﻿G. 
  C. 
  ComstocJc 
  — 
  The 
  Secular 
  Vcwiation 
  of 
  Latitudes. 
  471 
  

  

  of 
  ice 
  present 
  in 
  the 
  northern 
  hemisphere 
  at 
  the 
  time 
  of 
  maxi- 
  

   mum 
  glaciation 
  was 
  distributed 
  in 
  a 
  manner 
  very 
  different 
  

   from 
  the 
  present 
  arrangement, 
  and 
  this 
  different 
  distribution 
  

   will 
  be 
  fully 
  explained 
  by 
  shifting 
  the 
  terrestrial 
  pole 
  from 
  its 
  

   present 
  position 
  to 
  the 
  center 
  of 
  Greenland. 
  Opposed 
  to 
  this 
  

   explanation, 
  however, 
  stands 
  the 
  common 
  belief 
  of 
  astronomers 
  

   that 
  the 
  position 
  of 
  the 
  pole 
  if 
  not 
  absolutely 
  fixed 
  is 
  subject 
  

   only 
  to 
  very 
  inconsiderable 
  changes. 
  

  

  To 
  guard 
  against 
  any 
  possible 
  misapprehension 
  let 
  it 
  be 
  

   stated 
  once 
  for 
  all 
  that 
  the 
  questions 
  here 
  raised 
  do 
  not 
  relate 
  

   to 
  the 
  direction 
  of 
  the 
  earth's 
  axis 
  in 
  space, 
  i. 
  e. 
  to 
  the 
  phe- 
  

   nomena 
  grouped 
  under 
  the 
  names 
  precession 
  and 
  nutation, 
  but 
  

   to 
  the 
  position 
  of 
  the 
  points 
  in 
  which 
  the 
  rotation 
  axis 
  inter- 
  

   sects 
  the 
  earth's 
  surface. 
  

  

  If 
  any 
  such 
  change 
  in 
  the 
  position 
  of 
  the 
  pole 
  as 
  is 
  supposed 
  

   above 
  has 
  occurred 
  within 
  recent 
  geologic 
  time 
  it 
  may 
  be 
  

   fairly 
  presumed 
  that 
  some 
  motion 
  will 
  still 
  remain 
  although 
  

   nothing 
  can 
  be 
  predicated 
  a 
  priori 
  in 
  regard 
  to 
  its 
  amount 
  or 
  

   direction, 
  and 
  the 
  problem 
  which 
  I 
  have 
  proposed 
  to 
  myself 
  is 
  

   to 
  determine 
  whether 
  there 
  is 
  any 
  such 
  motion 
  of 
  the 
  pole 
  of 
  

   sufficient 
  magnitude 
  to 
  be 
  shown 
  by 
  existing 
  astronomical 
  data. 
  

  

  Theoretically 
  there 
  are 
  three 
  classes 
  of 
  observations 
  which 
  

   may 
  contribute 
  to 
  the 
  solution 
  of 
  the 
  problem 
  : 
  determinations 
  

   of 
  latitude, 
  of 
  azimuth 
  and 
  of 
  longitude 
  ; 
  but 
  for 
  the 
  present 
  

   at 
  least 
  only 
  the 
  first 
  of 
  these 
  can 
  furnish 
  available 
  data 
  and 
  

   the 
  amount 
  of 
  satisfactory 
  data 
  of 
  this 
  kind 
  is 
  exceedingly 
  

   small. 
  I 
  do 
  not 
  wish 
  to 
  enlarge 
  here 
  upon 
  the 
  inherent 
  diffi- 
  

   culties 
  which 
  stand 
  in 
  the 
  way 
  of 
  determining 
  a 
  change 
  in 
  the 
  

   latitude 
  of 
  a 
  given 
  station, 
  but 
  some 
  consideration 
  of 
  them 
  is 
  

   necessary 
  for 
  the 
  proper 
  appreciation 
  of 
  the 
  conclusions 
  which 
  

   are 
  subsequently 
  reached. 
  

  

  To 
  take 
  a 
  concrete 
  instance, 
  the 
  following 
  determinations 
  of 
  

   latitude 
  at 
  Greenwich 
  seem 
  to 
  indicate 
  a 
  progressive 
  change 
  in 
  

   the 
  position 
  of 
  that 
  observatory 
  : 
  

  

  Date. 
  Latitude. 
  Authority. 
  

  

  1693 
  

  

  51° 
  28' 
  41 
  // 
  -7 
  

  

  Peters. 
  

  

  Flamsteed. 
  

  

  1751 
  

  

  38-72 
  

  

  Auwers. 
  

  

  Bradley. 
  

  

  1826 
  

  

  38*59 
  

  

  Pond. 
  

  

  Gr. 
  Obs. 
  1834 
  

  

  1838 
  

  

  38-23 
  

  

  Airy. 
  

  

  Gr. 
  Cat. 
  1860 
  

  

  1845 
  

  

  38-17 
  

  

  a 
  

  

  a 
  u 
  u 
  

  

  1855 
  

  

  38-15 
  

  

  a 
  

  

  u 
  a 
  a 
  

  

  1881 
  

  

  38-07 
  

  

  Christie. 
  

  

  Ten 
  Tear 
  Catalogue. 
  

  

  1889 
  

  

  37-95 
  

  

  a 
  

  

  Annual 
  Report. 
  

  

  Am. 
  Jour. 
  Sci.-— 
  Third 
  Series, 
  Yol. 
  XLII, 
  No. 
  252.— 
  December, 
  1891. 
  

   32 
  

  

  