ON  THE  PEAK  OE  TENEEIEFE. 
479 
sphere  would  have  been  impracticable;  for  the  calmness  of  the  atmosphere  and  the 
radiation  of  the  sky,  inasmuch  as  depended  on  them,  tended  powerfully  to  the  forma- 
tion of  dew.  Thus  at  Guajara,  on  the  4th  of  August,  when  an  unsurpassable  definition 
seemed  to  reign  all  night  long,  and  there  was  no  wind  and  no  cloud ; and  in  so  far 
there  ought  to  have  been  dew  on  the  object-glass,  whose  temperature  must  have  been 
brought  do\\Ti  far  below  that  of  the  surrounding  air, — for  the  radiation  of  the  sky 
amounted  to  17°  (that  is,  the  temperature  under  cover  was  50°,  and  the  temperature  of 
a black-bulb  thermometer  exposed  to  the  sky  was  33°  at  2 A.M.), — yet  the  temperature  of 
the  dew-point  being  at  the  same  moment  only  19°,  no  clouding  of  the  object-glass  took 
place. 
Comparing  our  observations  on  the  mountain  with  those  of  Mr.  Welsh  in  his  balloon 
ascents,  and  the  more  ancient  observations  from  De  Luc  and  Saussuee  downward,  this 
excessive  dryness  above  the  clouds  may  be  considered  a universal  phenomenon ; though 
the  absolute  height  at  which  it  most  signally  prevails,  depends  on  season  and  other 
effects.  Thus  in  Tenerifie,  throughout  the  summer,  the  dry  atmosphere  may  be  secured 
at  so  small  an  elevation  as  5000  feet;  but  in  autumn  and  winter  (see  the  Mountain 
Meteorological  Journal  for  September  the  14th),  we  should  have  to  rise  occasionally  so 
high  as  12,000  feet,  in  order  to  reach  it. 
(3.)  Daylight  Ohsermtions  of  Stars. 
Sundry  notices  of  stars  picked  up  duilng  the  day  with  the  Sheepshanks  telescope  on 
Guajara  are  scattered  through  the  Astronomical  Joiu’nal,  but  its  performances  in  this 
way  were  not  strikingly  better  than  in  Edinburgh.  Some  allowance  must  undoubtedly 
be  made  for  the  ver}'  untoward  circumstance  of  the  telescope  on  the  mountain  being 
exposed  in  the  open  air  and  bright  sunlight,  while  in  Scotland  it  had  been  employed  in 
a dark  room,  with  only  a small  aperture  for  vision  in  the  roof.  Over  and  above  this 
cause,  much  of  the  want  of  greater  success  in  the  day  observations  must  be  attributed 
to  illumination  of  the  atmosphere  by  the  sun.  True,  that  the  atmosphere  becomes 
more  transparent  as  we  ascend,  but  the  brightness  of  the  sun  increases  at  the  same  time, 
and  the  multitudinous  reflexions  of  its  light  from  motes  in  the  air  increase  more 
intensely  still. 
Hence,  as  long  as  the  sun  is  above  the  horizon,  daylight  on  the  mountain  is  almost  as 
fatal  to  stars  as  on  the  plains.  The  blue  of  the  Alpine  sky,  which  has  been  spoken 
of  by  travellers  as  something  so  deep  as  to  verge  on  black,  we  did  not  find  even  on  the 
culminating  point  of  the  Peak,  at  12,200  feet  of  altitude,  to  be  anything  extraordinary. 
Wlien  observing  the  sky  in  immediate  proximity  to  the  sun,  as  for  the  eclipse  red  pro- 
minences, the  field  of  view  was  so  intensely  bright,  that  a very  dark  glass  was  necessary 
to  protect  the  eye ; and  even  when  using  the  faint  reflexion  from  the  transparent  mirror 
of  a sun-eyepiece  of  the  Pattinson  equatorial,  considerable  practice  was  necessary  before 
one  s eye  could  withstand  the  glare. 
This  rendered  a search  for  stars  very  near  the  sun  quite  hopeless ; and  even  when 
MDCCCLVIII.  3 s 
