POWDER MAGAZINE, is that place where the powder is kept in very large quantities. Authors differ greatly both with regard to their situation and construction; but all agree that they ought to be arched and bomb-proof. In fortifications, they are frequently placed in the rampart; but of late they have been built in different parts of the town. The first powder magazines were made with Gothic arches: but M. Vauban finding them too weak, constructed them in a semicircular form; whose dimensions are 60 feet long within, and 25 broad; the foundations are eight or nine feet thick, and eight feet high from the foundation to the spring of the arch, the floor is two feet from the ground, which keeps it from dampness.
One of our engineers of great experience some time since had observed, that after the centres of semicircular arches are struck, they settle at the crown and rise
up at the hanches, even with a straight horizontal extrados, and still much more so in powder magazines, whose outside at top is formed like the roof of a house, by two inclined planes joining in an angle over the top of the arch, to give a proper descent to the rain; which effects are exactly what might be expected agreeable to the true theory of arches. Now, as this shrinking of the arches must be attended with very ill consequences, by breaking the texture of the cement after it has been in some degree dried, and also by opening the joints of the voussoirs at one end, so a remedy is provided for this inconvenience with regard to bridges, by the arch of equilibration in Mr Hutton's book on bridges; but as the ill effect is much greater in powder magazines, the same ingenious gentleman proposed to find an arch of equilibration for them also, and to construct it when the span is 20 feet, the pitch or height 10 (which are the same dimensions as the semicircle), the inclined exterior walls at top forming an angle of 113 degrees, and the height of their angular point above the top of the arch equal to seven feet. This very curious question was answered in 1775 by the reverend Mr Wildbore, to be found in Mr Hutton's Miscellanea Mathematica.