o anything that will bear the weight. If a smooth and neat
face is wanted other precautions must be taken. The shutters must be
planed, and coated with a mixture of soap and oil, so as to come away
easily after the concrete is set. Moreover, when depositing the
concrete, a shovel or other tool must be worked between the wet concrete
and the shutter. This draws sand and water to the face and prevents the
rough stones from showing themselves. Sometimes rough concrete is
rendered over with a plaster of cement and sand after the shutters have
been removed, but this is liable to peel off and should be avoided.

Depositing.

The method of depositing depends on the situation. If for important
walls, or for small scantlings such as steel concrete generally
involves, the concrete should be deposited in quite small quantities and
very carefully rammed into position. If for massive walls, it is usual
to tip it out in large quantities from a barrow or wagon, and simply
spread it in layers about a foot thick. Depositing concrete under water
for breakwaters and bridge foundations requires special skill and
special appliances. It is usually done in one of three ways:--(a) By
moulding the concrete ashore into large blocks, which, when sufficiently
hard, are lowered through the water into position by a crane or similar
machine with the aid of divers. The most notable instance of this type
of construction was at the port of Dublin, where Mr B. B. Stoney made
blocks no less than 350 tons in weight. Each block formed a piece of the
quay wall 12 ft. long and 27 ft. high, being made on shore and then
deposited in position by floating sheers of special design. (b) By
moulding the concrete into what are called "bag-blocks." In this system
the concrete is filled into bags, which are at once lowered through the
water like the blocks. But in this case the concrete being still wet can
adapt itself more or less to the shape of the adjoining bags, and strong
rough walls can be built in this way. Sometimes the bags are made of
enormous size, as at Aberdeen breakwater, where the contents of each bag
weighed 50 tons. The canvas was laid in a hopper barge and there filled
with the concrete and sewn up. The enormous bag was then dropped through
a door in the bottom of the barge upon the breakwater foundation. (c) By
depositing the wet concrete through the water between temporary upright
timber frames which form the two faces of the wall. In this case