ON  THE  PEAK  OF  TENERIFFE. 
475 
“ 1.  The  determination  of  heights  by  barometric  observations  is  liable  to  be  influenced, 
to  an  extent  at  present  unknown,  by  the  state  of  motion  of  the  air,  and  other  similar 
disturbing  causes.  Much  useful  information  on  this  point  might  be  obtained  by  taking- 
out  three  or  four  barometers,  to  be  planted,  one  at  the  station  chosen  near  the  top  of  the 
mountain,  and  the  others  in  different  directions  round  the  base,  especially  one  towards 
the  windward  and  another  toward  the  lee  side  of  the  mountain,  with  reference  to  the 
prevading  winds.  The  barometers  should  be  observed  at  the  same  hour  several  times 
during  the  day,  and  the  temperature  of  the  air,  and  likewise  that  of  the  mercury,  unless 
the  two  may  be  assumed  to  be  the  same,  as  well  as  the  dew-point,  should  be  registered, 
the  direction  of  the  -wind  noted,  and  its  velocity  estimated,  at  each  observation.  It  is 
supposed  that  persons  might  be  found,  who,  either  from  an  interest  in  the  subject  or  for 
a small  pecuniary  consideration,  would  undertake  the  registration  of  the  barometers 
placed  at  the  base  of  the  mountain.  One  or  more  barometers  might  be  placed  at 
different  altitudes,  should  habitations  and  suitable  observers  be  found.  It  is  to  be 
hoped  that  time  may  be  allowed  for  an  independent  determination  of  the  difference  of 
altitude  of  the  stations  by  triangulation  or  levelling, 
“ 2.  The  temperature  and  hygrometric  state  of  the  ah  might  be  determined  at  inter- 
vals in  ascending  or  descending  the  mountain,  a portable  barometer  being  read  at  the 
same  time,  so  as  to  give  the  altitude,  and  the  transitions  from  one  aerial  current  to 
another,  whenever  they  occurred,  being  noted.  These  results  would  be  especially 
interesting  for  comparison  wdth  those  obtained  in  the  recent  balloon  ascents  of  Mr. 
Welsh,  undertaken  under  the  direction  of  the  British  Association,  and  the  more  ancient 
observations  of  the  same  kind. 
“ 3.  Interesting  information  might  be  obtained  relating  to  the  absorption  of  the  solar 
rays  by  the  atmosphere,  considered  with  reference  to  their  total  thermic  effect,  by  making 
observ’ations  at  different  altitudes  with  Sh  John  Hekschel’s  actinometer. 
“ 4.  As  some  of  the  fixed  lines  of  the  spectrum  appear  to  owe  their  existence  to  the 
absorption  of  light  by  the  earth’s  atmosphere,  it  would  be  interesting  to  compare  the 
lines  seen  at  the  mountain  top  station  when  the  sun  is  low,  with  those  seen  about  the 
middle  of  the  day,  and  those  again  with  the  lines  seen  at  a small  elevation  above  the 
level  of  the  sea ; and  it  would  add  much  to  the  interest  of  the  investigation  if  photo- 
graphic impressions  of  the  lines  could  be  taken. 
“ 5.  Certain  observations  seem  to  show  that  the  atmosphere  is  to  a certain  extent 
opake  -with  regard  to  the  more  refrangible  of  the  solar  rays,  so  that  it  seems  likely  that 
the  spectrum  would  be  found  to  be  of  greater  extent,  on  the  more  refrangible  side,  on 
the  top  of  a high  mountain  than  below.  This  point  could  easily  be  decided  by  forming 
a pure  spectrum  with  a quartz  apparatus,  and  receiving  it  on  a piece  of  glass  coloured 
by  uranium,  or  on  some  other  substance  possessing  a similar  property. 
“ 6.  Observations  might  be  made  on  the  polarization  of  the  light  of  the  clear  sky  as 
seen  from  the  top  of  a mountain,  especially  with  reference  to  the  determination  of  tlie 
neutral  point  or  points,  if  any  exist. 
