CONSTRUCTION XI. THE ARCH MASONRY 173

curve of the outer arch, as at m. This the reader assuredly thinks an ugly arch. There are a great many of them in Venice, the ugliest things there, and the Venetian builders quickly began to feel them so. What could they do to better them? The arch at m has a central piece of the form r. Substitute for it a piece of the form s, and we have the arch at n.

§ 20. This arch at n is not so strong as that at m; but, built of good marble, and with its pieces of proper thickness, it is quite strong enough for all practical purposes on a small scale. I have examined at least two thousand windows of this kind and of the other Venetian ogees, of which that at y (in which the plain side piece d is used instead of the cusped one) is the simplest; and I never found one, even in the most ruinous palaces (in which) they had had to sustain the distored weight of falling walls) in which the central piece was fissured; and this is the only danger to which the window is exposed; in other respects it is as strong an arch as can be built.

It is not to be supposed that the change from the r keystone to the s keystone was instantaneous. It was a change wrought out by many curious experiments, which we shall have to trace hereafter, and to throw the resultant varieties of form into their proper groups.¹

§ 21. One step more: I take a mid-cusped side piece in its block form at t, with the bricks which load the back of it. Now, as these bricks support it behind, and since, as far as the use of the cusp is concerned, it matters not whether its weight be in marble or bricks, there is nothing to hinder us from cutting out some of the marble, as at u, and filling up the space with bricks. (Why we should take a fancy to do this, I do not pretend to guess at present; all I have to assert is, that, if the fancy should strike us, there would be no harm in it.) Substituting this side piece for the other in the window n, we have that at w, which

¹ [This subject is worked out in ch. vii. of the next volume, §§ 24-49.]

[Version 0.04: March 2008]