﻿470 
  Mr. 
  F. 
  W. 
  Very 
  on 
  the 
  Greenhouse 
  

  

  determining 
  a 
  planet's 
  temperature, 
  and 
  shows 
  that 
  a 
  planet's 
  

   albedo 
  is 
  largely 
  atmospheric. 
  This 
  being 
  so, 
  since 
  about 
  a 
  

   third 
  of 
  the 
  sun's 
  radiant 
  energy 
  resides 
  in 
  the 
  visible 
  

   spectrum 
  where 
  selective 
  scattering 
  is 
  most 
  effective, 
  the 
  

   earth, 
  with 
  its 
  greater 
  albedo, 
  receives 
  a 
  smaller 
  proportion 
  

   of 
  heat 
  at 
  its 
  surface 
  than 
  Mars 
  on 
  this 
  account. 
  In 
  the 
  

   visible 
  region 
  of 
  the 
  spectrum, 
  the 
  radiant 
  absorption 
  by 
  

   gases 
  is 
  small, 
  and 
  the 
  blanketing 
  action 
  of 
  the 
  atmosphere 
  

   nil, 
  because 
  there 
  are 
  no 
  rays 
  of 
  this 
  wave-length 
  emitted 
  by 
  

   its 
  heated 
  surface; 
  but 
  the 
  larger 
  proportion 
  of 
  heat 
  received 
  

   from 
  this 
  part 
  of 
  the 
  spectrum 
  by 
  the 
  ground 
  of 
  Mars 
  pro- 
  

   duces, 
  if 
  the 
  absorbent 
  and 
  conducting 
  properties 
  of 
  the 
  

   surface 
  are 
  the 
  same, 
  a 
  relatively 
  larger 
  radiation 
  of 
  infra-red 
  

   rays 
  outward, 
  which 
  is 
  at 
  the 
  same 
  time 
  proportionally 
  better 
  

   preserved. 
  

  

  Land 
  temperatures 
  on 
  this 
  earth 
  exceed 
  those 
  over 
  water. 
  

   Mars 
  has 
  the 
  land 
  climate. 
  The 
  heat-storing 
  powers 
  of 
  the 
  

   soil 
  are 
  also 
  greater 
  with 
  a 
  dry 
  climate. 
  Again, 
  the 
  conditions 
  

   on 
  Mars 
  favour 
  a 
  greater 
  thermal 
  effect 
  from 
  the 
  same 
  amount 
  

   of 
  solar 
  radiation. 
  That 
  the 
  cumulative 
  effect 
  of 
  so 
  many 
  

   factors 
  acting 
  in 
  one 
  direction 
  can 
  largely 
  overpower 
  a 
  defi- 
  

   ciency 
  of 
  that 
  due 
  to 
  remoteness 
  from 
  the 
  sun, 
  is 
  a 
  conclusion 
  

   which 
  is 
  at 
  least 
  permissible 
  ; 
  and 
  if 
  computations 
  founded 
  on 
  

   reasonable 
  assumptions 
  lead 
  to 
  results 
  which 
  are 
  in 
  good 
  

   agreement 
  with 
  the 
  seasonal 
  indications 
  deduced 
  from 
  the 
  

   fluctuations 
  of 
  a 
  snow-cap 
  whose 
  existence 
  is 
  far 
  more 
  pro- 
  

   bable 
  than 
  some 
  of 
  the 
  hypotheses 
  made 
  in 
  arguments 
  against 
  

   its 
  possibility, 
  it 
  seems 
  to 
  me 
  that 
  the 
  balance 
  of 
  the 
  evidence 
  

   inclines 
  strongly 
  towards 
  Professor 
  Lowell's 
  conclusions. 
  

  

  In 
  treating 
  the 
  subject 
  of 
  atmospheric 
  selective 
  depletion 
  

   of 
  radiation, 
  a 
  more 
  accurate 
  result 
  might 
  be 
  obtained 
  by 
  

   treating 
  the 
  diffraction 
  by 
  air 
  molecules, 
  and 
  the 
  selective 
  

   reflexion 
  from 
  dust 
  separately; 
  but 
  without 
  going 
  into 
  this 
  

   refinement, 
  an 
  approximate 
  knowledge 
  of 
  the 
  reflective 
  

   depletion 
  of 
  the 
  solar 
  rays 
  falling 
  on 
  an 
  air-covered 
  planet 
  

   can 
  be 
  reached 
  if 
  we 
  admit 
  that 
  the 
  transmission, 
  so 
  far 
  as 
  it 
  

   depends 
  on 
  reflexion, 
  varies 
  as 
  tfj, 
  where 
  t 
  r 
  is 
  the 
  reflective 
  

   transmission 
  for 
  rays 
  at 
  the 
  zenith, 
  and 
  e 
  is 
  the 
  air 
  mass. 
  

   The 
  depletion 
  will 
  then 
  be 
  1— 
  t 
  6 
  r 
  , 
  and 
  the 
  total 
  reflective 
  

   depletion 
  for 
  the 
  entire 
  surface 
  of 
  the 
  sunlit 
  atmosphere 
  is 
  

  

  2(1 
  -Ox 
  A 
  cos?, 
  

   where 
  ?is 
  the 
  sun's 
  zenith 
  distance. 
  For 
  ^ 
  = 
  0*6, 
  

   2(1- 
  ^A 
  cos 
  ?= 
  0-677, 
  

  

  