=Line of Departure.=--The line in which the projectile is moving when it leaves the gun. [Illustration: _Elevation._ FIG. 102.] [Illustration: _Plan._ FIG. 103.] =Angle of Departure.=--The angle made by the line of departure with the horizontal. (_d_ in Fig. 102.) =Angle of Elevation.=--The angle made by the axis of the piece with the horizontal. (_q_ in Fig. 102.) =The Jump.=--The difference between the angle of elevation and the angle of departure, owing to the movement of the gun at discharge. (_j_ in Fig. 102.) (The jump of 3.2 in. and 3.6 in. guns varies from 20 minutes at 1 degree to 30 minutes at 10 degrees elevation.) =Initial Velocity.=--The velocity of the projectile at the muzzle. =Remaining Velocity.=--The velocity at any point of the trajectory. =Final Velocity.=--The velocity at the end of the range. =Range.=--The horizontal distance from the muzzle to the point where the projectile strikes. =Drift.=--The departure of the projectile from the plane of fire. With guns having a right-handed twist it is to the right, and its extent varies nearly as the square of the range. =Direct Fire= is from guns with service charges at all angles of elevation not exceeding 15°. =Indirect or Curved Fire= is from guns with less than service charges, and from howitzers and mortars, at all angles of elevation not exceeding 15°. =High-angle Fire= is from guns, howitzers, and mortars, at all angles exceeding 15°. =Front or Frontal Fire= is that which is directed perpendicularly, or nearly so, to the general line of troops fired at. [Illustration: FIG. 104.] =Oblique Fire= is that which is directed obliquely to the line fired at; it is more searching than front fire. =Enfilade Fire= is that which rakes the enemy's line of troops, the gun being on the prolongation of the line. It is the most effective fire. =Flanking Fire= is one directed along the front of, or nearly parallel to, the line to be flanked or defended. =Reverse Fire= is when the object is fired at from the rear. [Illustration: FIG. 105.] =Cross-fire= is where the projectiles from guns in different positions cross one another at a particular point of ground. (See Fig. 105 for these cases.) PROBABILITY OF FIRE. Absolute certainty of hitting the same spot at each round is impossible of attainment; and accuracy of fire is therefore a comparative term. The =Probability of Fire= measures the chance of hitting a given target. It is determined for any gun by firing a certain number of shot at a given range, measuring carefully the ranges and dividing the sum by the number of shots, which gives the _mean range_.