When  a  coelostat  mirror  is  used  in  this  way  we
can  fix  the  telescope  in  any  position  we  like  within
certain  limits.  One  position  which  very  naturally
occurs  to  us  is  the  horizontal  one.  If  we  lay  the
big  telescope  on  the  ground,  there  is  no  danger  of


JOURNEYING  BY  TELESCOPE  121

its  falling  :  and  it  is  in  other  respects  convenient
to  work  with.  Hence  several  large  telescopes  have
already  been  built  in  this  way.  A  good  example
is  the  Snow  telescope  on  Mount  Wilson.  You  see
in  the  picture  the  long  shed  which  is  really  the  tele-
scope itself  :  and  at  one  end  there  is  the  coelostat
mirror,  reflecting  the  light  (in  this  case  usually  the
light  of  the  Sun,  not  that  of  a  star)  into  the  telescope.
But  there  is  one  great  disadvantage  about  this
horizontal  position.  The  ground  gets  heated  during
the  day  and  causes  air  currents  to  ascend  from  it.
