A. Turn the pinion on the gun-arm to move the scale of the range correction-device until 2150 is set. (The zero of this scale = 2000.) (By doing this it is readily seen that the gun-arm range-scale is just 150 yards nearer the gun center; consequently all ranges read on this scale will be 150 yards more than if the range correction-device were at zero.) The azimuth correction is set by turning the micrometer until the number of even degrees of the azimuth correction (in this case one degree less) is read on the main scale, and the hundredths on the micrometer. (Thus it is seen that the gun-arm will read as many degrees and hundredths more or less than the true azimuth as the number of degrees and hundredths of the azimuth correction determined.)
Note.—Having determined by the ballistic board the range and azimuth corrections, they will usually answer for some time and thus avoid continual setting of these corrections on the gun-arm.
Q. What is the object of the travel-devices for range and azimuth correction on the gun-arm?
A. These are to determine the amount of change of range and azimuth between each observation of the target. The results thus obtained are given to the range- and deflection-board operators, who use it in finding the total range correction and the total azimuth correction.
Q. What are all plotting-boards principally used for?
A. For finding the position of a target whereby the range and the azimuth of it from any other point (as a directing-gun of a battery) can be determined.
Q. What is meant by the scale of a plotting-board?
A. By the scale of a plotting-board is meant, one inch on the board is equal to one, two, or so many yards on the ground; e.g., a scale of "one inch equals 300 yards" means, one inch distance on the board equals 300 yards on the ground.
Q. How can you determine the distance between two points on a plotting-board?