architecture in France, England, and Italy . 24 THE GOTHIC VAULT [CH. II The groin-ing line Let us take first an example of a cross-vault overa square bay of moderate size, say that of the crypt atWinchester which is about 14/X14/. The four equalbounding arches are represented by A, B, C, D in Fig. 4,of which one A, E, B is set up in elevation over its two half-cylinders of the vault being equal in span. fV Fig. 4. the groins will lie true in a straight line between ACand DB. On the base AC we set up the elevationof the diagonal groin, of which the crown M will ofcourse be leve
Gothic architecture in France, England, and Italy . 24 THE GOTHIC VAULT [CH. II The groin-ing line Let us take first an example of a cross-vault overa square bay of moderate size, say that of the crypt atWinchester which is about 14/X14/. The four equalbounding arches are represented by A, B, C, D in Fig. 4,of which one A, E, B is set up in elevation over its two half-cylinders of the vault being equal in span. fV Fig. 4. the groins will lie true in a straight line between ACand DB. On the base AC we set up the elevationof the diagonal groin, of which the crown M will ofcourse be level with E. The curve of the oroin is foundby what are called ordinates. The direct arch A, E, B,which is the true section of the half-cylinder, is dividedon the base line AB into equal parts at F, G, H, I, andthe diagonal likewise at J, K, L, M. Then if J—J is ch. n] THE GOTHIC VAULT 25 made equal in height to F—F, K—K to G—G, L—Lto H—H, and M—M to I—E, a line drawn through theirtops will give the true curve of the groin, which will bean ellipse. This is the usual vault of Norman crypts, and Ellipse ofNorman aisles and all Romanesque vaults of small dimen-sions ; and the crown of the arches being all on thesame level its convenience is obvious, for in one caseit carries the floor of the church, and in the other that ofthe triforium. A glance, however, tells us that the ellipse of the Weaknessdiago
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