ASTRONOMICAL  ESSAYS.
21
Or  THe  MOTIoN  Or  THe  PLANETS  ROUND
THEIR  AXIS.
By  attentively  considering,  with  a  teleſcope,
the  surface  of  the  primary  planets,  our  solar  observer ¬
  will  find,  that  some  parts,  or  SPOTS,  are
more  obscure  than  others.  By  continued  observation ¬
  he  will  find,  that  these  spots  change  their
places,  and  move  from  one  side  of  the  planet  to
the  other;  then  disappear  for  a  certain  space
of  time;  after  which,  they  again,  for  a  while,
become  visible  on  the  side  where  they  were  first
seen,  always  continuing  the  same  motion  nearly
in  an  uniform  manner.
The  distance  between  the  ſpots  grows  wider  as
they  advance  from  the  edge  towards  the  middle
of  the  planet,  and  then  grows  narrow  again  as
they  pass  from  the  middle  to  the  other  edge.  The
time  they  are  seen  on  the  planet's  disk,  is  somewhat ¬
  leſs  than  the  time  of  their  disappearance.
From  these  circumstances  he  will  conclude,
first,  that  these  spots  adhere  to  the  body  of  the
planet,  and  that  each  planet  is  a  globe  turning  on
it's  axis,  and  has  consequently  two  motions,  one
whereby  it  is  moved  round  it's  axis  in  à  ſhort
time,  the  other  by  which  it  revolves  round  the
B  3
sun.