ry precisely
what is a man's true line of moral conduct; but this, in arch morality
and in man morality, is a very simple and easily to be understood
principle,--that if either arch or man expose themselves to their
special temptations or adverse forces, _outside_ of the voussoirs or
proper and appointed armor, both will fall. 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.

Sec. VIII. There are, therefore, properly speaking, two arch lines. 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 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.

Sec. IX. Now when the reader is told that the line of resistance varies
with every change either in place or quantity of the weight above the
arch, he will see at once that we have no chance of arranging arches by
their moral characters: we can only take the apparent arch line, or
visible direction, as a ground of arrangement. We shall consider the
possible or probable forms or contours of arches in the present Chapter,
and in the succeeding one the forms of voussoir and other help which
may best fortify these visible lines against every temptation to lose
their consistency.

[Illustration: Fig. XXX.]

Sec. X. Look back to Fig. XXIX. Evidently the abstract or ghost line of
the arrangement at A is a plain horizontal line, as here at _a_, Fig. XXX.
The abstract line of the arrangement at B, Fig. XXIX., is composed of
two straight lines, set against each other, as here at _b_. The abstract
line of C, Fig. XXIX., is a curve of some kind, not at present
determined, suppose _c_, Fig. XXX. Then, as _b_ is two of the straight
lines at _a_, set up against each other, we may conceive an arrangement,
_d_, made up of two of the curved lines at _c_, set against each other.
This is called a poi