between the diameters of the gun at the base-ring
and swell of the muzzle, or at any intermediate point on the line of
metal, will give the proper height of the dispart-sight at the point
where the least diameter was taken, to which must be added the height
of the lock-piece above the base-ring, in order to get a line of sight
over it, parallel to the axis of the bore.

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: