ure of which is that it enables the thickness of the wall to be
gauged accurately, and also provides a fixing for the first course of
slabs. Figs. 4 and 5 show such slabs for internal and external angles,
and Fig. 6 shows one for straight work. The use of a wall-base slab is
not essential, although it is the more accurate method of building, for
in cases where it is desirable to economize labor, or from other causes,
the slabs forming the first course may be made with a thicker base, and
may be fixed by a deposition of concrete, which is allowed to set behind
them. The second course of slabs is laid upon the first course with
breaking joints of half-slab bond, each course being keyed to the other
by means of a quick-setting cementing material poured into the key-holes
provided in the edges of the slab for that purpose, a bituminous cement
being preferred. The key-holes are made in several ways, those shown in
the illustrations being of a dovetail shape; circular, square, or indeed
holes of any other shape formed in the edges of the slab and in an
oblique direction are also employed. Special slabs for cants, or
squint-quoins (Figs. 17 and 18) and angles (Figs. 12, 13, 14, 15, and
16) are manufactured, the angle occurring (if we omit the hexagonals and
take the 18 inch slab) at three-quarters the length of each slab. This
gives a half-slab bond to each course, as on one face of the quoin in
one course will appear a quarter slab and in the course above a
three-quarter slab superimposed upon it, or _vice versa_. Thus are the
walls in Figs. 19 and 20 built up. For openings, the jambs and lintels
(and in window-openings the sill) are made solid with a provision for a
key-hole to the mass of concrete filling behind them. That portion of
the jambs against which the slabs butt has a groove coinciding with a
similar one in the edge of the slab, for the purpose of forming a joggle
joint by squeezing the bedding material into them or by joggling them in
with a cement grout. All the slabs are joggled together in a similar
way.

[Illustration: FIG. 21.-FIG 25.]

The plastic concrete filling or beton which the shells are made to
contain may be deposited between the slabs when any number of courses
(according to convenience) have been built up, and when set practically
forms with the solid work introduced a monolith, to which the face slabs
are securely keyed. With over-clayed Portland cements, which are known
to contract in setting,