2gO SERVICE OF ARTILLERY

O, P, where the walls intersect, and under the pilasterswhich support the fronts AB, CD, and on which the roofrests (97, No. 4). The chambers in the wall FF whichis without any break, are distant from each other, and fromthe principal mines, 12 or 15 times the lenght of the fideor diameter (97, No. 5); and if in the faces A B, CD,the chambers A, H, I, be too far distant to have propereffect, two intermediate ones S, S may be made. The wallGG in which are the chimnies and doors of communi-cation, need not be undermined ; but the size of the princi-pal chambers K, L, M, should be increased by about J. Themines thus disposed may be divided into two classes ; viz.those made in single walls, as Q_R, 8, and those made incross walls, as A H, &c.

100. In this profile P P represents the plane of the gidundsurrounding a building : (PI. 4, Fig. 5) the walls are single,and the chambers are to be of a cubical form : to makethe excavation in the foundation D A E, below the planeP P (97, No. 1, 2), let a long trench be dug 1 £ of thethickness of the wall D E in depth as D B, and 3s feetin width as CD. Let the chamber G of a cubical form,be made } of BD distant from the bottom B F, and awooden box of proper dimensions for containing the quan-tity of powder ascertained by preceding experiments (98)placed in it; and if there be any reason to apprehend thatthe powder may be damaged by the moisture of the wall,let the box be lined with oil-cloth. At the mouth of thechamber make a ledge H, 3 inches broad, and of a depthproportioned to the size of the planks that are to close theopening. Any projections or cavities in the wall wherethe trench is dug, should be smoothed or filled up withwell worked plaister. The planks for blocking up thechamber should be 2 inches, and the beams 5 or 6 inchesthick, and a quantity of clay and rotten dung, (which isexcellent for preventing the escape of the inflamed fluid)prepared. The box being filled with powder, and a sau-cisson laid in a pitched wooden trough, communicatingwith it, the opening is exactly closed ; planks called mant-lets as C F K L, B D I M, are placed vertically alongthe faces of the trench, and the beams N are driven inhorizontally at right angles to the mantlets, and all theinterstices filled with clay and dung well rammed.

101. To