01^  THE  PEAK  OF  TENEEIFFE. 
499 
for  observation  on  a high  mountain  should  be  graduated  to  above  212°,  such  a tempera- 
ture of  radiation  having  been  reached  on  Guajara  in  the  middle  of  the  day. 
Towards  the  chief  astronomical  end  of  the  Expedition,  there  is  yet  a more  interesting 
conclusion  to  be  drawn.  The  days  of  highest  radiation  are  those  of  least  temperature, 
and  vice  versa ; and  this  difference  obtains  in  a signal  degree  on  days  when  there  was  no 
visible  disturbing  action  of  wind  or  clouds.  What  then  causes  the  radiation  of  one  day 
to  be  greater  than  that  of  another,  and  the  temperature  lessl  The  immediate  agent 
appears  to  be  the  atmospheric  dust,  which  has  already  been  spoken  of  in  the  Astrono- 
mical Journal  as  so  prejudicial  a medium  to  telescopic  vision,  weakening  direct  light 
and  multiplying  general  hght ; acting,  in  fact,  by  light  precisely  as  the  measures  o: 
radiation  and  temperature  prove  that  it  does  by  heat. 
Hence,  then,,  we  may  easily  understand  why,  Avith  the  dust  strata  rising  to  a vei}' 
limited  height,  say  11,000  feet,  the  small  difference  of  altitude  between  Alta  Vista  anc 
Guajara  produced  as  great  an  increase  in  the  radiation  as  did  the  great  difference,  nearly 
four  times  as  great,  between  Guajara  and  Orotava.  Hence  also  we  are  furnished  with 
a veiy  portable  apparatus  for  ascertaining  on  a high  and  distant  mountain  the  principal 
elements  that  produce  a good  astronomical  site. 
(2.)  Radiation  hy  Actinometer. 
In  the  accurate  determination  of  solar  radiation  for  physical  inquiries,  the  black-bulb 
must  peld  to  Sir  John  Heeschel’s  actinometer.  That,  in  principle,  has  been  stated  by 
an  able  judge  to  be  perfect ; but  the  instruments  themselves,  as  furnished  by  the  best 
maker  in  1856,  according  to  our  Teneriffe  experiences,  have  not  arrived  at  equal  practical 
perfection.  One  of  the  most  improved,  the  only  one  procurable  at  the  time  we  left 
England,  was  veiy  kindly  lent  by  Mr.  Aiea%  and  Avith  great  generosity  he  ordered 
another  to  be  constructed  by  the  same  artist  as  quickly  as  possible,  and  sent  after  us. 
Of  these  two,  the  internal  thermometer  of  the  first,  Avhen  unpacked  on  Guajara,  Avas 
found  broken,  and  the  actinometer-bulb  had  leaked,  under  circumstances  of  carriage, 
where  barometers,  thermometers,  and  eAen  the  gold  leaves  of  an  electrometer,  and  the 
silk  fibre  of  a thermo-multiplier,  traA'elled  Avith  perfect  safety : the  second  actinometer, 
only  arriAing  in  Teneriffe  at  the  conclusion  of  the  summer,  never  got  beyond  the  Con- 
sul’s office  in  Santa  Cruz ; neA^ertheless — though  admirably  packed,  first  in  its  oAvn  box, 
and  then  with  this  placed  in  the  centre  of  a much  larger  box,  and  filled  in  on  every  side 
Avith  paper  shaAings — it  had  leaked,  and  to  such  an  extent,  that  the  fluid  could  not  be 
brought  into  the  graduated  tube.  No  additional  stock  of  fluid,  and  no  solid  materials 
for  preparing  it  Avith  AA^ater,  had  been  supplied  Avith  either  instrument.  I mention  these 
things  only  as  indications  of  improA^ements  to  be  made  on  the  next  occasion. 
Trying  to  make  up  for  the  broken  internal  thermometer  of  the  first  actinometer  on 
Guajara,  by  placing  a small  one,  Avrapped  up  in  blue  calico,  under  the  glass  covering 
the  great  blue  bulb,  I began  to  make  observations  on  August  7 ; they  were  continued 
throughout  the  day,  except  during  a cloudy  interval.  Wind  and  haze  much  interfered 
