ses 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.

Sec. 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.

[Illustration: Plate V.
Arch Masonry.
BRULETTO OF COMO.]

Sec. XXII. In the above reasonings, the inclination of the joints of the
voussoirs to the curves of the arch has not been considered. It is a
question of mu