ment, or by a coping of any other waterproof and
fire-resisting material, properly secured."

Arches are constructions built of wedge-shaped blocks, which by reason of
their shape give support one to another, and to the [Sidenote: Arches.]
super-imposed weight, the resulting load being transmitted through the
blocks to the abutments upon which the ends of the arch rest. An arch
should be composed of such materials and designed of such dimensions as to
enable it to retain its proper shape and resist the crushing strain imposed
upon it. The abutments also must be strong enough to take safely the thrust
of the weighted arch, as the slightest movement in these supports will
cause deflection and failure. The outward thrust of an arch decreases as it
approaches the semicircular form, but the somewhat prevalent idea that in
the latter form no thrusting takes place is at variance with fact.

[Illustration: FIG. 14.]

Arches in brickwork may be classed under three heads: plain arches,
rough-cut and gauged. Plain arches are built of uncut bricks, and since the
difference between the outer and inner periphery of the arch requires the
parts of which an arch is made up to be wedge-formed, which an ordinary
brick is not, the difference must be made in mortar, with the result that
the joints become wedge-shaped. This obviously gives an objectionable
inconsistency of material in the arch, and for this reason to obtain
greatest strength it is advisable to build these arches in independent
rings of half-brick thickness. The undermost rings should have thin joints,
those of each succeeding ring being slightly thickened. This prevents the
lowest ring from settling while those above remain in position, which would
cause an ugly fissure. In work of large span bonding blocks or "lacing
courses" should be built into the arch, set in cement and running through
its thickness at intervals, care being taken to introduce the lacing course
at a place where the joints of the various rings coincide. Stone blocks in
the shape of a voussoir (fig. 14) may be used instead. Except for these
lacing courses hydraulic lime mortar should be used for large arches, on
account of its slightly accommodating nature.

Rough-cut arches are those in which the bricks are roughly cut with an axe
to a wedge form; they are used over openings, such as doors and windows,
where a strong arch of neat appearance is desired. The joints are usually
made equal in width to those