OF  THE  GLOBES.
359
lels,  reckoned  from  either  pole  towards  the  polar
circle;  which  you  are  to  remember.
Set  the  globe  upon  your  horizontal  plane  in  the
sun-ſhine,  and  put  it  nearly  north  and  south  by
the  mariner's  compass,  it  being  first  elevated  to
the,  latitude  of  the  place,  and  the  place  itself
brought  under  the  graduated  side  of  the  strong
brass  meridian;  then  move  the  frame  and  globe
together,  till  the  ſhade  of  extuberancy,  or  term
of  illumination,  just  touches  the  polar  parallel
for  the  day,  and  the  globe  will  be  settled  as  before; ¬
  and  if  accurately  performed,  the  variation
of  the  magnetic  needle  will  be  ſhewn  by  the  degree ¬
  to  which  it  points  in  the  compass  box.
And  here  observe,  if  the  parallel  for  the  day
ſhould  not  happen  to  fall  on  any  one  of  those
drawn  upon  the  globe,  you  are  to  eſtimate  a  proportionable ¬
  part  between  them,  and  reckon  that
the  parallel  of  the  day.  If  we  had  drawn  more,
the  globe  would  have  been  confused.
The  reason  of  this  operation  is,  that  as  the  sun
illuminates  half  the  globe,  the  ſhade  of  extuberancy ¬
  will  constantly  be  90  degrees  from  the
point  wherein  the  sun  is  vertical.
If
Z  4