The guns of the Dahlgren pattern are cylindrical for a certain distance forward of the base-line, always giving a line of sight parallel to the axis of the bore.
298. All the new guns are marked on the top of the lock-piece, base-ring, the reinforce sight-mass, and the swell of the muzzle, by notches which indicate a vertical plane passing through the axis of the bore, at right angles to the axis of the trunnions.
299. Pivot-guns have been supplied with trunnion-sights, designed to be used when the ordinary sights do not give the required elevation. This instrument, however, gives but a rude approximation in either elevation or direction.
300. The rifled cannon in service have the breech-sight on the side of the breech and the front sight on the rimbase, which permits the gun to be accurately aimed and the object kept in view at all elevations. It is intended in future to apply this arrangement to all cannon mounted on pivot-carriages.
301. Various modes have been practised to ascertain at sea the distance from the object aimed at, so as to regulate the elevation of guns, but none can be depended upon for giving it with minute accuracy, and even when obtained it is continually varying; therefore, when the projectile is seen to exceed or fall short of the object considerably, the sight-bar must be readjusted accordingly. It thus becomes, under ordinary circumstances, the best instrument for approximating distances. In correcting the elevation, however, the variation of range to the first graze, attributable to eccentricity, differences of windage, and other causes, must be taken into consideration, as, under the most favorable circumstances, at the Experimental Battery of the Ordnance Yard, this variation is found to equal fifty yards, more or less.
302. In addition to the errors arising from these sources, we have also those due to the direction and force of the wind, the movement of the ship across the line of fire, and to sheering round a pivot when performing evolutions.
303. They can be obviated or diminished by the following means:
1st. Allow the Gun Captain to estimate the distance to windward or to leeward, right or left, to be allowed for the deflection; or,
2d. Indicate the number of yards right or left of the object; which, after all, depends on his estimation of distance.
3d. Furnish a sight which, in addition to the elevation, allows for the deviation, and permits the Gun Captain in all cases to aim directly at the target.