POWDER MAGAZINE is that place where gunpowder is kept in large quantities. The first powder magazines were made with Gothic arches; but Vauban finding them too weak, constructed them in a semicircular form. Their usual dimensions are sixty feet long within, and twenty-five broad; the foundations are eight or nine feet thick, and eight feet high from the foundation to the spring of the arch; and the floor is two feet from the ground, which secures it from dampness.