hey are nailed close
together and the joints planed off. When centres are required to be
taken down, the wedges upon which the centre rests are first removed so
as to allow the arch to take its bearing gradually. Centres for brick
sewers and vault arching are formed in the same way as previously
mentioned, with ribs and laggings, but the thickness of the timbers
depends upon the weight to be carried.

[Illustration: FIG. 22.--Centering for Stone Arch.]

[Illustration: FIG. 23.--Single Floor.]

_Floors._--For ordinary residential purposes floors are chiefly
constructed of timber. Up to about the year 1895 nearly every modern
building was constructed with wood joists, but because of evidence
adduced by fire brigade experts and the serious fires that have occurred
fire-resisting floors have been introduced. These consist of steel
girders and joists, filled in with concrete or various patented brick
materials in accordance with such by-laws as those passed by the London
County Council and other authorities. The majority of the floors of
public buildings, factories, schools, and large residential flats are
now constructed of fire-resisting materials. There are two descriptions
of flooring, single and double.

[Illustration: FIG. 24.--Floor pugged to resist passage of sound.]

Single flooring.

Single flooring (fig. 23) consists of one row of wood joists resting on
a wall or partition at each end without any intermediate support, and
receiving the floor boards on the upper surface and the ceiling on the
underside. Joists should never be less than 2 in. thick, or they are
liable to split when the floor brads are driven in; the thickness varies
from 2 to 4 in. and the depth from 5 to 11 in. (see _By-laws_, below),
the distance between each joist is usually 12 in. in the clear, but
greater strength is obtained in a floor by having deep joists and
placing them closer together. These floors are made firm and prevented
from buckling by the use of strutting as mentioned hereafter.

The efficiency of single flooring is materially affected by the
necessity which constantly occurs in practice of trimming round
fireplaces and flues, and round well holes such as lifts, staircases,
&c. Trimming is a method of supporting the end of a joist by tenoning it
into timber crossing it; the timber so tenoned is called the trimmer
joist, and the timber morticed for the tenon of the trimmer is called
the trimming joist, while the interme