The steel pin on the under surface of the slide moves in the groove on the upper surface of the arc, and, by abutting against the steel stop-pins _e_ at each end of the groove, prevents the slide from being moved off the arc. The arc is graduated from 0 to 20 degrees, but readings to 2 minutes are obtained by means of the vernier, _f_, at the rear of the slide. Plates, graduated in yards for shell or shrapnel, are on either side of the arc. They are removable, being graduated for the piece with which the arc is used. The zero index-marks on the sides of the slide are supplemented by two others equidistant, plus 2 degrees and minus 2 degrees. Any one of the three may be used in setting the slide to a desired range in yards. The top of the slide has a scale, graduated 3 degrees on either side of the zero, with sub-divisions of 6 minutes; the markings plus and minus are the reverse of those on the side of the slide. The vernier, _g_, on the level-piece permits of a least reading of 2 minutes being obtained. Motion is given the level-piece by the deflection-screw _h_. To set the pointing arc, make the zero of the level-piece coincide with the zero of the slide-scale, and then take the required elevation in yards on the side desired. Should the piece and object fired at be on different levels, and the angle known, the index of the level is moved to indicate this angle--to the plus side if an elevation, and to the minus side if a depression; then take the elevation in yards as before. The correction is made in this way for 3 degrees or less, but for angles above 3, up to 5 degrees, the 2-degree marks on the side of the slide must be used, the index mark of the level being moved to coincide with the mark corresponding to the number of degrees--elevation or depression--less 2 degrees. The plus 2-degree mark is used as an index when the object has an elevation, and the minus 2-degree mark when a depression. To obtain the angle corresponding to difference of level, sight the piece on the object by the ordinary sight, using any elevation; set the pointing arc at this elevation, and move the level-piece by the deflecting-screw until the air-bubble is centred. The index of the level-piece then points to the desired angle--plus if an elevation, and minus if a depression. If the wheels of the carriage be not on the same level the gun will shoot towards the lower wheel. A correction for this is made by a deflection-scale. The correction for deflection due to inclination of trunnions, _expressed in units of the deflection-scale_, is, at any range, equal to the product of the angle of elevation and the angle of inclination of the trunnions, both expressed in degrees, into a constant multiplier. When the divisions of the deflection scale are 1/500 of the distance between the sights this multiplier is 0.1523; when the divisions are 1/1000 it is 0.3046. The rear sight must be moved towards the higher wheel.