﻿128 
  JOURNAL 
  OF 
  THE 
  WASHINGTON 
  ACADEMY 
  OF 
  SCIENCES 
  VOL. 
  12, 
  NO. 
  5 
  

  

  elements 
  but 
  to 
  ascertain 
  the 
  secular 
  change 
  of 
  these 
  elements, 
  to 
  

   outline 
  the 
  disturbed 
  areas, 
  and 
  to 
  discover 
  the 
  effect 
  of 
  ocean 
  depths. 
  

  

  As 
  far 
  as 
  I 
  am 
  aware 
  only 
  two 
  specifically 
  organized 
  magnetic 
  

   surveys 
  of 
  the 
  oceans 
  have 
  ever 
  been 
  established, 
  that 
  of 
  Edmund 
  

   Halley 
  in 
  1698, 
  and 
  that 
  at 
  the 
  present 
  time 
  of 
  the 
  Department 
  of 
  

   Terrestrial 
  Magnetism 
  of 
  the 
  Carnegie 
  Institution 
  of 
  Washington; 
  

   the 
  latter 
  being 
  by 
  far 
  the 
  most 
  comprehensive 
  and 
  extensive, 
  covering 
  

   the 
  seas 
  from 
  latitude 
  60 
  ° 
  south 
  to 
  60 
  ° 
  north, 
  this 
  sea 
  work 
  being 
  supple- 
  

   mented 
  by 
  many 
  hundreds 
  of 
  stations 
  on 
  land 
  in 
  countries 
  where 
  such 
  

   work 
  is 
  not 
  being 
  actively 
  carried 
  out 
  by 
  the 
  nations 
  themselves. 
  

  

  Most 
  excellent 
  work 
  is 
  being 
  done, 
  and 
  the 
  first 
  magnetic 
  survey 
  

   of 
  the 
  oceans 
  may 
  be 
  said 
  now 
  to 
  be 
  nearing 
  completion, 
  but 
  obser- 
  

   vations 
  for 
  the 
  secular 
  change 
  data 
  must 
  be 
  kept 
  up. 
  To 
  keep 
  the 
  

   secular 
  change 
  data 
  up 
  to 
  date, 
  and 
  to 
  fill 
  in 
  as 
  yet 
  unsurveyed 
  areas, 
  

   will 
  probably 
  require 
  the 
  time 
  of 
  one 
  expedition 
  continuously. 
  

  

  Additional 
  magnetic 
  stations 
  are 
  especially 
  needed 
  in 
  the 
  polar 
  

   regions, 
  especially 
  in 
  the 
  north 
  polar 
  area. 
  No 
  opportunity 
  should 
  

   be 
  neglected 
  by 
  any 
  surveying 
  or 
  exploring 
  expedition 
  to 
  obtain 
  

   magnetic 
  and 
  allied 
  observations 
  wherever 
  needed. 
  Such 
  electrical 
  

   observations 
  as 
  may, 
  be 
  found 
  necessary 
  and 
  practicable 
  should 
  also 
  

   be 
  carried 
  out 
  in 
  connection 
  with 
  the 
  magnetic 
  observations 
  at 
  sea. 
  

  

  Gravity 
  observations. 
  — 
  A 
  problem 
  of 
  much 
  importance 
  in 
  the 
  study 
  

   of 
  the 
  physics 
  of 
  the 
  earth 
  is 
  the 
  determination 
  of 
  the 
  intensity 
  of 
  

   gravity 
  at 
  sea 
  first, 
  to 
  furnish 
  further 
  information 
  that 
  wiir 
  enable 
  

   us 
  to 
  ascertain 
  more 
  accurately 
  the 
  shape 
  of 
  the 
  earth, 
  and 
  second, 
  to 
  

   determine 
  the 
  distribution 
  of 
  the 
  densities 
  in 
  the 
  so-called 
  "isostatic 
  

   shell" 
  of 
  the 
  lithosphere. 
  

  

  Researches 
  in 
  this 
  and 
  other 
  countries 
  have 
  made 
  it 
  certain 
  that 
  

   the 
  outer 
  seventy 
  miles 
  of 
  the 
  earth's 
  material 
  is 
  in 
  a 
  state 
  of 
  approxi- 
  

   mate 
  isostatic 
  equilibrium. 
  If 
  we 
  assume 
  a 
  surface 
  seventy 
  miles 
  

   below 
  sea 
  level 
  under 
  the 
  continent 
  and 
  on 
  this 
  surface 
  lay 
  out 
  squares 
  

   approximately 
  one 
  hundred 
  miles 
  on 
  a 
  side 
  and 
  extend 
  vertical 
  planes 
  

   from 
  these 
  sides 
  to 
  the 
  surface 
  of 
  the 
  earth, 
  we 
  should 
  have 
  the 
  same 
  

   mass 
  in 
  each 
  of 
  the 
  columns, 
  though 
  some 
  of 
  the 
  columns 
  would 
  be 
  

   a 
  mile 
  or 
  more 
  longer 
  than 
  others. 
  In 
  other 
  words, 
  each 
  column 
  of 
  

   equal 
  cross 
  section 
  is 
  found 
  to 
  have 
  about 
  the 
  same 
  pressure 
  on 
  the 
  

   nucleus 
  at 
  a 
  depth 
  of 
  seventy 
  miles 
  below 
  sea 
  level 
  as 
  any 
  other 
  column. 
  

  

  Do 
  these 
  conditions 
  exist 
  under 
  the 
  ocean? 
  The 
  answer 
  to 
  this 
  

   question 
  requires 
  the 
  obtaining 
  of 
  observations 
  for 
  the 
  intensity 
  of 
  

   gravity 
  over 
  ocean 
  areas. 
  

  

  