﻿610 
  Gradient 
  of 
  Penetrating 
  Radiation 
  from 
  the 
  Earth. 
  

  

  Hess's 
  observations 
  : 
  a 
  diminution 
  in 
  the 
  ionization 
  due 
  to 
  

   penetrating 
  radiation 
  was 
  first 
  observed 
  followed 
  by 
  an 
  

   increase 
  probably 
  due 
  to 
  the 
  gradual 
  accumulation 
  of 
  active 
  

   deposit. 
  

  

  Sect. 
  4. 
  Note 
  on 
  the 
  Origin 
  of 
  the 
  Penetrating 
  Radiation 
  

   and 
  Conditions 
  to 
  be 
  fulfilled 
  in 
  the 
  Correct 
  Measure- 
  

   ment 
  of 
  the 
  Gradient. 
  

  

  Chauveau 
  * 
  concludes, 
  from 
  a 
  review 
  of 
  existing 
  data, 
  that 
  

   three 
  sources 
  may 
  contribute 
  to 
  the 
  penetrating 
  radiation 
  : 
  — 
  

  

  (i.) 
  The 
  effect 
  of 
  radio-active 
  matter 
  suspended 
  in 
  the 
  

   atmosphere 
  contributing 
  *1 
  to 
  "2 
  ion 
  per 
  c.c. 
  per 
  sec. 
  

  

  (ii.) 
  The 
  contribution 
  of 
  a 
  layer 
  of 
  active 
  deposit 
  carried 
  

   down 
  by 
  the 
  electrostatic 
  potential 
  gradient 
  of 
  the 
  atmo- 
  

   sphere 
  : 
  contribution 
  1 
  to 
  2 
  ions 
  and 
  subject 
  to 
  large 
  fluctua- 
  

   tions 
  depending 
  on 
  meteorological 
  conditions. 
  

  

  (iii.) 
  The 
  largest 
  contribution 
  of 
  4-5 
  ions 
  due 
  to 
  the 
  radium 
  

   content 
  of 
  the 
  soil 
  : 
  we 
  have 
  already 
  seen 
  that 
  the 
  whole 
  

   effect 
  is 
  almost 
  entirely 
  due 
  to 
  the 
  surface-layer 
  of 
  a 
  few 
  

   centimetres' 
  depth. 
  

  

  In 
  the 
  light 
  of 
  the 
  information 
  brought 
  forward 
  in 
  the 
  

   present 
  paper, 
  the 
  following 
  conditions 
  should 
  obtain 
  in 
  a 
  

   measurement 
  of 
  the 
  gradient 
  : 
  — 
  

  

  (i.) 
  Observations 
  should 
  be 
  taken 
  over 
  a 
  level 
  plain, 
  con- 
  

   sisting 
  of 
  a 
  homogeneous 
  soil 
  in 
  the 
  surface-layer 
  : 
  the 
  

   radium 
  content 
  of 
  samples 
  taken 
  at 
  a 
  lew 
  centimetres' 
  depth, 
  

   but 
  over 
  as 
  wide 
  an 
  area 
  as 
  possible, 
  should 
  be 
  determined 
  by 
  

   a 
  separate 
  experiment 
  as 
  well 
  as 
  their 
  coefficients 
  of 
  absorp- 
  

   tion 
  to 
  7-rays. 
  

  

  (ii.) 
  The 
  ionization 
  in 
  a 
  cavity 
  a 
  few 
  centimetres 
  deep 
  

   should 
  be 
  measured 
  : 
  from 
  a 
  practical 
  point 
  of 
  view 
  this 
  is 
  

   most 
  easily 
  accomplished 
  by 
  placing 
  a 
  shallow 
  wooden 
  box, 
  

   about 
  25 
  centimetres 
  deep, 
  and 
  as 
  wide 
  and 
  broad 
  as 
  possible, 
  

   on 
  two 
  trestles 
  and 
  measuring 
  the 
  ionization 
  beneath 
  the 
  

   box 
  as 
  closely 
  as 
  possible 
  to 
  the 
  lower 
  surface. 
  By 
  filling 
  

   the 
  box 
  to 
  various 
  depths 
  with 
  soil 
  typical 
  of 
  that 
  found 
  in 
  

   the 
  locality, 
  measurements 
  equivalent 
  to 
  the 
  measurements 
  

   of 
  the 
  gradients 
  in 
  the 
  earth 
  can 
  be 
  obtained. 
  Corrections 
  

   for 
  the 
  finite 
  size 
  of 
  the 
  box 
  may 
  be 
  made 
  by 
  means 
  of 
  an 
  

   analysis 
  similar 
  to 
  that 
  given 
  by 
  the 
  writer 
  for 
  a 
  circular 
  

   plate 
  f 
  , 
  in 
  which 
  case 
  the 
  formulae 
  are 
  comparatively 
  simple 
  : 
  

   this 
  corresponds 
  to 
  making 
  the 
  box 
  referred 
  to 
  circular 
  in 
  

   shape. 
  

  

  * 
  Chauveau, 
  he. 
  cit. 
  p. 
  72. 
  

   § 
  King, 
  loc. 
  cit. 
  p. 
  249, 
  

  

  