piston. They are internally mounted in a single assembly. The hydrostatic piston acts to open and close electrical switches. The explosive piston aligns the explosive train when the hydrostatic piston and the explosive piston are forced in by water pressure after the mine is laid. These pistons are held in the retracted or safe position by safety pins during ground handling. The safety pins are replaced by arming wire assemblies after the mines are loaded aboard the aircraft. After planting, a clock-delay mechanism prevents firing   until   the   mine   has   been   submerged   for   a predetermined period of time. A switch in the detonator circuit of the mine controls the firing mechanism. This switch  remains  open  until  the  clock-delay  device  has run its predetermined time. This time could vary from 8 minutes to 10 days, depending upon the type of clock installed.   Sometimes   another   clock   is   also   used   to sterilize  the  mine  after  a  preselected  period  of  armed life. Firing Components Firing components include elements that detect the target,  analyze  target  information,  and  act  to  fire  the mine.  In  simple  systems,  a  single  device  does  these functions.  However,  in  most  systems,  these  functions are performed by two or more components. The types of firing mechanisms used in mines vary widely in appearance and configuration. In older mines, the firing mechanism might be a circuit arrangement. In newer   mines,   the   firing   mechanism   might   be   a color-coded rectangular box. Since influence-actuated mechanisms are the only type  of  firing  mechanisms  currently  used  in  aircraft mines, they are discussed in the following paragraphs. Influence-actuated firing mechanisms may be subdivided   into   three   general   categories—acoustic, pressure, and magnetic. ACOUSTIC.—An  acoustic  mine  is  fired  by  the initiation of an enclosed microphone. The microphone picks up the sound waves generated by a nearby ship's rotating  screws  or  other  operating  machinery.  These mines are equipped with an anticountermine device that prevents detonation of the mine from explosions set off during minesweeping operations. PRESSURE.—The pressure mine firing mechanism is actuated by a decrease in water pressure immediately   surrounding   the   mine.   Normally,   this occurs  only  when  a  large  ship  passes  over  it.  The pressure mine firing mechanism is used in conjunction with   a   magnetic-influence   mechanism.   The   com- bination   of   these   two   mechanisms   makes   effective minesweeping operations nearly impossible. MAGNETIC TYPE.—Magnetic mines are induction  mines  actuated  by  changes  in  the  earth's magnetic  field.  Their  actuation  depends  primarily  on the  rate  at  which  the  field  changes  rather  than  the amount  of  change.  A  change  in  the  magnetic  field induces  an  electromagnetic  field  in  the  winding  of  a coil.   This   electromagnetic   field   and   the   resulting current  are  proportional  to  the  rate  of  change  of  the magnetic field. When the magnetic field increases to a predetermined  level,  a  relay  actuates  and  closes  the firing circuit. REVIEW NUMBER 2 ANSWERS A1. Mines classified by their intended use are classified as service, exercise (recoverable), and training mines. A2. Service mines are fully loaded with explosives and designated for use in wartime. A3. Moored or bottom mines are classified by the position  they  take  in  the  water  after  being laid. A4. The   effective   depth   of   a   bottom   mine   is controlled by the amount of charge contained in the mine relative to the depth to which the mine is planted. A5. A cable connected to an anchor resting on the ocean floor stabilizes moored mines. A6. Influence-actuated   mines   are   classified   as magnetic, acoustic, and pressure. Parachute Packs Some  mines  laid  from  aircraft  use  parachutes  to prevent them from building up too much speed before entering  the  water.  The  parachute  reduces  the  impact velocity   and   protects   the   mine   components   from damage. The use of parachute packs permits the mines to be released from an aircraft at high altitudes, which may be required when ports or sea passages are heavily armed. The parachute pack currently used is the delayed-opening type. In this type of parachute pack, a control  mechanism  is  used  to  open  the  parachute  at some point in the air trajectory of the mine. A typical delayed-opening  parachute  pack  contains  a  drogue- type parachute, release gear, and a control device. The 5-10