ON  THE  PEAK  OP  TENEEIPFE. 
503 
Sir  John  Hekschel  has  explained  the  inability  of  observers  at  low  levels  to  obtain 
beat  indications  of  the  moon’s  rays,  by  considering  that  the  heat  is  spent  in  dissipating 
vapours  in  the  upper  regions  of  the  atmosphere,  and  so  explains  the  preponderance  of 
clear  over  cloudy  nights  at  the  time  of  full  moon.  On  Guajara,  there  appeared  to  be 
a strong  tendency  to  “upper”  clouds  during  the  several  days  preceding  full  moon,  but 
on  that  night  every  particle  of  them  disappeared ; the  lower  clouds,  however,  were 
constant  through  the  Avhole  lunation.  This  does  seem  to  confirm  Sir  John’s  idea;  and 
to  show  too,  that  the  moon’s  heat,  though  effective  at  great  heights,  is  entirely  expended 
before  arriving  at  the  lowest  strata  of  cloud,  2300  feet  above  the  sea.  The  elevation  of 
the  upper  clouds  which  were  apparently  so  effectually  acted  on,  we  had  no  means  of 
accurately  judging  of,  but  I should  suspect  that  it  could  not  have  been  less  than  15,000 
feet;  at  that  height  then,  or  higher,  should  be  made  the  next  observations  on  the 
radiation  of  the  moon. 
(4.)  Lines  in  the  Spectrum. 
Towards  obser\ing  the  black  lines  in  the  solar  spectrum,  we  had  a speculum  to  reflect 
sunlight,  kindly  furnished  by  Mr.  J.  Nasmyth,  C.E.,  and  an  apparatus  consisting  of  a 
prism,  a fine  adjustable  slit  placed  in  the  focus  of  a 2-inch  object-glass,  and  a telescope 
of  the  same  size,  with  magnifying  powers  as  high  as  30,  prepared  for,  and  lent  to  us, 
by  Mr,  Aiey.  There  was  no  angular  measurement  contemplated ; only  eye  observation, 
and  comparison  of  diflferences  between  the  spectrum  seen,  and  that  engraved  by  Fraun- 
HOFER.  To  employ  these  instruments  on  Guajara,  a small  chamber,  some  10  feet  square, 
was  built  of  rude  stone,  roofed  in  with  planks  and  old  canvas,  and  further  covered  with 
a quantity  of  “ retama"  branches  to  keep  out  every  particle  of  the  sun’s  light:  while  a 
square  tube  of  wood,  5 feet  long,  ulth  a moderate  aperture  at  the  end,  was  thrust  through 
the  wall  in  the  dh’cction  of  the  speculum  mounted  on  a small  stone  pier  outside.  In 
this  manner  a considerable  degree  of  darkness  was  secured,  even  when  the  sun’s  rays 
were  being  reflected  into  the  instrument. 
A cursory  examination  of  the  spectrum  showed  much  general  correspondence  with 
Fraunhofer’s  view  as  to  the  principal  lines ; but  so  great  discordance  as  to  detail,  that 
I thought  it  better  to  proceed  on  an  entirely  independent  footing,  and  make  original 
drawings  of  what  I saw ; and  when  well  satisfied  with  them,  to  compare  them  with  the 
engra\ing.  One  evening  trying  the  sun  unusually  low,  and  finding  new  features  worth 
following  up  further  than  could  be  done  while  using  the  reflected  ray  from  the  speculum, 
I took  the  prism  apparatus  out  into  the  open  air,  and  by  means  of  a theodolite  stand 
and  photographic  black  bag,  was  enabled  to  continue  the  observations  until  the  sun  set 
at  the  usual  mountain  zenith  distance  of  91°  11',  These  direct  observations  of  the  sun 
were  repeated  on  many  occasions  with  the  sun  both  east  and  west.  The  lunar  spectrum 
and  that  of  the  blue  sky  were  also  similarly  examined. 
So  far  for  the  Guajara  experiences.  At  Alta  Vista  a similar  optical  dark  room  -was 
prepared ; but  profiting  by  hints  procured  from  the  direct  use  of  the  line  instrument  at 
MDCCCLVIII.  3 X 
