order  by  moving  an  answering  pointer  into  the same  sector  as  the  requesting  pointer  from  the bridge. Engine-Revolution Indicator Near the engine-order telegraph, you normally will   find   another   device,   the   engine-revolution indicator (fig. 18-6). On the face of this instrument are three small windows   with   two   rows   of   numbers   in   each window.   The    lower    row    of    numbers    is    set individually  by  three  hand  knobs  located  directly below  the  windows.  For  example,   if   110   shaft revolutions  per  minute  is  required  for  a  ship  to proceed  at  15  knots,   the   lee   helmsman   would indicate the numbers 1, 1, and 0 in the lower row of  numbers.  These  lower  numbers  give  a  visual indication of the  shaft  revolutions  ordered  by  the conning   officer   to   the   engine   room.   Through electrical   transmission,   corresponding   numbers appear  on  a  similar  instrument  in   the   engine room(s).  In  the  engine  room(s),  these  orders  are receipted   and   acknowledged   when   the   engine- room instrument is set on the same settings. Once again  this  indication  is  transmitted  back  to  the bridge electrically and is shown on the upper row of  numbers.  Thus,  the  lee  helmsman  is  able  to report  to  the  conning  officer  the  receipt  of  the order  for  engine  speed  and  see  that  the  order  is being carried out. 7.122 Figure 18-6.-Engine revolution indicator. The number of revolutions per minute required to   travel   at   the   various   speeds   (in   knots)   is calculated in advance and is posted on a table near the lee helm. On older  ships  the  helm  and  the  lee  helm  are located  in  different  consoles,  usually  near  each other.  On  newer  ships,  however,  a  ship-control console houses all  the  equipment  for  steering  the ship  and  for  controlling  its  speed  in  one  central location. Additionally, on some ships you  will find lighting,   steering,   and   general-alarm   controls housed in the ship-control console. COMPASSES The best known and most widely used naviga- tional   instrument   is   the   compass.   Without   it, precise  information  on  headings  and  directions would  be  almost  impossible  to  obtain.  Compasses were used even before the days of Columbus, and they remain indispensable to today’s Navy. A compass is an instrument that indicates  the fixed  point  or  direction  of  north.  It  allows  you  to judge  all  other  directions  by  this  fixed  point  to determine the direction in which you are heading. The  Navy  uses  two  main  types  of  compasses: gyroscopic    and    magnetic.    The    gyrocompass operates  on  the  principle  that  a  rapidly  spinning object is balanced at its center of gravity, much as a    spinning    top    stands    on    its    point.    The gyrocompass   is   designed   to   point   toward   true north,  although  it  may  have  a  slight  mechanical error  (for  which  an  allowance  is  made).  On  the other  hand,  the  magnetic  compass  is  controlled primarily by the magnetic properties of the earth; therefore,  it  tends  to  point  toward  the  magnetic north pole. GYROCOMPASS The   gyrocompass   is   unaffected   by   magnetic influence. When in proper working order, it points constantly  to  the  true  rather  than  the  magnetic north pole. The  gyrocompass  may  have  a  slight  mechani- cal   error   of   1   or   2   degrees,   but   the   error   is computed easily. Since the error remains constant 18-6