its true place is in heavy
foundations, retaining walls, and such like, and then perfectly
independent of other material. Arches, thin walls, and such like are
very questionable structures in continuous concrete, and are on record
rather as failures than otherwise. This may to a certain degree be due
to the high coefficient of expansion Portland cement concrete has by
heat. This was found by Cunningham to be 0.000005 of its bulk for one
degree Fahrenheit. It is a matter which any intelligent observer may
remark, the invariable breakage of continuous concrete sidewalks,
while those made in small sections remain good. This may be traced to
expansion and contraction by heat, together with friction on the lower
side.

In foundations, according to the same authority above quoted, properly
made Portland cement concrete may be trusted with a safe load of 25
tons per square foot.

In large masses concrete should be worked continuously, while in small
masses it should be moulded in small sections, which should be
independent of each other and simply form artificial stones.

The facility with which concrete can be used in founding under water
renders it particularly suitable for subaqueous structures. The method
of dropping it from hopper barges in masses of 100 tons at a time,
inclosed in a bag of coarse stuff, has been successfully employed by
Dyce Cay and others. This can be carried on till the concrete appears
above water, when the ordinary method of boxing can be employed to
complete the work. This method was employed in the north pier
breakwater at Aberdeen, the breakwater being founded on the sand, with
a very broad base. The advantage of bags is apparent in the leveling
off of an uneven foundation. In breakwater works on the Tay, in
Scotland, where the writer was engaged, large blocks perforated
vertically were employed. These were constructed below high water
mark, and an air tight cover placed over them. They were lifted by
pontoons as the tide rose, and conveyed to and deposited in place, the
hollows being filled with air, serving to give buoyancy to the mass.
After placing in position the vertical hollows were filled with
concrete, so binding the whole together--they being placed vertically
over each other.

As mentioned before, continuous stretches of concrete in small
sections should be guarded against, owing to expansion by heat; but
the fact of a few cracks appearing in heavy masses of concrete should
not ca