s
the cement; and these two must be shoveled together several times.
While this is being done, the broken stone or gravel must be wet, and
now it is put on top of the sand and cement and well shoveled
together, with just enough water added so that the mass will almost
bear the weight of a man.

Concrete is impatient to be hardening, and if it is not put into the
right place, it will begin promptly to harden in the wrong place, and
nothing can be done with it afterwards. If it is to be made in blocks,
the moulds must be ready and the concrete put into them at once and
well tamped down. For such uses as beams and the sides of tanks where
great strength is needed, the cement is often "reinforced," that is,
rods of iron or steel are embedded in it. For floors, a sheet of woven
wire is often stretched out and embedded. At first only solid blocks,
made to imitate rough stone, were used for houses, but the hollow
block soon took their place. This is cheaper; houses built this way
are warmer in winter and cooler in summer; and it prevents moisture
from working through the walls. Many cities have regulations about the
use of hollow blocks, all the more strict because concrete is
comparatively new as a building material. In Philadelphia the blocks
must be composed of at least one barrel of Portland cement to five
barrels of crushed rock or gravel. They must be three weeks old or
more before being used; the lintels and sills of the doors must be
reinforced; and every block must be marked, so that if the building
should not prove to be of proper strength, the maker may be known.
There would seem, however, to be little question of the quality of the
blocks, for samples must pass the tests of the Bureau of Building
Inspection.

Even better than the hollow block is the method of making the four
walls of a house at once by building double walls of boards and
pouring in the concrete. When this has hardened, the boards are
removed, and whatever sort of finish the owner prefers is given to the
walls. They can be treated by spatter-work, pebble dash, or in other
ways before the cement is fully set, or by bush hammering and tool
work after the cement has hardened. Coloring matter can be mixed with
the cement in the first place; and if the owner decides to change the
color after the house is completed, he can paint it with a thin cement
of coloring matter mixed with plaster of Paris.

A concrete house has several advantages. In the first