ARCHITECTURE. 



figures, having their centres in the same 

 vertical line or axis, and such that the 

 plans of any two sections may have the 

 sides of similar inscribed figures parallel 

 to each other, or that the figures of these 

 plans may be concentric. If the dome is a 

 portion of a sphere, that is, if its base be a 

 circle, and its vertical section through the 

 centre of its base the segment of a circle, 

 then it is called a cupola. 



When the portion of a sphere, or cupo- 

 la, springs from a wall on a polygonal plan, 

 and the vertical axis of the sphere passes 

 through the middle of the plan, then the 

 spandrels, or triangular spheric portions, 

 comprehended between the springing 

 lines and a horizontal plane passing 

 through the different summits of the walls, 

 are called pendentives. 



When two or more plain vaults pene- 

 trate or intersect each other, the figure 

 of the intradoes formed by the several 

 branches is called a gvoin, or cross vault. 

 When two opposite equal branches 

 meet other two opposite equal branches 

 in two intersecting vertical planes, pass- 

 ing through the diagonal lines, joining 

 the four exterior angles of the plane, the 

 groin may be called an equal pitched qua- 

 drilateral groin. 



If two opposite branches of an equal 

 pitched groin have cylindrical intradoes, 

 and their plan of less breadth than that 

 of the other two branches, the groin may 

 be called cylindro-cylindroidal, or cylin- 

 droido cylindric groin, according as the 

 cylindric branches or the other two are 

 of the greatest breadth. 



When a groin consisting of four bran- 

 ches is made by two equal portions of 

 cylindric surfaces, with the axis of the one 

 cutting that of the other, it is called an 

 equal pitched cylindric groin. 



When two opposite branches of a cylin- 

 dric groin are of less breadth than the 

 other two, it may be called an unequal 

 pitched cylindric groin. This is called by 

 workmen a Welch groin. 



When the branches of a cylindric groin 

 are of equal breadth in the plan, the groin 

 may be called an equilateral cylindric 

 groin. 



It is not easy to give a geometrical de- 

 finition that will extend to all properties 

 of vaulting, called, by writers of the first 

 eminence, groins. The first given is al- 

 most universal. It applies not only to 

 plain vaults intersecting each other, but 

 also to those that are annular, or in the 

 form of semi-cylindric rings, intersected 

 by cylindric or cylindroidal plain vaults, 

 the axis of which tends to that of the an- 



nulus. It does not, however, comprehend 

 that species used in King Henry VII's 

 chapel, Westminster, and King's College 

 chapel, Cambridge. 



This species of groins, instead of the 

 horizontal sections of the curved surfaces 

 presenting exterior right angles, as is ge- 

 nerally the case, present convex arches 

 of circles. There is yet one property 

 that is common to every species of groins, 

 that is, the several branches intersect and 

 form arches upon each inclosing wall, 

 and the perpendicular surface of the wall 

 upon each side is continued till it is inter- 

 cepted by the intradoes of the arches; 

 consequently the upright of each wall is 

 equal in height to the summit of the arch- 

 es. Hence the difference between groins 

 and domes. A groin is a branched vault, 

 and each branch terminates against the 

 enclosing walls ; whereas a- dome is a 

 vault without branches, and the curves 

 spring from the wall, or walls, from all 

 points around its bottom circumference, 

 whether the walls stand upon a polygonal, 

 circulaF, or elliptic plan. 



The Greeks, it would appear, had few 

 or no arches or vaults much prior to the 

 reign of Augustus, from which time they 

 sometimes employed plain vaults with 

 cylindrical intradoes ; we also find that 

 they used quadrilateral, equal pitched 



froined vaults, with cylindrical or cylin- 

 roidal intradoes, or mixed of both, over 

 the passages of the theatres and gym- 

 nasia. 



The Romans, as would appear also, 

 did not employ vaults mor6 early than the 

 Greeks. The Pantheon, one of the earli- 

 est remaining structures with arches, was 

 probably built by Agrippa, the son-in-law 

 of Augustus, though some maintain that 

 he only added the portico ; but of this 

 there is no proof, as no mention is made 

 of this celebrated building before his time. 

 We find from Vitruvius (lib. iii.c. 5,) that 

 the floors of temples were frequently sup- 

 ported by vaults, and (lib. v. c. 1,) that 

 the roofs of basilicas were vaulted of the 

 tortoise form, which he distinguishes by 

 the name of testudo. This form of vault- 

 ing is very flat, with four curved sides 

 springing from each of the four walls, and 

 it approaches nearly to that of a flat dome 

 upon a rectangular plan. 



We also find, from the remains of Ro- 

 man buildings, the ceilings of their apart- 

 ments vaulted. The side apartments, or 

 chapels, of the Temple of Peace, and of 

 the baths of Dioclesian, have vaults with, 

 cylindrical intradoes, while the great rec- 

 tangular apartment in each of these edifl- 



