﻿the 
  Viscosity 
  of 
  Glacier 
  Ice. 
  101 
  

  

  between 
  the 
  surface 
  velocity 
  and 
  bottom 
  velocity 
  that 
  must 
  

   be 
  used 
  in 
  the 
  calculations. 
  

  

  The 
  various 
  estimates 
  of 
  slip 
  are, 
  in 
  per 
  cent, 
  of 
  mean 
  

   velocity 
  : 
  — 
  

  

  1. 
  Slip 
  at 
  side 
  of 
  glacier 
  25 
  per 
  cent. 
  

  

  2. 
  By 
  extrapolation 
  of 
  surface 
  velocity 
  curve 
  . 
  . 
  68*7 
  ,, 
  

  

  3. 
  Above, 
  corrected 
  for 
  probable 
  greater 
  slip 
  at 
  

  

  sides 
  52-2 
  „ 
  

  

  4. 
  Mean 
  of 
  direct 
  comparison 
  of 
  actual 
  with 
  

  

  hypothetical 
  depth 
  and 
  parabolic 
  velocity 
  

  

  curve 
  16 
  ,, 
  

  

  5. 
  Above, 
  taking 
  77 
  per 
  cent, 
  as 
  the 
  hypothetical 
  

  

  depth 
  compared 
  with 
  actual 
  depth 
  and 
  

  

  parabolic 
  velocity 
  curve 
  31 
  ,, 
  

  

  6. 
  As 
  above, 
  but 
  with 
  lesser 
  surface 
  viscosity 
  . 
  . 
  43 
  ,, 
  

  

  7. 
  As 
  above, 
  for 
  winter 
  51 
  ,, 
  

  

  The 
  last 
  result 
  appears 
  to 
  give 
  the 
  most 
  accurate 
  result, 
  

   and 
  we 
  shall 
  use 
  in 
  our 
  calculations 
  a 
  winter 
  velocity 
  of 
  

   42*1 
  metres 
  per 
  annum 
  and 
  a 
  slip 
  of 
  21*46 
  metres 
  per 
  annum, 
  

   giving 
  a 
  differential 
  shear 
  velocity 
  of 
  20*64 
  metres 
  per 
  

   annum, 
  or 
  0*0000654 
  cm. 
  per 
  second. 
  

  

  Force 
  Producing 
  Motion. 
  — 
  The 
  force 
  producing 
  motion 
  

   results 
  from 
  the 
  slope 
  of 
  the 
  glacier. 
  If 
  the 
  glacier 
  were 
  of 
  

   equal 
  thickness 
  longitudinally 
  the 
  slope 
  of 
  the 
  bed 
  would 
  be 
  

   equal 
  to 
  that 
  of 
  the 
  surface. 
  But 
  in 
  the 
  region 
  of 
  ablation 
  

   the 
  glacier 
  generally 
  gets 
  thinner 
  and 
  thinner 
  the 
  farther 
  it 
  

   is 
  followed 
  down 
  tbe 
  valley. 
  It 
  is 
  a 
  question, 
  therefore, 
  

   which 
  figure 
  should 
  be 
  used, 
  that 
  giving 
  the 
  slope 
  of 
  the 
  

   glacier 
  bed 
  or 
  that 
  giving 
  the 
  slope 
  of 
  the 
  glacier 
  upper 
  

   surface. 
  

  

  AVith 
  regard 
  to 
  the 
  slope 
  of 
  the 
  bed, 
  this 
  can 
  only 
  be 
  

   obtained 
  by 
  empirical 
  methods. 
  But 
  even 
  when 
  the 
  bed 
  is 
  

   flat, 
  if 
  there 
  is 
  a 
  slope 
  of 
  the 
  upper 
  surface 
  the 
  ice 
  flows 
  in 
  

   the 
  direction 
  of 
  the 
  slope. 
  It 
  is 
  clear 
  from 
  this 
  that 
  it 
  is 
  

   the 
  slope 
  of 
  the 
  upper 
  surface 
  of 
  the 
  ice 
  which 
  is 
  most 
  

   important, 
  and 
  as 
  this 
  can 
  readily 
  be 
  obtained 
  with 
  accuracy, 
  

   we 
  shall 
  consider 
  the 
  downward 
  component 
  of 
  gravity 
  as 
  

   being 
  proportional 
  to 
  the 
  slope 
  of 
  the 
  upper 
  surface 
  of 
  the 
  

   glacier. 
  The 
  difference 
  between 
  the 
  slope 
  of 
  the 
  upper 
  

   surface 
  and 
  that 
  of 
  the 
  glacier 
  bed 
  cannot 
  differ 
  very 
  much, 
  

   so 
  the 
  error 
  introduced 
  cannot 
  be 
  large. 
  

  

  Fig. 
  10 
  shows 
  a 
  plan, 
  after 
  Bliimcke 
  and 
  Hess, 
  of 
  the 
  

   Hintereis 
  Glacier 
  tongue 
  both 
  above 
  and 
  below 
  section 
  V. 
  

   Sections 
  IV. 
  and 
  VI. 
  are 
  600 
  metres 
  apart, 
  and 
  four 
  lines 
  

  

  