III.  When  the  ship  is  in  a  closed  harbor  and  no  distant
object  can  be  seen,  a  standard  compass  is  taken  on  shore  and
placed  in  such  a  position  as  to  be  free  from  the  influences
of  magnetic  attraction,  and  where  it  may  be  seen.    As  the  ship
swings  around  the  bearing  of  the  compass  on  shore  is  observed
from  the  ship  as  her  head  comes  to  each  point,  and  at  the  same
instant,  indicated  by  signals,  the  ship's  compass  is  observed
from  shore.     The  observations  on  shore  are  first  reversed  to
bring  them  into  the  same  direction  as  those  taken  from  the
ship,  and  are  compared  with  the  latter;  the  difference  is  the
deviation.     Should  there  be  a  suspicion  of  local  attraction  to
the  compass  on  shore  a  plane  table  may  be  used  instead  of  the
shore  compass.
IV.  When  at  sea  or  out  of  sight  of  land  if  a  distant  sail  be  in
sight,  advantage  may  be  taken  of  a  calm  or  light  airs  to  swing


LO


SIM  I'M-!    l-il.KM  \  \VH..\TION.
ship  by  it  as  in  the   -ccond  case  or   liy  the   true    hearing  of  the
MIM  explained  in  Chapter  X.
Thr  deviation  for  the  ship's  head  at  eaeli  of  the  points  of  the
compass  having  been  obtained  by  any  of  the  processes  which
have  been-  described,  a  table  of  the  reMilN  should  be  con
-(meted,  marking  the  deviation  east  when  the  north  end  of  the
needle  has  been  drawn  to  the  eastward,  and  west  when  it  ha-
been  drawn  to  the  westward,  of  the  magnetic  north.  The-.
Imuld  be  constructed  and  made  use  of  even  though  the
compasses  be  or  be  not  corrected  by  mairnets  or  masses  of  iion
U  i-  tlie  practice  in  iron  ship-.
