ON  THE  PEAK  OF  TENEEIFFE. 
477 
extraordinai-y  importance  in  repairing  damages  to  instruments  caused  by  the  dry  atmo- 
sphere above  the  clouds,  and  in  adapting  materials  and  means  to  the  novel  circumstances 
in  which  they  were  placed. 
For  the  sea-level  observations  taken  during  the  same  period,  all  acknowledgements 
are  due  to  the  captain  of  the  yacht  (Loving  Coeke),  who  made  the  whole  of  the  said 
observations  himself,  and  with  strict  attention  to  the  principles  laid  down  to  him. 
Very  lately  I have  had  the  pleasiu’e  of  receiving  from  Teneriffe  a large  mass  of  tide 
observations,  taken  under  the  immediate  superintendence  of  Don  Feancisco  Aguilae,  a 
Spanish  civil  engineer,  engaged  on  the  repair  of  the  Mole,  and  inspired  with  the  most 
laudable  enthusiasm  for  the  promotion  of  science.  Finding  that  these  observations 
have  been  made  exactly  in  accordance  with  the  desired  instructions,  and  having  per- 
sonally examined  the  tide-gauge,  erected  for  the  purpose  under  the  care  of  the  Don  and 
Mr.  L.  Hamilton,  I have  great  satisfaction  in  including  the  Spanish  contribution  in  the 
present  report. 
CHAPTER  II. 
DEDUCED  IMPEOVEMENT  OF  ASTEONOMICAL  VISION  WITH  HEIGHT. 
(I.)  Vision  and  Definition. 
This  important  question  was  quickly  and  satisfactorily  settled  on  Guajara;  for  by  fre- 
quent trials  during  several  years  in  Edinburgh,  I had  ascertained  the  range  of  vision 
with  the  Sheepshanks  telescope  to  extend  to  the  lOth  magnitude;  I had  never,  for 
instance,  been  able  to  see  the  companion  of  a Lyrse  (Ilth  magnitude),  even  when  select- 
ing the  most  favourable  nights,  and  with  the  star  only  5 degrees  from  the  zenith ; while 
with  the  same  telescope  and  the  same  eye  on  Guajara,  at  8903  feet  of  height,  and  with 
a Lyree  25  degrees  from  the  zenith,  the  companion  was  always  and  easily  visible,  more 
so  than  the  companion  of  Polaris  (9th  mag.)  used  to  be  in  Edinburgh.  Smaller  stars 
still  were  also  obsen'ed,  as  C of  5 xAquilee  (I4th  mag.),  D of  13  Lyree  (I2th  mag.),  B of 
S Aquilse  (12th  mag.),  B of  128  Anseris  (13th  mag.),  B and  C of  |8  Equulei  (13th  and 
14th  mags.).  Stars  of  the  15th  and  16th  magnitudes,  looked  for,  were  not  seen,  as  was 
the  case  also  with  one  of  the  13th  magnitude. 
An  extension  of  telescopic  rision  through  four  magnitudes  is  thus  made  out,  and  would 
be  an  inestimable  addition  to  our  larger  classes  of  telescopes ; for  much  of  the  advance 
seemed  to  be  owing  to  improved  definition,  as  well  as  to  the  transparency  of  the  air. 
This  fine  definition  was  shown  in  the  perfection  of  the  images  of  the  stars,  which,  five 
nights  out  of  six,  exhibited  clear  little  disks  surrounded  by  regular  rings,  when  viewed 
under  a magnifying  power  of  150,  and  contrasted  most  favourably  with  the  amorphous 
figures  that  the  same  telescope  had  always  given  in  Edinburgh.  When  there,  indeed,  I 
had  been  at  a loss  to  understand  how  accurate  measure  could  be  applied  to  the  double 
stars ; but  on  Guajara  the  appearance  of  every  double  star  seemed,  by  its  finish  and 
exactness,  to  provoke  one  to  apply  a wire  micrometer  to  it. 
