﻿472 
  W. 
  Bowie 
  — 
  Distribution 
  of 
  Isostatic 
  Compensation. 
  

  

  the 
  regional 
  method 
  the 
  compensation 
  for 
  each 
  topographic 
  

   feature 
  should 
  v 
  be 
  distributed 
  separately 
  before 
  making 
  the 
  

   computations 
  to 
  obtain 
  its 
  effect. 
  This, 
  of 
  course, 
  would 
  be 
  

   possible, 
  but 
  it 
  would 
  be 
  such 
  a 
  laborious 
  process 
  that 
  it 
  would 
  

   not 
  be 
  practicable. 
  While 
  the 
  method 
  of 
  making 
  the 
  compu- 
  

   tations 
  for 
  the 
  regional 
  distribution 
  of 
  the 
  compensation 
  is 
  

   somewhat 
  illogical, 
  yet 
  it 
  is 
  probable 
  that 
  none 
  of 
  the 
  conclu- 
  

   sions 
  reached 
  in 
  the 
  study 
  of 
  the 
  question 
  would 
  be 
  materially 
  

   changed 
  if 
  the 
  ideal 
  method 
  had 
  been 
  employed. 
  

  

  The 
  compensation 
  of 
  the 
  topography, 
  beyond 
  the 
  areas 
  

   selected 
  for 
  the 
  regional 
  distribution, 
  was 
  assumed 
  to 
  be 
  

   directly 
  under 
  the 
  topographic 
  features, 
  as 
  is 
  the 
  case 
  with 
  the 
  

   local 
  distribution 
  computations. 
  

  

  In 
  each 
  test 
  the 
  gravity 
  anomaly 
  was 
  computed 
  for 
  each 
  of 
  

   the 
  124 
  stations. 
  An 
  anomaly 
  is 
  the 
  difference 
  between 
  the 
  

   observed 
  value 
  of 
  gravity 
  and 
  the 
  computed 
  value. 
  The 
  com- 
  

   puted 
  value 
  is 
  the 
  theoretical 
  value, 
  at 
  sea-level, 
  for 
  the 
  latitude 
  

   of 
  the 
  station, 
  corrected 
  for 
  the 
  elevation 
  of 
  the 
  station, 
  for 
  

   the 
  attraction 
  of 
  the 
  topography 
  of 
  the 
  whole 
  world 
  (this 
  is 
  

   nearly 
  always 
  positive 
  for 
  land 
  areas 
  and 
  is 
  always 
  negative 
  for 
  

   ocean 
  areas), 
  and 
  for 
  the 
  attraction 
  of 
  the 
  isostatic 
  compensa- 
  

   tion 
  (this 
  is 
  negative 
  for 
  land 
  areas 
  and 
  positive 
  for 
  ocean 
  areas). 
  

  

  The 
  value 
  of 
  gravity 
  at 
  the 
  base 
  station 
  in 
  Washington 
  is 
  

   980*112 
  dynes 
  per 
  gram 
  as 
  a 
  force 
  or 
  centimeters 
  per 
  second 
  

   per 
  second 
  as 
  an 
  acceleration. 
  The 
  anomaly 
  is 
  given 
  in 
  the 
  

   third 
  place 
  of 
  decimals 
  of 
  dynes. 
  Thus 
  the 
  anomaly 
  of 
  *001 
  

   dyne 
  is 
  about 
  one 
  millionth 
  of 
  the 
  force 
  of 
  gravity. 
  

  

  A 
  disc 
  of 
  material 
  2*67 
  in 
  density, 
  100 
  feet 
  thick, 
  and 
  20 
  

   kilometers 
  in 
  diameter, 
  has 
  an 
  attractive 
  effect 
  of 
  '0034 
  dyne 
  

   on 
  a 
  gram 
  mass 
  located 
  on 
  the 
  surface 
  of 
  the 
  disc 
  at 
  its 
  center. 
  

   We 
  may 
  therefore 
  interpret 
  the 
  average 
  anomaly 
  (0*020) 
  as 
  

   equivalent 
  to 
  an 
  excess 
  or 
  deficiency 
  of 
  mass 
  of 
  average 
  density 
  

   which 
  is 
  20 
  kilometers 
  in 
  diameter 
  and 
  600 
  feet 
  thick. 
  * 
  

  

  There 
  are 
  given 
  in 
  the 
  following 
  table 
  the 
  mean 
  anomalies 
  

   with 
  and 
  without 
  regard 
  to 
  sign 
  for 
  all 
  of 
  the 
  124 
  stations 
  used 
  

   in 
  the 
  tests. 
  

  

  Eegional 
  Anomalies 
  

   Local 
  , 
  * 
  n 
  

  

  Anomaly 
  Zone 
  K 
  Zone 
  M 
  Zone 
  O 
  

   Mean 
  with 
  regard 
  to 
  sign 
  —0-002 
  —0-001 
  —0*001 
  —0-002 
  

  

  Mean 
  without 
  regard 
  to 
  sign 
  0*020 
  0*019 
  0*020 
  0*020 
  

  

  The 
  mean 
  anomalies 
  for 
  the 
  various 
  methods 
  of 
  distribution 
  

   of 
  the 
  compensation 
  are 
  practically 
  the 
  same. 
  This 
  is 
  no 
  

   doubt 
  due 
  to 
  the 
  fact 
  that 
  most 
  of 
  the 
  stations 
  are 
  on 
  topog- 
  

   raphy 
  with 
  little 
  relief. 
  

  

  * 
  A 
  table 
  of 
  attractions 
  for 
  various 
  masses 
  is 
  given 
  on 
  page 
  73 
  of 
  Special 
  

   Publication 
  No. 
  40. 
  

  

  