SET-FORWARD RULER.
Q. Describe the set-forward ruler and explain its use.
A. First find the travel in yards per minute. Set the pointer (a) on the slide (b) at the number of yards on the scale of "yards travel per minute (c)." Then on gun-arm get time of flight for that point. The "set-forward point" will be the reading opposite time of flight on the scale of "yards travel during time of flight +1 minute (d)." (See Fig. 81.)
Example.—After taking four observations on a target we find that in one minute's time it has traveled 200 yards. Set the pointer (a) at 200 yards on the scale (c). On the gun-arm we see that the time of flight for this point is sixty seconds. Therefore our set-forward point is 400 yards, as this is the reading exactly opposite the time of flight on scale (d). (See Fig. 81.)
Q. Describe and explain the use of the prediction scale.
A. The prediction scale is graduated in the same manner as the gun-arm (1" = 300 yards), and is used for finding the predicted point. After having marked four points on the board, showing the course of the target, place the prediction scale so that zero (0) is on the last point, or reading, and then mark off as many yards in advance of the last point as the first reads from zero. This point is known as the predicted point, and is used by the range officer only. As soon as the predicted point is found he sets his azimuth instrument at the given azimuth and when the target crosses the vertical wire in the instrument, he gives the signal "Fire." (See Fig. 82.)
GUN ARM FOR MORTARS.