[9] Six-lined craters (_r_ = 3_l_) are generally considered as the practical limit of overcharged mines, although at Chatham mines have been fired giving _r_ = (3-3/4)_l_. The published data concerning them indicate that they required charges larger than the formulas above given call for. [10] Describe the semicircle _BMNF_. Then _OD_ : _ON_ :: _CH_ : _CM_; _l_ : _ON_ :: _v_ : _h_. _ON_^2 = (_h_^2 _l_^2) / _v_^2. (1) _BO_ : _ON_ :: _ON_ : _OF_; _h_ + _r_ : _ON_ :: _ON_ : _h_ - _r_. _ON_^2 = _h_^2 - _r_^2 (2) _CF_ : _CH_ :: _CD_ : _CK_; _h_ : _v_ :: √(_r_^2 + _l_^2) : _l_. (_h_^2)(_l_^2)/_v_^2 = _r_^2 + _l_^2 (3) ∴ Eqs. (1), (2), and (3), _r_^2 + _l_^2 = _h_^2 - _r_^2. _h_^2 = _l_^2 +2_r_^2 (4) _h_ = √(1 + 2(_r_/_l_)^2). * * * * * Eqs. (3) and (4), (_l_^2 + 2_r_^2)_l_^2 = _v_^2(_r_^2 + _l_^2). _v_^2 = (_l_^2 +2_r_^2)/(1+_r_^2/_l_^2) = _l_^2(1+2(_r_^2/_l_^2))/(1+_r_^2/_l_^2); _v_ = _l_(√((1+2(_r_/_l_)^2)/(1+(_r_/_l_)^2))). [11] General Abbot’s experiments in submarine mining fix the relative values of cannon powder and dynamite No. 1 in water, measured by the pressure exerted by them (not by the craters formed), as 1 : 2.45. The characters of the media and the explosives would naturally lead to the inference that the superiority of dynamite over powder would be greater in water than in earth.--J. M. [12] The 82 lb. dynamite-mine at Willet’s Point produced almost exactly the same effect upon the gallery of access as the 200-lb. cannon-powder mine, while its external crater was considerably less in diameter. Its crater was surrounded, however, by concentric cracks spaced at intervals of 3 or 4 feet to a distance of about 40 feet from the centre of the crater. No such effect was produced by the powder-mine. On the other hand, the actual radii of rupture produced by five experimental mines fired at Olmutz in 1871-2 agree very closely with the values which result from applying Lebrun’s formulas to craters of the same size and shape produced by gunpowder, and indicate that charges of dynamite and gunpowder which produce identical craters will also have identical radii of rupture. The somewhat contradictory results given by the Willet’s Point and Olmutz mines show the necessity for further experiments. [13] To pass a 5" siege-gun, mounted upon a high or “overbank” carriage (model 1887), requires a gallery 7' × 7' in clear. [14] Four of these may be unskilled laborers. [15] No. required at commencement of gallery. Beyond 4 feet add one man, and one additional for every 20 feet of gallery. [16] Instead of a truck a canvas bag may be used. A large hoe or drag may be used to draw back the earth from the face of the gallery. [17] One mason’s level. [18] These numbers are for small shafts of about 2' by 4'; large shafts require a larger force. They advance at about the same rate as galleries of equal cross-section.