The =Breech-sight= (Fig. 41) consists of a tangent scale affixed to a circular base revolving in a plane normal to the axis of the piece to compensate for inclination of trunnions when on uneven ground. The scale moves laterally on its base to correct for wind and drift, and bears a spirit-level, _f_. Along the vertical limb, _C_, of the sight, which is slotted, moves an eye-piece, _D_, actuated by a screw, _d_. This eye-piece (bearing the peep-sight _d′_) extends on either side of the slot, the lower edge being horizontal and bevelled. The one on the right is graduated into ten equal parts. The vertical limb being graduated into degrees (0 to 15°), and then sub-divided by diagonals into sixths, a diagonal scale of equal parts reading to minutes is obtained. The rear sights now being issued have range scales in _yards_ for shell and shrapnel engraved upon the sight in addition to the degree scale. The scale of yards for shrapnel on the rear face of the sight, left side, is arranged with diagonal lines, to be read in the same manner as the degree scale. Each diagonal embraces 250 yards of range, and there are five divisions on the upper edge of the slide, by which this space is sub-divided, so that the intervals of 50 yards in range are read directly from the scale. The scale of yards for shell, on the front face of the sight, is marked for each 100 yards of range, and is read from an index line on the slide. The horizontal limb, or cross-bar, _e_, which is actuated by the screws _e′e′_, is graduated into spaces, each of which corresponds to a deviation equal to 1/345 of the range. (In future constructions this division will correspond to 1/1000 of the range.) This variation is tabulated in the range table under the heading "Deflection for one division of the horizontal scale." By means of a pivot, _B_, the sight proper is attached to the stem _A_, the axis of rotation being at the zero of the scale. The stem tapers so as to fit into the sight-socket fixed at the breech. A pin projecting from the stem, and fitting in a recess in the socket, insures the sight being properly placed on the gun. The sight is first levelled and clamped by means of the clamp-screw _a_ before sighting. Vertical and horizontal changes for elevation, drift, etc., are given by the thumb-screws _d_ and _e′e′_. THE POINTING ARC. (Fig. 41_a_.) This instrument, now under consideration for use with field-artillery, to replace wholly or in part the rear sight, is made principally of bronze, and comprises the arc _a_, the slide _b_, and the level-piece _c_. The arc has two small seating plates, _d d_, secured to its base by countersunk screws. The slide is connected to the arc by a dovetail-joint, and moves over it, the friction of a spring keeping the slide in any desired position. The tension of this spring is adjusted by the two screws on the side of the slide. [Illustration: FIG. 41_a_.]