ded with difficulty, especially when the
vessel is rolling and the line of fire can only be approximate.

496. On shore, the plan of giving the direction is to determine
practically two fixed points, which shall be in a line with the piece
and the object, and sufficiently near to be readily distinguished by
the eye. These points being covered by a plummet, is the vertical
plane including the line of metal, which becomes the plane of fire.

497. In mortar-vessels other expedients are resorted to, such as
trunnion-sights, or a white line painted on the mortar-bed parallel to
the axis of the bore when level; but the first plan is preferable.

498. The circles on which the mortars stand being fitted with
eccentrics, are made to revolve so as to point the mortar at the
object without the trouble of swinging the vessel or moving the mortar
round with handspikes.

499. Before firing, care must be taken that the eccentrics are thrown
out of gear, and the circle flat upon the platform on which it
revolves.

TABLES OF CHARGES, ELEVATIONS, AND RANGES FOR 13-INCH MORTARS.

500. CHARGES FOR 13-INCH MORTAR-BOMBS.

---------------------------------------------+-------------+
Charge. | 13-Inch. |
---------------------------------------------+-------------+
| lbs. oz. |
Of shell filled | 11 0 |
To burst shell | 6 0 |
To blow out fuze | 0 6 |
Ordinary service-charge | 7 0 |
Incendiary, match, or other composition | 0 8 |
---------------------------------------------+-------------+

501. RANGES WITH SEA-COAST 13-INCH MORTARS, 20 deg. ELEVATION.

-----------------------------------------------------------+
Charge. | Mean Time of | Least | Greatest | Mean |
| Flight. | Range. | Range. | Range. |
---------+--------------+------------+----------+----------+
Lbs. | Seconds. | Yards. | Yards. | Yards. |
4 | 8. | 840 | 877 | 869 |
6 | 9.5 | 1209 | 1317 | 1263 |
8 | 11.66 | 1653 | 1840 | 1744 |
10 | 12.50 | 2010 | 2128 | 2066 |
12 | 14.25 | 2369 | 2688 | 2528 |
14 | 15.25 | 2664 | 2780