Q. Describe how to obtain the range of a target from the primary or secondary station when the azimuth-angles from the primary and secondary stations are given to you.
A. First: Set the auxiliary-arm index to read the number of even degrees the target is from the secondary station, setting the arm-clamp in the V-shaped notch on the azimuth-circle corresponding to that number of degrees.
THE MODERN PLOTTING-BOARD.
Fig. 61.
Second: Set the index-disc to read the hundredths by turning the index-knob and clamp the index. The auxiliary arm is now set; therefore the secondary arm is set automatically in azimuth by virtue of its always remaining parallel to the auxiliary arm.
Third: Set the primary arm to read the number of degrees and hundredths the target is from the primary station. (The point of intersection of the fiducial or bevel edges of the primary and secondary arms is the position of the target on the plotting-board.)
Fourth: Slide the metal intersection-block or "targ" along the secondary arm until it touches the edge of the primary. The range in yards can now be readily read on the scales marked on these arms. (Fig. 62.)