In the 19th century blockhouses were usually designed to give partial
protection against field artillery; the walls being built of two
thicknesses of logs with earth between them, the roof flat and covered
with 2 or 3 ft. of earth, and earth being piled against the walls up
to the loopholes. Nowadays they are employed only in positions where
it is not likely that artillery will be brought against them: but they
may be made tenable for a while even under artillery fire if they are
surrounded by a trench and parapet.
Blockhouses are especially useful for small posts protecting such
points as railway bridges, which the enemy may attempt to destroy by
cavalry raids. The essential feature is a bullet-proof loopholed wall,
arranged for all-round fire, with enough interior space for the
garrison to sleep in. The roof may be simply weatherproof. Some
arrangement for storing water must be provided. Circular blockhouses
were very popular in South Africa. They were made of sheets of
corrugated iron fastened 6 in. apart on a wooden framework, the space
between the sheets being filled with small stones. The loopholes were
made of sheet-iron frames inserted in the walls. Fig. 97 shows a
section of one of these blockhouses.
[Illustration: By permission of the Controller H.M. Stationery Office.