y in hot weather, also when absorbent stone is used or when the
concrete is not rammed. In these cases the concrete should be allowed to
take all it can, but an excess of water which would flow away, carrying
the cement with it, should be avoided.

Mixing.

The thorough mixing of the constituents is a most important item in the
production of good concrete. Its object is to distribute all the
materials evenly throughout the mass, and it is performed in many
different ways, both by hand and by machine. The relative values of hand
and machine work are often discussed. Roughly it may be said that where
a large mass of concrete is to be mixed at one or two places a good
machine will be of great advantage. On the other hand, where the mixing
platform has to be constantly shifted, hand mixing is the more
convenient way. In hand mixing it is usual to measure out from gauge
boxes the sand, stones and cement or lime in a heap on a wooden
platform. Then they are turned once or twice in their dry state by men
with shovels. Next water is carefully added, and the mixture again
turned, when it is ready for depositing. For important work and
especially for thin structures the number of turnings should be
increased. Many types of mixing machines are obtainable; the favourite
type is one in which the materials are placed in a large iron box which
is made to rotate, thus tumbling the matrix and aggregate over each
other again and again. Another simple apparatus is a large vertical pipe
or shoot in which sloping baffle plates or shelves are placed at
intervals. The materials are fed in at the top of the shoot and fall
from shelf to shelf, the mixing being effected by the various shocks
thus given. When mixed the concrete is carried at once to the position
required, and if the matrix is quick-setting Portland cement this
operation must not be delayed.

Moulds.

One of the few drawbacks of concrete is that, unlike brickwork or
masonry, it has nearly always to be deposited within moulds or framing
which give it the required shape, and which are removed after it is set.
Indeed, the trouble and expense of these moulds sometimes prohibit its
use. It is essential that they shall be strong and stiff, so as not to
yield at all from the pressure of the wet concrete. The moulds for the
face of a wall consist generally of wooden shutters, leaning against
upright timbers which are secured by horizontal or raking struts to firm
ground, or t