ON  THE  PEAK  OF  TENEEIFFE. 
497 
Of  the  value  of  ir  I can  form  but  a very  remote  idea,  perhaps  10°  or  perhaps  30°. 
The  circumstance  of  there  being  such  a quantity,  while  illustrating  the  powerful  radia- 
tion of  the  region,  prohibits  any  attempt  at  exactitude  in  the  conclusions  to  be  drawn : 
but  we  may  notice  the  earlier  hour  of  the  day,  at  which  the  maximum  would  seem  to 
take  place,  as  the  station  is  higher,  as  well  as  the  greater  intensity  of  the  radiation : 
while,  that  the  increase  should  appear  to  be  nearly  as  great  between  Guajara  and  Alta 
Vista,  as  between  the  sea-level  and  Guajara,  is  a circumstance  that  bears  notably  on  the 
question  of  its  being  practically  advantageous  to  secure  for  astronomical  instruments, 
heights  greater  still  than  any  yet  experimented  on. 
Radiation  with  the  Sun  on  and  below  the  Horizon. 
At  Guajara  the  radiation  at  night  was  found  to  be  — 11°'4,  with  a temperature  of  62°; 
at  sunrise  — 4°'0,  with  a temperature  of  51°'2 ; and  at  sunset  +12°'0,  with  a tempera- 
ture of  59°-8,  for  the  beginning  of  August. 
That  the  excessive  radiation  of  the  day  should  heat  up  the  air  in  the  west  is  not  to 
be  wondered  at ; but  that  the  sun’s  influence  is  so  weak  at  rising,  that  the  joint  effect 
of  its  rays  and  the  general  eastern  exposure  should  produce  a negative  effect,  is  very 
remarkable. 
The  individual  observations  are  as  follows : — 
Night  radiation. 
Night  radiation. 
Mean  . 
August  1,  at  2 28  a.m.,  radiation  = — 5-3,  temperature  =53-3 
August  4,  at  1 43  a.m.,  radiation  = — 17' 4,  temperature  =50' 8 
August  2,  at  2 6 a.m.,  radiation  = — 11-4,  temperature  = 52 '0 
Sunrise  radiation,  August  2, 
Sunrise  radiation,  August  4, 
Sunrise  radiation,  August  8, 
Mean  . . August  5, 
radiation  = — 1 • 9 , 
radiation  = — 5 '5, 
radiation  = — 5'0, 
radiation  = — 4-0, 
temperature  = 52  • 6 
temperature  = 47'2 
temperature  = 53-8 
temperature  = 51  '2 
Sunset  radiation,  August  1, 
Sunset  radiation,  August  8, 
Mean  . . August  5, 
radiation  = fl-  6'8, 
radiation  = + 1 7 ' 2 , 
radiation  = + 12'0, 
temperature  =58 '8 
temperature  = 60‘8 
temperature  =59'8 
Horary  Variation  of  Radiation. 
For  the  purpose  of  obtaining  the  march  of  the  radiation  through  the  twenty-four 
hours,  the  observations  of  August  1 and  4 seem  safer  to  be  employed  than  any  of  the 
others;  the  first  day  having,  however,  the  drawback,  that  the  amount  of  radiation 
appears  to  have  been  lowered  throughout  the  twenty-four  hours,  by  the  violent  and 
unusual  wind  which  was  blowing  at  the  time.  This  effect  was  strongest  about  sunrise ; 
but  for  this  period  we  are  enabled  to  supplement  the  observations  of  August  1 by  those 
of  August  4,  which  appears  to  have  been  an  unexceptionable  day.  As  usual  on  such 
days,  the  mercury  mounted  soon  after  9 a.m.  above  the  graduation  of  the  thermometer. 
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