[Illustration] Q. How are they formed or described? A. The semicircular arch is described from a centre in the same line with its spring; the stilted arch in the same manner, but the sides are carried downwards in a straight line below the spring of the curve till they rest upon the imposts; the segmental arch is described from a centre lower than its spring; and the horse-shoe arch from a centre placed above its spring. Q. During what period of time do we find these arches generally in use? A. The semicircular arch, which is the most common, we find to have prevailed from the time of the Romans to the close of the twelfth century, when it became generally discarded; and we seldom meet with it again, in its simple state, till about the middle of the sixteenth century. It is in some degree considered as a characteristic of the Anglo-Saxon and Norman styles. The stilted arch is chiefly found in conjunction with the semicircular arch in the construction of Norman vaulting over a space in plan that of a parallelogram. The segmental arch we meet with in almost all the styles, used as an arch of construction, and for doorway and window arches; whilst the form of the horse-shoe arch seems, in many instances, to have been occasioned by the settlement and inclination of the piers from which it springs. Q. Into how many classes may the pointed arch be divided? A. Into two, namely, the simple pointed arch described from two centres, and the complex pointed arch described from four centres. Q. What are the different kinds of simple pointed arches? A. The LANCET, or acute-pointed arch; the EQUILATERAL pointed arch; and the OBTUSE-ANGLED pointed arch. Q. How is the lancet arch formed and described? A. It is formed of two segments of a circle, and its centres have a radius or line longer than the breadth of the arch, and may be described from an acute-angled triangle. (fig. 5.).[TN-1] Q. How is the equilateral arch formed and described? A. From two segments of a circle; the centres of it have a radius or line equal to the breadth of the arch, and it may be described from an equilateral triangle. (fig. 6.) [Illustration] Q. How is the obtuse-angled arch formed and described? A. Like the foregoing, it is formed from two segments of a circle, and the centres of it have a radius shorter than the breadth of the arch; it is described from an obtuse-angled triangle. (fig. 7.) Q. During what period were these pointed arches in use? A. They were all gradually introduced in the twelfth century, and continued during the thirteenth century; after which the lancet arch appears to have been generally discarded, though the other two prevailed till a much later period. Q. What are the different kinds of complex pointed arches? A. Those commonly called the OGEE, or contrasted arch; and the TUDOR arch. Q. How is the ogee, or contrasted arch, formed and described?