KNOWLEDGE AND USE OF THE PLOTTING-BOARD AND AZIMUTH INSTRUMENT.
Q. Describe the plotting board.
A. It is the same as that used for guns except the gun arm and its parts are not used. The scale is ordinarily 100 yards = 1 inch.
Q. How is it used for mine firing?
A. The ship is tracked by plotting points of intersection of the primary and secondary arm every 20 seconds. After several positions are plotted a point is predicted at a certain interval of time ahead and by means of a "combination prediction and speed scale" the time of the arrival of the ship over the mine which was previously plotted on the board is obtained and this is the time of firing that mine. This is called "judgment firing."
Q. Describe and use the azimuth instrument.
A. See Examination for First-class Gunners for Gun-companies.
BATTERIES, GENERATORS, AND SEARCH-LIGHTS.
Q. Name the batteries used in mine work.
A. Casemate battery (storage—40 cells) and boat telephone battery (either storage or dry cells sufficient to give 15 volts).