CIVIL ARCHITECTURE. 



629 



practice. 



Bectang'.i- 

 In groiu. 



Multangu- 

 lar jjroiii. 



Equianpu- 

 Ur groin. 



Equilateral 



the several branches intersect and form arches of equal 

 height upon each inclosing wall, the perpendicular sur- 

 face of which is continued on both sides, till intercepted 

 by the intrados of the arches ; consequently, the upright 

 of each wall is equal in height to that of the common 

 apex of the arches. This forms a striking and charac- 

 teristic difference between domes and groins : the latter 

 is a branched vault, terminating in each branch against 

 the inclosing wall ; whereas the former is a vault without 

 branches, having its curves springing from all points of 

 the wall or walls around the bottom of its circumference, 

 ivhether upon a polygonal, circular, or elliptical plan. 

 Another property of the dome is, that all its horizontal 

 sections are similar figures, whether made by the exterior 

 or interior surface. 



Groins are variously denominated, according to the 

 surfaces of the geometrical bodies which form the simple 

 vault, viz. 



When the axes of the simple vaultl are in two vertical 

 planes, crossing at right angles to each other, they form 

 a rectangular groin. 



When three or more simple vaults of one common 

 height pierce each other, and form a complex vault, in 

 such a manner that if the surfaces of the several solids 

 of which each is formed were respectively applied, one 

 at a time, to succeeding portions of the surface of the 

 complex vault, each portion of the complex vault would 

 come in contact with certain corresponding portions of 

 the surface of each of the solids : the complex surface 

 thus generated is called a multangular groin. 



When the several axes of the simple vaults form equal 

 angles around the same point, and when each of the 

 vaults are of the same width, the surface of the solid is 

 called an equiangular groin. 



When the breadths of the cross vaults, or openings of 

 a groined vault are equal, the groin is said to be equila- 

 teral. 



The species of every groin, formed by the intersec- 

 tion of two vaults of unequal width, is denoted by two 

 preceding words ; the first ending in o, indicates the 

 simple vault of the greater width, and the second ter- 

 minating with ic, denotes the simple vault of the less 

 width. 



Thus, a cylindro-cylindric groin, is a groin in which 

 the cylindric portion is wider than the cylindroid. In 

 this ipecies of groin, the section of the cylindroidic part 

 has its lesser axis horizontally posited. 



A cylindroido-cylindric groin is one which has the 

 greater axis of the cylindroidic part horizontally posi- 

 ted. 



When the two portions of a groin are of equal 

 width, the groin is either cylindric or cylindroidic, ac- 

 cordingly as the portions are both cylinders or both cy- 

 liudroids. 



When one vault pierces another of less height, the 

 angle formed at the intersection is called an arch, and 

 its species is indicated by the two preceding words, as 

 in groins ; that ending in o, indicates the simple vault 

 of the greater height, and the other ending in ic, deno- 

 minates that of the lesser height. 



A cylindro- cylindric arch, is an arch made of two 

 cylindric portions ; but the portion indicated by the 

 word ending in o, is higher than the portion ending in 

 ic. 



A sphero-cylindric arch is that in which the spheric 

 portion surmounts the cylindric portion, or that the 

 principal or master vault is a sphere, and the other which 

 perforates it a cylinder. 



A cyliodro-spheric arch is one in which the higher 

 3 



arch is a portion of a cylinder, and the other a portion Practice* 

 of a sphere, as must be the case with a spherical headed N ~ "V"''' 

 niche in a cylindric wall, or in a cylindric vault. 



It does not appear that the Greeks made use of vaults Creeks did 

 or arches prior to the Roman conquest ; but from that fl0t make 

 period they employed not only plain vaults with cylin- vaul "> * c - 

 drical intradosses, but also quadrilateral equal-pitched 

 groined vaults, with cylindrical or cylindroidal intrados- 

 ses, or a mixture of both, as may be observed over the 

 passages of the theatres and gymnasia. 



The dome was invented by the Romans or Etrurians. Dome R- 

 The Pantheon, which is generally reputed to have been man. ' 

 built by Agrippa, son in-law to Augustus, (though some 

 writers maintain that he ouly added the portico), is one 

 of the earliest remaining structures with arches : it con- 

 sists of a hemispherical cavity enriched with coffers, and 

 terminates upwards in an aperture called the eye of the 

 dome. The exterior side rises from degrees or steps, 

 placed in a sloping direction, and forming nearly a tan- 

 gent to the several internal groins of the steps, present- 

 ing to the eye a truncated segment of a sphere, much 

 less than a hemisphere. This forms the general charac- 

 ter of the Roman dome. 



Domes were of very frequent use among the Romans, 

 as may be deduced from their groins, and the remains 

 of their ancient edifices. But ancient Greece does not 

 furnish a single example of a dome, if we except that 

 which covers the monument of Lysicrates, but which 

 being only of a single stone, may rather be deemed a lin- 

 tel than a built dome. 



Vitruvius observes, (Bookiii. chap. 3.) that the floors 

 of temples were frequently supported by vaults, and 

 (book v. chap. 1.) that the roofs of basilicas were vault- 

 ed in the tortoise form, which he distinguished by the 

 term Icsluilo. This mode of vaulting is very flat, and 

 has four curved sides springing from the four walls, ap- 

 proaching nearly to the form of a flat dome upon a rect- 

 angular plan. 



From the remains of Roman buildings we also ob- 

 serve that their ceilings were vaulted over their apart- 

 ments, as may be seen in the chapels of the temple of 

 Peace, and the side apartments of Dioclesian's baths, 

 which are furnished with vaults having cylindrical intra- 

 dosses ; while the great rectangular apartments, in 

 both these edifices, are vaulted with groins. Nor is it 

 a little remarkable that these groins are not formed by 

 the intradosses of the vaults of the chapel, whose sum- 

 mits rife but a small distance above the springing of the 

 middle groins. The piers between the chapels also have 

 small arcades, the summits of which are considerably be- 

 low the cylindrical intradosses of the side vaults ; a mode 

 to be discovered in many other buildings. 



The Romans used annular vaults, as in the temple of Annular 

 Bacchus, where, as in the temple of Peace and the baths vaults, 

 of Dioclesian, the summits of the arcades, supporting rcn n>le of 

 the cylindric wall and dome of the central apartment, do B * cchul ' 

 not intersect the annular intrados, but this convex tide 

 of the cylindric wall which supports it, consequently 

 they do not form groins. 



The intrado.Hses of the Roman domes, as we have al- General 

 ready hinted, are of a semicircular section, as in the Pan- observa- 

 theon, and the temple of Bacchus at Rome, the temple f ' oul 

 of Jupiter, and the vestibule of the palace of Dioclesian 

 at Spalatro ; while the vertical section of the extrados 

 through the axis, exhibits a much less segment, as may 

 be seen in the first and last of the examples quoted. 

 The latter observation, however, only applies to edifices 

 built prior to the reign of the emperor Justinian ; after 

 which period, from the completion of the dome of St. 



