Bomb-proof protection.
The penetration of projectiles varies according to the nature of the
soil--the lighter the better for protection. Sand offers the greatest
resistance to penetration, clay the least. Since, however, the
penetration of heavy shells fired from long ranges with high elevation
may be 20 ft. or more in ordinary soil, we can no longer look to earth
alone as a source of protection against bombardment. Again a moderate
quantity of earth over a casemate increases the explosive effect of a
shell by "tamping" it, that is by preventing the force of the explosion
from being wasted in the open air. We find therefore that in most modern
designs the tops of casemates are left uncovered, or with only a few
inches of earth over them, in which grass may be grown for concealment.
For the materials of casemates and revetment walls exposed to fire,
concrete (q.v.) has entirely replaced masonry and brickwork, not because
of its convenience in construction, but because it offers the best
resistance. The exact composition of the concrete is a matter that
demands great care and knowledge. It should be, like an armour plate,
hard on the surface and tough within. The great thickness of 10 ft. of
concrete for casemate arches, very generally prescribed on the continent
in important positions, is meant to meet the danger of several
successive shells striking the same spot. To stop a single shell of any
siege calibre in use at present, 5 ft. of good concrete would be enough.
A good deal is expected from the use of "reinforced concrete" (that is
concrete strengthened by steel) both for revetment walls and casemates.
Parapets.
Parapets are frequently made continuous or glacis-wise, that is the
superior slope is prolonged to the bottom of the ditch so that the whole
rampart can be swept by the fire of the defenders from the crest, and
there is no dead ground in front of it. It is also common to build the
crest of the parapet in solid concrete, with sometimes a concrete
banquette, so that bombardment shall not destroy the line the defenders
have to man in repelling an assault. This concrete parapet may be
further reinforced by hinged steel bullet-proof plates, to give head
cover; which when not in use hang down behind the crest.
[Illustration: From General Rocchi's _Traccia per lo studio della
fortificazione_, by permission.