The Danger Area.—­That portion of the works that is devoted to the actual manufacture or mixing of explosive material is generally designated by the term “danger area,” and the buildings erected upon it are spoken of as “danger buildings.”  The best material of which to construct these buildings is of wood, as in the event of an explosion they will offer less resistance, and will cause much less danger than brick or stone buildings.  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 specified distance apart; a license also must be obtained.  The application for a license must give a plan (drawn to scale) of the proposed factory or magazine, and the site, its boundaries, and surroundings, and distance the building will be from any other buildings or works, &c., also the character, and construction of all the mounds, and nature of the processes to be carried on in the factory or building.[A]

[Footnote A:  Explosives Act, 38 Vict. ch. 17.]

[Illustration:  FIG. 1.—­SECTION OF NITRO-GLYCERINE CONDUIT. a, lid; b, lead lining; c, cinders.]