ether, whatever their number, place, or size, as at _n_; and the
great value of the arch is, that it permits small stones to be used with
safety instead of large ones, which are not always to be had. Stones cut
into the shape of _k_, _l_, and _m_, whether they be short or long (I
have drawn them all sizes at _n_ on purpose), are called Voussoirs; this
is a hard, ugly French name; but the reader will perhaps be kind enough
to recollect it; it will save us both some trouble: and to make amends
for this infliction, I will relieve him of the term _keystone_. One
voussoir is as much a keystone as another; only people usually call the
stone which is last put in the keystone; and that one happens generally
to be at the top or middle of the arch.

Sec. V. Not only the arch, but even the lintel, may be built of many
stones or bricks. The reader may see lintels built in this way over
most of the windows of our brick London houses, and so also the
gable: there are, therefore, two distinct questions respecting each
arrangement;--First, what is the line or direction of it, which gives it
its strength? and, secondly, what is the manner of masonry of it, which
gives it its consistence? The first of these I shall consider in this
Chapter under the head of the Arch Line, using the term arch as including
all manner of construction (though we shall have no trouble except about
curves); and in the next Chapter I shall consider the second, under the
head, Arch Masonry.

Sec. VI. Now the arch line is the ghost or skeleton of the arch; or rather
it is the spinal marrow of the arch, and the voussoirs are the vertebrae,
which keep it safe and sound, and clothe it. This arch line the
architect has first to conceive and shape in his mind, as opposed to, or
having to bear, certain forces which will try to distort it this way and
that; and against which he is first to direct and bend the line itself
into as strong resistance as he may, and then, with his voussoirs and
what else he can, to guard it, and help it, and keep it to its duty and
in its shape. So the arch line is the moral character of the arch, and
the adverse forces are its temptations; and the voussoirs, and what else
we may help it with, are its armor and its motives to good conduct.

Sec. VII. This moral character of the arch is called by architects its
"Line of Resistance." There is a great deal of nicety in calculating it
with precision, just as there is sometimes in finding out ve