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Hence, if the piers at the ends are of large dimensions, they will hold the voussoirs together, and make such works durable. Having taken heed in these matters to see that proper attention is paid to them, we must also be equally careful that all walls are perfectly vertical, and that they do not lean forward anywhere.
But the arch head has a natural organism, which separates its ornament into distinct families, broadly definable. LXIX. It is easily conceivable that when two ranges of voussoirs were used, one over another, it would be easier to leave those beneath, of a smaller diameter, than to bevel them to accurate junction with those outside.
In the farther development of this, which we have next to consider, the voussoirs, in consequence of their own negligence or over-confidence, sustain a total and irrecoverable defeat. That archivolt is in its earliest conditions perfectly pure and undecorated, the simplest and rudest of Gothic forms.
The other line, which is the line of resistance, or line of good behavior, may or may not be consistent with the outward and apparent curves of the arch; but if not, then the security of the arch depends simply upon this, whether the voussoirs which assume or pretend to the one line are wide enough to include the other.
An arch whose line of resistance is in the middle of its voussoirs is perfectly safe: in proportion as the said line runs near the edge of its voussoirs, the arch is in danger, as the man is who nears temptation; and the moment the line of resistance emerges out of the voussoirs the arch falls. § VIII. There are, therefore, properly speaking, two arch lines.
Now so long as an arch is pure circular or pointed, it does not matter how many joints or voussoirs you have, nor where the joints are; nay, you may joint your keystone itself, and make it two-pieced.
§ XVII. We have traced the various conditions of treatment in the archivolt alone; but, except in what has been said of the peculiar expression of the voussoirs, we might throughout have spoken in the same terms of the jamb.
One is the visible direction or curve of the arch, which may generally be considered as the under edge of its voussoirs, and which has often no more to do with the real stability of the arch, than a man's apparent conduct has with his heart.
The proportion of voussoirs to arches naturally depended upon the size of the arch, large voussoirs to large arches, small voussoirs to small arches.
Whatever its form, if we merely let the ends of the bars into the voussoirs of its heading, the least settlement of the masonry would distort the arch, or push up some of its voussoirs, or break the window bars, or push them aside.
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