CONSTRUCTION X. THE ARCH LINE 159

studying b and d, as subordinate to and connected with the simple arch at c.

§ 11. Many architects, especially the worst, have been very curious in designing out of the way arches,-elliptical arches, and four-centred arches, so called, and other singularities. The good architects have generally been content, and we for the present will be so, with God’s arch, the arch of the rainbow and of the apparent heaven, and which the sun shapes for us as it sets and rises. Let us watch the sun for a moment as it climbs: when it is a quarter up, it will give us the arch a, Fig. 31; when it is half up, b, and when three quarters up, c. There will be an infinite number of arches between these, but we will take these as sufficient representatives of all. Then a is the low arch, b the central or pure arch, c the high arch, and the rays of the sun would have drawn for us their voussoirs.

§ 12. We will take these several arches successively, and fixing the top of each accurately, draw two right lines thence to its base, d, e, f, Fig. 31. Then these lines give us the relative gables of each of the arches; d is the Italian or southern gable, e the central gable, f the Gothic gable.

§ 13. We will again take the three arches with their gables in succession, and on each of the sides of the gable, between it and the arch, we will describe another arch, as at g, h, i. Then the curves so described give the pointed arches belonging to each of the round arches; g, the flat pointed arch, h, the central pointed arch, and i, the lancet pointed arch.

§ 14. If the radius with which these intermediate curves are drawn be the base of f, the last is the equilateral pointed arch, one of great importance in Gothic work. But between

[Version 0.04: March 2008]