When an explosion of nitro-glycerine or dynamite occurs in one of these
buildings, the sides are generally blown out, and the roof is raised some
considerable height, and finally descends upon the blown-out sides. If, on
the other hand, the same explosion had occurred in a strong brick or stone
building, the walls of which would offer a much larger resistance, large
pieces of brickwork would probably have been thrown for a considerable
distance, and have caused serious damage to surrounding buildings.

It is also a very good plan to surround all danger buildings with mounds
of sand or earth, which should be covered with turf, and of such a height
as to be above the roof of the buildings that they are intended to protect
(see frontispiece).[A] These mounds are of great value in confining the
force of the explosion, and the sides of the buildings being thrown
against them are prevented from travelling any distance. In gunpowder
works it is not unusual to surround the danger buildings with trees or
dense underwood instead of mounds. This would be of no use in checking the
force of explosion of the high explosives, but has been found a very
useful precaution in the case of gunpowder.

[Footnote A: At the Baelen Factory, Belgium, the danger buildings are
erected on a novel plan. They are circular in ground plan and lighted
entirely from the roof by means of a patent glass having wire-netting in
it, and which it is claimed will not let a splinter fall, even if badly
cracked. The mounds are then erected right up against the walls of the
building, exceeding them in height by several metres. For this method of
construction it is claimed that the force exerted by an explosion will
expand itself in a vertical direction ("Report on Visits to Certain
Explosive Factories," H.M. Inspectors, 1905).]

In Great Britain it is necessary that all danger buildings should be a