§ XIX. Now let us take a pair of side pieces, made to order, like that at _l_, and see what we can make of them. We will try to fit them first with a keystone which continues the 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_. § XX. 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 distorted 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. § XXI. 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 may, perhaps, be of some service to us afterwards; here we have nothing more to do with it than to note that, thus built, and properly backed by brickwork, it is just as strong and safe a form as that at _n_; but that this, as well as every variety of ogee arch, depends entirely for its safety, fitness, and beauty, on the masonry which we have just analysed; and that, built on a large scale, and with many voussoirs, all such arches would be unsafe and absurd in general architecture. Yet they may be used occasionally for the sake of the exquisite beauty of which their rich and fantastic varieties admit, and sometimes for the sake of another merit, exactly the opposite of the constructional ones we are at present examining, that they seem to stand by enchantment.