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THE  ADMIEALTT  ASTEONOmCAL  EXPEEDIEXT 
I. — Letter  from  G.  B.  Airy,  Astronomefr  Itoyal,  to  the  Secretary  of  the  Admiralty,  dated 
13^A  May,  1856. 
“ Sir, — I have  the  honour  to  acknowledge  your  letter  of  the  2nd  instant,  acquainting 
me  with  the  sanction  of  the  Lords  Commissioners  of  the  Admiralty  to  a grant  of  £500  to 
Professor  Piazzi  Smyth  for  defraying  the  expenses  of  an  astronomical  visit  to  the  Peak 
of  Teneriffe,  and  requesting  me  to  communicate  any  suggestions  for  the  due  carrying 
out  of  this  project,  which  may  occur  to  me. 
“ 2.  In  reply,  I would  first  submit  to  My  Lords,  that  though  it  is  desirable  that  some 
document  of  the  character  of  Instructions  should  be  issued,  as  indicating  their  Lordships’ 
general  understanding  of  the  grounds  on  which  they  have  sanctioned  this  appropriation 
of  public  money,  yet  on  the  other  hand  it  is  most  desuable  that  the  several  heads  should 
be  so  lightly  stated,  as  to  leave  Professor  Smyth  in  the  most  absolute  freedom  as  to  his 
general  course  of  action. 
“ 3.  First,  I would  state  as  the  recommendation  of  their  Lordships,  that  Professor 
Smyth  should  consider  the  object  of  his  expedition  to  be  not  so  much  to  obtain  specific 
and  determinate  results,  as  to  ascertain  what  may  be  done,  or  what  may  be  expected 
in  future  expeditions  to  places  under  the  same  atmospheric  cucumstances.  The  main 
thing  is,  to  discover  how  much  astronomical  observations  may  be  benefited  by  the 
removal  of  the  injurious  infiuence  of  the  lower  third  part  of  the  atmosphere.  At  the 
same  time,  there  are  some  specific  observations,  requiring  httle  time,  and  natui'aUy 
falling  in  with  the  general  series,  which  it  is  well  to  mention  by  name. 
“ 4.  Among  these,  the  first  beyond  all  doubt  is  to  endeavom-  to  ascertain  whether  the 
red  prominences  which  have  been  seen  on  occasions  of  total  echpses  of  the  sun,  and  which 
seem  to  be  connected  with  the  sun’s  body,  can  be  seen  upon  the  imeclipsed  sun,  when 
the  atmosphere  is  so  rare  and  so  pure  that  the  difiused  light  in  the  proximity  of  the 
sun’s  disk  becomes  practically  insensible. 
“ 5.  It  is  very  desirable  that  careful  observations  should  be  made  on  the  zodiacal 
light,  and  that  these  should  be  continued  through  all  hours  of  the  night,  and  espe- 
cially about  midnight.  Observations  lately  pubhshed  have  led  several  persons  to 
suppose  that  the  matter  whose  illumination  exhibits  the  zodiacal  hght  suiTomids  not 
the  sun,  but  the  earth ; and  this  could  probably  be  definitively  settled  on  the  Peak  of 
Teneriffe. 
“ 6.  The  scrutiny  of  the  appearances  of  some  double  stars  and  nebulae,  and  more 
especially  that  of  the  disks  of  the  moon,  the  planets,  and  theu-  satellites,  ob\iously  pre- 
sents itself  as  an  important  object.  Perhaps,  however,  it  is  to  be  desired  that  the  obser- 
vations should  be  so  framed  as  to  determine  what  can  be  done,  rather  than  at  present 
to  carry  out  any  laborious  series  of  special  observations. 
“7.  In  the  related  subject  of  optics,  it  is  to  be  desu'ed  that  measures  of  the  polarization 
of  the  light  and  determinations  of  the  plane  of  polarization,  be  made  in  different  parts 
of  the  sky,  and  be  compared  with  similar  observations  made  at  the  bottom  of  the 
mountain. 
