﻿Theory 
  and 
  Planetary 
  Temperatures. 
  469 
  

  

  bable 
  that 
  a 
  much 
  wider 
  range 
  of 
  possibilities 
  may 
  appear. 
  

   Even 
  if 
  the 
  complete 
  differential 
  equations 
  cannot 
  now 
  be 
  

   attempted, 
  the 
  problem 
  can 
  be 
  taken 
  up 
  in 
  sections, 
  as 
  he 
  has 
  

   done 
  for 
  a 
  part 
  of 
  the 
  process, 
  investigating 
  one 
  influence 
  

   after 
  another 
  separately, 
  but 
  of 
  course 
  recognizing 
  that 
  these 
  

   will 
  have 
  mutual 
  modifications 
  which 
  must 
  be 
  considered 
  in 
  

   the 
  final 
  summing 
  up. 
  

  

  Take, 
  for 
  example, 
  the 
  climatic 
  influences 
  involved 
  in 
  the 
  

   evaporation 
  and 
  condensation 
  of 
  water. 
  M. 
  Liais 
  many 
  

   years 
  ago 
  estimated 
  that 
  this 
  process 
  has 
  a 
  very 
  large 
  part 
  in 
  

   determining 
  terrestrial 
  thermal 
  conditions. 
  1 
  will 
  suppose, 
  

   in 
  illustration, 
  that 
  we 
  seek 
  the 
  effect 
  of 
  this 
  factor 
  on 
  the 
  

   climate 
  of 
  Mars. 
  Assuming 
  that 
  the 
  barometric 
  pressure 
  is 
  

   much 
  smaller 
  on 
  Mars, 
  a 
  supposition 
  for 
  which 
  there 
  is 
  good 
  

   reason, 
  evaporation 
  of 
  moisture 
  at 
  a 
  given 
  temperature 
  will 
  

   be 
  more 
  rapid, 
  and 
  condensation 
  more 
  difficult 
  than 
  here. 
  

   Consequently, 
  at 
  that 
  temperature, 
  more 
  moisture 
  can 
  exist 
  

   in 
  the 
  atmosphere 
  of 
  Mars, 
  diffusing 
  to 
  a 
  great 
  height, 
  and 
  

   still 
  the 
  relative 
  humidity 
  will 
  remain 
  low*. 
  The 
  smaller 
  

   relative 
  humidity 
  diminishes 
  the 
  strength 
  of 
  the 
  broad 
  diffuse 
  

   bands 
  of 
  aqueous 
  absorption, 
  but 
  does 
  not 
  affect 
  the 
  line 
  

   absorption. 
  Since 
  condensation 
  of 
  moisture 
  is 
  favoured 
  by 
  

   the 
  presence 
  of 
  free 
  ions 
  to 
  serve 
  as 
  nuclei 
  of 
  condensation, 
  

   we 
  should 
  like 
  to 
  know 
  something 
  as 
  to 
  this 
  condition 
  on 
  

   Mars. 
  We 
  do 
  know 
  that 
  here, 
  with 
  abundance 
  of 
  moisture 
  

   in 
  the 
  air, 
  rain 
  does 
  not 
  fall 
  unless 
  some 
  widespread 
  general 
  

   condition 
  of 
  the 
  atmosphere, 
  which 
  appears 
  to 
  be 
  associated 
  

   with 
  its 
  electrical 
  state, 
  is 
  added 
  to 
  the 
  other 
  favouring 
  

   factors. 
  Telescopic 
  observation 
  of 
  Mars 
  shows 
  a 
  very 
  con- 
  

   siderable 
  degree 
  of 
  persistence 
  in 
  the 
  surface 
  markings 
  which 
  

   indicates 
  comparative 
  absence 
  of 
  obscuring 
  cloud, 
  that 
  is 
  to 
  say 
  > 
  

   there 
  is 
  less 
  liability 
  of 
  rain, 
  moisture 
  remains 
  long 
  in 
  the 
  air, 
  

   and 
  the 
  continuity 
  of 
  the 
  protecting 
  aqueous 
  envelope 
  is 
  more 
  

   complete. 
  Though 
  less 
  heat 
  is 
  received, 
  the 
  final 
  result, 
  if 
  

   more 
  is 
  stored, 
  may 
  be 
  not 
  very 
  different 
  in 
  the 
  two 
  planets 
  

   so 
  far 
  as 
  this 
  factor 
  is 
  concerned. 
  

  

  Professor 
  Lowell 
  t 
  emphasizes 
  the 
  importance 
  of 
  albedo 
  in 
  

  

  * 
  Since 
  this 
  was 
  written, 
  Mr. 
  Slipher 
  has 
  obtained 
  at 
  the 
  Flagstaff 
  

   Observatory 
  spectrograms 
  of 
  Mars 
  showing 
  intensification, 
  not 
  of 
  the 
  com- 
  

   paratively 
  feeble 
  " 
  rain 
  band 
  " 
  near 
  D 
  which 
  has 
  been 
  the 
  subject 
  of 
  much 
  

   contention 
  in 
  the 
  past, 
  but 
  of 
  the 
  much 
  more 
  powerful 
  water-vapour 
  

   band 
  " 
  a 
  " 
  in 
  the 
  extreme 
  red. 
  I 
  have 
  had 
  the 
  privilege 
  of 
  examining 
  

   the 
  original 
  photographs, 
  and 
  can 
  testify 
  to 
  the 
  certainty 
  of 
  the 
  evidence 
  

   they 
  bring 
  of 
  the 
  presence 
  of 
  water-vapour 
  in 
  the 
  atmosphere 
  of 
  Mars. 
  

  

  | 
  Percival 
  Lowell, 
  " 
  Temperature 
  of 
  Mars,"' 
  Proc. 
  American 
  Acad. 
  

   Arts 
  and 
  Sciences, 
  vol. 
  xlii. 
  p. 
  

  

  