Column 15 is headed "Change of range for motion of target in plane of fire of 10 knots,’ and column 18 is headed “Deviation for lateral motion of target perpendicular to line of fire, speed of 10 knots.” Actually, column 15 gives the distance that the target will move in the line of fire, if its speed in the line of fire is 10 knots, during the time of flight of the projectile. Column 18 gives the same information for motion across the line of fire. The values in the columns are derived by multiplying the speed of 10 knots, expressed as 16.89/3 yards per second, by the time of flight of the projectile for the given range, as found in column 4 of the range table.
Example: Find the errors in range and deflection caused by motion of a target, the components of whose speed are 16 knots in the line of fire and 13 knots across the line of fire, the firing ship being on the starboard bow of the target, range 10,000 yards.
Solution: Since the values in both columns 15 and 18 are found by multiplying speed components of 10 knots by time of flight, these values are identical, and are in this case 124 yards.
Then, in range, 16/10 X 124 = 198 yards that the target will move toward the firing ship during the time of flight. The correction to compensate for this motion must be DROP 198 YARDS.
Across the line of fire the target will move 13/10 X 124 = 161 yards to the right, and the correction to compensate for this motion must be to the right also.
It should be noted that target motion toward the firing ship will cause an error which is over in direction; also that target motion to the right will cause the projectile to fall to the left of the target position at the end of the time of flight. The correction, in each case, is in the direction of motion of the target.