CIVIL ARCHITECTURE. 



iron cramp*. For more copious information, Sec MA- 



mnur. 



VAULTS, DOMES, or GROINS. A vault is an interior 

 roof, ruing in concave direction from the walU on 

 which it rests, either in a continued arch from side to 

 tide, in which case the section is semicircular, or a sec- 

 tion of a circle less than a semicircle, or else meeting 

 the vertex in a point, or line, as when the section is Go- 

 thic. 



The concavity, or interior surface of the vault, is call- 

 ed the intrailos ; which, 'in simple vaulting, generally con- 

 sists of the portion of the surface of a cylinder, cylin- 

 druid, or sphere, never exceeding half the solid ; while 

 the springing lines, from which the vault rises, and by 

 which the walls are terminated, are generally straight, 

 and parallel to the axis. 



Vaults having a horizontal straight axis, are called 

 straight vaults ; and those with their axes horizontal are 

 termt-d horizonlul vaulls. 



The exterior, or convex curve of the superior surface 

 of an arch, is called the i:rtnulox. 



The vertical sections of the intradosscs of vaults may 

 be formed in curves of an infinite variety ; but the most 

 elegant are circular or elliptic, and have been adopted 

 among the Romans, by some of our ancestors of the 

 middle ages, and by the nations of modern Europe. To 

 these, therefore, we shall at present confine our observa- 

 tions. 



A cylindrical, or cradle vault, consists of a plain arch ; 

 the figure of whose extrados is a portion of a cylindrical 

 surface, terminating on the top of the walls which sup- 

 port it, in a horizontal plane parallel to the axis of the 

 cylinder. 



A cylindroidal vault consists also of a plain arch, the 

 figure of whose extrados springs from a horizontal plane, 

 but its section perpendicular to those lines is every where 

 a semiellipsis, equal and similar throughout, having its 

 base that of either axis ; otherwise, it is sometimes the 

 segment of an ellipsis less than a semiellipsis, having an 

 ordinate parallel to the axis for its base. 



A vault, rising from a circular, elliptical, or polygo- 

 nal plan, with a concavity within and a convexity with- 

 out, so that all horizontal sections of the intrados may 

 be of similar figures, having their centres in the same 

 vertical line, or common axis, is called a dome. 



Various names are given to domes, according to the 

 figure of their plan, as polygonal, circular, or elliptic. 

 Circular domes may be either spherical, spheroidal, el- 

 lipsoidal, hyperbolical, parabolical, &c. Such as rise 

 higher than the radius of the base are called surmounted 

 dome*, and such as are below this altitude are termed 

 diminit/ied, or suriased domes. If a dome be a portion 

 of a sphere, that is, if its base be a circle, and its verti- 

 cal section through the centre of its base the segment of 

 a circle, it is called a cupola. A spherical dome, or cu- 

 pola, may be intersected by a cylindric vaulting in any 

 direction 5 and the intersection will always be circular, 

 provided the axis of the cylinder tend to the centre of 

 the sphere, because every aection of a sphere made by 

 a plane is a circle, as is also every section of a right cy- 

 linder perpendicular to the axis. Suppose, therefore, 

 the sphere to b cut by a plane forming a section equal 

 to that of the cylinder, and the two sections applied to- 

 gether, the right line drawn from the centre of the cir- 

 cle, which is the section of the sphere, to the centre of 

 such sphere, will be perpendicular to the plane of this 

 section ; and, since the axis of the cylinder is also per- 

 pendicular to the same plane, it will be in the same right 

 Lee with the remainder of the radius of the sphere. 



From this we deduce, that, when the axis of a eylindri- P.-aciir*. 

 cat vaulting is horizontal, and tend* to that of a spheri- '""^i * 

 cal vault, their intersection must be in the circumference 

 o! a circle, whose plane will be perpendicular to the ho- 

 rizon ; and hence those beautiful sphero-cylindrical 

 groins, so greatly and justly admired in our principal 

 buildings. 



Upon this principle, any building that has a polygo- 

 nal base may be made to terminate a circle, and sustain 

 a cupola, or cylindric wall ; for, if the tops of the side 

 walls of the polygon be brought to a level, and equal 

 segments of circles, whether semicircles or less portions, 

 be raised on the top, meeting in the lines of intersection 

 of the sides of the polygon, and if the angular spaces 

 between the circular-headed walls be made good to the 

 level of the summit of the arches, so as to coincide with 

 the circumference of a great circle of the sphere, they 

 will terminate in a ring at the level of the summit of the 

 arches, and be portions of the sphere, called by our work- Spandrtls 

 men sfxiiidrels, and by the French jxntlentivcs. On the . r linden 

 ring so formed, a cornice is usually laid, on which the tlTl - 

 cylindric wall or dome is raised. 



The plans of apartments intended to be covered with 

 cupolas, are, in general, either square or octangular. 

 The pendentives are likewise commonly equal in number 

 to the angles of the walls ; but this is not essential, be- 

 cause, in polygonal plans, arches may be thrown across 

 the angles, to double the number of the sides of the po- 

 lygon, still preserving the equal sides. Over the middle 

 of the walls, equal and similar arches may be built, that 

 shall touch those across the angles at the bottom, and 

 have their tops in the same level : Or, instead of walls, 

 piers may be carried to an adequate height upon each 

 angle of the polygon, and return upon either side of it. 

 Archivolts may then be turned over every two adjacent 

 piers, and the spandrels be filled in to the level of the 

 summits of the arches or archivolts, as before, and the 

 termination will be a circle on the inside, as already 

 stated. 



There do not appear any instances among the Roman No amin 

 buildings, of pendentives or spandrels being supported penden- 

 by four pillars, or by quadrangular or polygonal walls, tiv ** 

 and which support themselves on a spherical dome or 

 a cylindrical wall. Pendentives rising from pillare, and 

 surmounted with a dome, were originally introduced in 

 the celebrated church of St Sophia at Constantinople. 

 St Paul's, and St Stephen's, Walbrook, London, are 

 beautiful specimens of this sort. 



When two or more plain vaults penetrate or intersect Croim 

 each other, with their summits in the same horizontal scribed, 

 plane or level, the figure of the intrados, formed by the 

 several branches of the vaults, is called a groin : In other 

 words, a groin is a vault in which two geometrical solids 

 may be transversely applied, one after another, so that a 

 portion of the groin will have been in contact with the 

 first solid, and the remainder with the second when the 

 first is removed, and that the summit of the one may 

 intersect that of the other. This definition will be 

 found almost universal, as it applies not only to plain 

 vaults intersecting each other, but also to such as are 

 annular, or in the form of eemi-cylindric rings, intersect- 

 ed by cylindric or cylindroidal plain vaults, whose axes 

 tend to that of the annuli ; but it does not include the 

 species used in the chapel of Henry VII. at Westmin- 

 ster, and in King's College chapel at Cambridge, where, 

 instead of the horizontal sections of the curved surfaces 

 presenting exterior right angles, as is generally the case, 

 they present convex arches of circles. 



A property common to every kind of groins, is, that 



