To ascertain when this frame is level, a circular spirit level is attached thereto, a telescope _d_ provided with one horizontal and three vertical cross wires, supported on Y's, admitting of vertical motion and attached to an upright _e_. A mill-headed screw _f_ enables the telescope _d_ to be raised or lowered; the telescope, which is rigidly connected to a vernier _g_, traversing over a graduated arc _h_, can be moved rapidly in a lateral direction by means of a rack and pinion arrangement _i_, and it can be clamped in any position by means of the screw _h_. Sights are fixed on the telescope in a vertical plane passing through its axis. To the outer rim of the frame of the arc, which is smooth, are secured the sights _l l_ (shown on a large scale at Fig. 68), to give the direction of the mines. These sights are provided each with a brass point of V form, _m_, and a binding screw, _n_, in metallic connection with each other, but insulated by means of an ebonite plate from the rest of the metal of the sight. One end of a short piece of insulated wire is attached to the binding screw _n_, and the other passes through a hole in the base of the sight and projects below it; _o_ is a brass tube rigidly connected to and moving with the upright carrying the telescope _d_, and projecting in front of this latter. A brass spring _p_ (see Fig. 69) is attached to, but insulated from the outer extremity of this tube, and is so arranged as to make contact with the V point _m_ on the sight, by means of a corresponding projection fitted to its under side. An insulated wire passing the tube _o_, the outer end of which is connected to a screw on the spring _p_, forms a metallic connection between this projection and the firing key. At Fig. 68 is shown an enlarged view of the front of the sight; in addition to the V projection _m_, and binding screw _n_, it is fitted with a capstan-headed screw to bear against the inner rim of the frame, and a thin wire upright _t_ for giving the alignment of the mine, to which a disc is attached, on which the number of the mine is affixed. When the distance between the station and the mine is only about one mile, an ordinary eyepiece is used in the place of the telescope _d_. At Fig. 67 is represented the arc employed at the converging station, which with the exception of there being no tube _o_, and only one sight, is precisely similar in construction to the one used at the firing station, and which has been described. [Illustration: APPARATUS FOR FIRING BY OBSERVATION. PLATE XVII]