470 
THE  ADMIBALTY  ASTEOXOMICAL  EXPEEBIEXT 
expenses  of  an  astronomical  expedition  to  the  Peak  of  Teneriffe,  to  be  undertaken  by 
Professor  Smyth  of  Edinburgh,  and  that  their  Lordships  wish  me  to  furnish  them  with 
any  suggestions  I may  have  to  offer  for  the  better  carrying  out  the  project; — 
“ 1st.  I have  to  state,  that  I consider  the  opportunity  a very  valuable  one  for  obtain- 
ing an  extensive  and  normal  series  of  comparative  actinometric  observations,  made 
smiultcmeously  (strictly  so)  on  the  summit  of  the  mountain  and  at  the  level  of  the  sea. 
with  actinometers  provided  with  interior  thermometers  (not  mercurial).  These  instru- 
ments, and  the  mode  of  using  them,  are  fully  described  in  the  section  on  Meteorology, 
and  fo]’ming  part  of  the  ‘ Manual  of  Scientific  Inquiry  ’ published  by  authority  of  theh 
Lordships  in  1849 ; the  object  being  the  determination  of  the  proportion  of  the  solar- 
heat  absorbed  by  the  atmosphere  between  the  two  limits  of  altitude.  Should  chcum- 
stances  permit,  an  intermediate  station,  about  half-way  up  the  mountain,  would  afford 
valuable  supplementary  observations.  Such  observations,  taken  at  the  time  of  the  sun 
being  vertical,  would  be  very  precious,  but  the  series  should  be  extended  to  every  alti- 
tude of  the  sun  down  to  the  horizon. 
“ 2nd.  It  has  been  stated,  that  at  a place  considerably  lower  than  the  actual  summit 
of  the  Peak,  there  occur  caverns  on  the  mountain  side,  in  which,  though  beneath  the 
limit  of  perpetual  sirow  proper  to  those  latitudes,  the  temperatrue  is  always  below  the 
freezing-point.  The  fact  (which  is  not  without  analogous  ones  elsewhere)  should  be 
inquired  into,  accurate  observations  made,  and  the  concomitant  cu-cumstances  carefully 
recorded. 
“ 3rd.  The  opportunity  will  of  course  not  be  lost  of  ascertaining  by  comparative 
observations,  with  one  and  the  same  telescope,  in  England,  at  Orotava,  and  on  the  sum- 
mit of  the  Peak,  what  degree  of  advantage,  in  point  of  optical  performance,  is  obtained 
by  change  of  chmate,  and  by  ascent  into  a clearer  and  rarer  atmosphere.  The  selection 
of  proper  objects  of  comparison,  such  as  nebulae,  clusters,  double  stars,  &c.,  must  be  left 
to  the  observer’s  judgment. 
4th.  The  spots  of  the  sun  will  probably  be  observed  to  very  much  greater  advantage 
at  so  elevated  a station,  and  by  the  aid  of  Mr.  Dawes’  eyepiece,  theh-  physical  pecu- 
liarities may  there  be  examined  with  every  prospect  of  obtaining  some  distinctly  new 
information.  There,  too,  if  anywhere,  it  may  be  possible,  by  careful  management,  to 
obtain  a sight  of  the  red  protuberances  from  the  sun’s  hmb,  which  on  the  plains  can 
never  be  seen  but  on  the  occasion  of  a total  eclipse. 
“ 5th.  As  Mr.  Smyth  is  an  expert  photographer,  he  should  be  provided  with  an  appa- 
ratus for  obtaining  photographic  impressions  of  everything  worthy  of  record,  inter  alia, 
the  great  Dragon  Tree  of  Orotava  (supposed  to  be  the  oldest  tree  in  the  world),  from 
several  points  of  view.  Of  course  accurate  girth  measurements  of  this  most  wonderful 
object,  at  several  levels  from  the  ground  (defined  by  marks  left  on  the  trunk),  will  not 
be  neglected. 
“ 6th.  The  polarization  of  the  sky  light,  at  the  summit  of  the  mouutam,  should  be 
carefully  examined,  and  the  point  of  maximum  polarization,  with  respect  to  the  sim’s 
