A. It is formed of four segments of a circle, and is described from four centres, two placed within the arch on a level with the spring, and two placed on the exterior of the arch, and level with the apex or point (fig. 8); each side is composed of a double curve, the lowermost convex and the uppermost concave. [Illustration] Q. When was the ogee arch introduced, and how long did it prevail? A. It was introduced early in the fourteenth century, and continued till the close of the fifteenth century. Q. How is the Tudor arch described? A. From four centres; two on a level with the spring, and two at a distance from it, and below. (fig. 9.) Q. When was the Tudor arch introduced, and why is it so called? A. It was introduced about the middle of the fifteenth century, or perhaps earlier, but became most prevalent during the reigns of Henry the Seventh and Henry the Eighth, under the Tudor dynasty, from which it derives its name. [Illustration] Q. What other kinds of arches are there worthy of notice? A. Those which are called foiled arches, as the round-headed trefoil (fig. 10), the pointed trefoil (fig. 11), and the square-headed trefoil (fig. 12). The first prevailed in the latter part of the twelfth and early part of the thirteenth century, chiefly as a heading for niches or blank arcades; the second, used for the same purpose, we find to have prevailed in the thirteenth century; and the latter is found in doorways of the thirteenth, fourteenth, and fifteenth centuries. In all these the exterior mouldings follow the same curvatures as the inner mouldings, and are thus distinguishable from arches the heads of which are only foliated within. [Illustration: DOORWAY. St. Thomas’s, Oxford, circa 1250.] [Illustration: Anglo-Saxon Doorway, Brixworth Church, Northamptonshire. (7th cent.)] CHAPTER III. OF THE ANGLO-SAXON STYLE. Q. During what period of time did this style prevail? A. From the close of the sixth century, when the conversion of the Anglo-Saxons commenced, to the middle of the eleventh century. Q. Whence does this style appear to have derived its origin? A. From the later Roman edifices; for in the most ancient of the Anglo-Saxon remains we find an approximation, more or less, to the Roman mode of building, with arches formed of brickwork. Q. What is peculiar in the constructive features of Roman masonry? A. Walls of Roman masonry in this country were chiefly constructed of stone or flint, according to the part of the country in which the one material or other prevailed, embedded in mortar, bonded at certain intervals throughout with regular horizontal courses or layers of large flat Roman bricks or tiles, which, from the inequality of thickness and size, do not appear to have been shaped in any regular mould. [Illustration: Portion of the Fragment of a Roman Building at Leicester.] Q. What vestiges of Roman masonry are now existing in Britain?