OF  THE  QUADRANT,  &c.
639
if  afternoon;  but  if  before  noon,  it  must  be  deducted ¬
  from  12  hours,  to  give  the  time.  This
apparent  time  must  be  corrected  by  the  equation,
as  in  problem  xii.
OF  THE  EQUATORIAL,  OR  UNIVERSAL  SUN-DIAL,
AND  IT’S  USES.
The  plumb-line,  or  direction  in  which  gravity
acts,  being  the  only  line  we  can  at  all  times  have
immediate  recourse  to,  for  determining  the
positions  of  objects,  is  the  chief  particular  to
which  the  circles  in  the  instrument  last  described
are  adapted;  and  accordingly,  their  planes  are
placed  the  one  parallel,  and  the  other  perpendicular ¬
  to  that  line.  But  as  there  are  few  places  on
the  earth  whoſe  vertical  or  horizontal  circles  correspond ¬
  with  thoſe  in  which  the  celestial  motions
are  performed,  it  was  found  necessary,  at  a  very
early  period,  to  construct  instruments  adapted
not  only  to  the  measurement  of  altitudes  and  azimuths, ¬
  but  also  to  follow  the  heavenly  bodies  in
their  respective  paths,  and  determine  their  right
ascensions  and  declinations,  more  immediately
than  can  be  done  by  the  quadrant  and  horizontal
circle.  The  equatorial  is  the  most  approved
modern  instrument  for  this  purpose.
It