Thus, a cylindro-cylindric groin, is a groin in which the cylindric portion is wider than the cylindroid. In ON species 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 posited.
When the two portions of a groin are of equal width, the groin is either cy lindric or cylindroidic, ac cordingly as the portions are both cylinders or both cy lindroids.
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, denominates 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-c•lindric arch is that in which the spheric portion surmounts the cylindric portion,or that the prin cipal or master vault is a sphere, and the other which perforates it a cylinder.
A cylindro•spherie arch is one in which the higher arch is a portimrof a cylinder, and the other a portion of a sphere, as must be the case with a spherical headed niche in a cylindric wall, or in a cylindric vault.
It does not appear that the Greeks made use of vaults or arches prior to the Roman conquest; but from that period they employed not only plain vaults with cylindri cal intradosses, but also quadrilateral equal-pitched groined vaults, with cylindrical or cylindroidal int•ados scs, 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. The Pantheon, which is generally reputed to have been built by Agrippa, son-in-law to Augustus, (though some writers maintain that he only added the portico), is one of the earliest remaining structures with arches : it consists of a hemispherical cavity enriched with coffers, and ter minates upwards in an aperture called the cyc 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 fur nish 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 lintel than a built dome.
Vitruvius observes, (Book iii. 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 foi in, which he distinguished by the tern, te•tudo. This mode of vaulting is very flat, and has four cm yed sides springing from the four walls, ap proaehing hearly to the form of a flat dome upon a rect angular plan.
From the remains of Roman buildings we also observe that their ceilings were vaulted over their apartments, as may be seen in the chapels of the temple of Peace, and the side apartments of Dioclesian's baths, which are fur nished with vaults having cylindrical intradosses ; 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 summits rise but a small dis tance above the springing of the middle groins. The piers between the chapels also have small arcades, the summits of which arc considerably below 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 Bacchus, where, as in the temple of Peace and the baths of Dioclesian, the summits of the arcades, supporting the cylindric wall and dome of the central apartment, du not intersect the annular intrados, but this convex side of the cylindric wall which supports it, consequently they do not form groins.