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XXV.  Astronomical  Experiment  on  the  Peak  of  Teneriffe^  carried  out  under  the  sanction 
of  the  Lords  Commissioners  of  the  Admiralty.  By  Professor  C.  Piazzi  Smyth. 
Communicated  hy  G.  B.  Airy,  Esq..,  Astronomer  Boyal. 
Eeceived  June  2, — Eead  June  18,  1857. 
CHAPTEE  I. 
INTEODTJCTOET. 
(1.)  Origin  and  Objects. 
The  principal  object  of  the  experiment  on  Teneriffe  in  the  summer  of  1856,  was  to 
ascertain  how  much  astronomical  observation  can  be  benefited,  by  eliminating  the  lower 
third  or  fourth  part  of  the  atmosphere.  That  the  amount  of  such  improvement  would 
be  large  and  form  a desirable  boon  to  practical  astronomy,  appears  to  have  been 
expected  by  Sir  Isaac  Newton  ; for  in  his  ‘ Optics  ’ he  expressly  says,  “ They  (telescopes) 
cannot  be  so  formed  as  to  take  away  that  confusion  of  rays  which  arises  from  the  tremors 
of  the  atmosphere.  The  only  remedy  is  a most  serene  and  quiet  air,  such  as  may  per- 
haps be  found  on  the  tops  of  the  highest  mountains  above  the  grosser  clouds.” 
After  having  enjoyed  some  slight  practical  experience  of  the  method  so  recommended, 
during  the  remeasurement  of  La  Caille’s  Southern  Arc  of  the  Meridian  under- 
Mr.  Maclear,  its  further  prosecution  was  brought  prominently  to  my  attention  by  the 
pecuhar  position  of  the  Edinburgh  Observatory.  Accordingly,  in  November  1852,  I had 
the  honour  of  presenting  to  the  Board  of  Visitors,  under  the  Presidency  of  the  Eight 
Hon.  Lord  Eutherpurd,  a scheme  for  realizing  Newton’s  idea  through  means  of  a summer 
expedition  to  the  Peak  of  Teneriffe ; where  there  appeared  a hope  that  telescopes  might 
be  elevated  more  than  10,000  feet  above  the  level  of  the  sea,  with  greater  facilities  of 
every  sort,  than  on  any  other  known  mountain.  The  same  project  was  also  brought 
before  Section  A.  of  the  British  Association,  three  years  later. 
In  the  autumn  of  1855,  a copy  of  the  Edinburgh  Observatory  Eeport  reached  the 
hands  of  E.  Stephenson,  Esq.,  M.P. ; and  he  found  its  indications  to  agree  so  well  with 
his  own  experience  in  early  life  on  South  American  Cordilleras,  that,  though  we  were 
entirely  unacquainted,  he  very  kindly  sent  me,  through  a mutual  friend,  an  offer  of  a 
passage  to  Teneriffe  in  his  yacht  ‘ Titania.’  Circumstances,  however,  prevented  my 
profiting  by  his  kindness  on  that  occasion. 
Finally,  in  the  spring  of  1856,  the  Astronomer  Eoyal,  G.  B.  Airy,  Esq.,  having 
himself  engaged  in  some  special  astronomical  inquiries,  where  success  was  rendered 
impossible  by  interference  of  the  atmosphere ; and  having  become  convinced  of  the  per- 
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