﻿100 
  Messrs. 
  R. 
  M. 
  Deeley 
  and 
  P. 
  H. 
  Parr 
  on 
  

  

  This 
  is 
  about 
  16 
  per 
  cent, 
  of 
  the 
  average 
  annual 
  velocity. 
  

  

  A 
  = 
  Borings 
  on 
  section 
  V. 
  

   B 
  = 
  Surface-speed 
  at 
  boring. 
  

   C 
  = 
  Percentage 
  of 
  slip. 
  

   D 
  = 
  Slip 
  in 
  metres 
  per 
  annum. 
  

  

  Borings 
  4 
  and 
  5 
  are 
  on 
  opposite 
  sides 
  of 
  the 
  thickest 
  

   portion 
  of 
  the 
  glacier, 
  and 
  yet 
  there 
  is 
  a 
  very 
  great 
  difference 
  

   between 
  the 
  calculated 
  slips 
  at 
  these 
  points. 
  The 
  slips 
  at 
  

   short 
  distances 
  from 
  the 
  two 
  sides 
  are 
  also 
  much 
  smaller 
  

   than 
  the 
  actual 
  slips 
  obtained 
  near 
  the 
  edges 
  of 
  the 
  glacier. 
  

  

  Two 
  borings 
  were 
  also 
  made 
  at 
  other 
  points, 
  and 
  Blumcke 
  

   and 
  Hess 
  consider 
  that 
  the 
  hypothetical 
  thickness 
  of 
  the 
  ice, 
  

   made 
  on 
  the 
  assumption 
  that 
  there 
  is 
  no 
  differential 
  motion, 
  

   is 
  77 
  per 
  cent, 
  of 
  the 
  actual 
  thickness. 
  Regarding 
  the 
  

   vertical 
  curve 
  of 
  velocity 
  as 
  a 
  parabola, 
  this 
  would 
  give 
  a 
  

   slip 
  a 
  little 
  in 
  excess 
  of 
  31 
  per 
  cent. 
  

  

  The 
  method 
  of 
  calculation 
  employed 
  to 
  obtain 
  the 
  values 
  

   given 
  in 
  Table 
  II., 
  column 
  C, 
  is 
  as 
  follows 
  : 
  — 
  

  

  In 
  the 
  diagram 
  (fig. 
  8) 
  the 
  area 
  AA'CC 
  gives 
  the 
  volume 
  

   of 
  ice 
  passing 
  when 
  there 
  is 
  no 
  differential 
  motion, 
  whilst 
  

   AA'DB 
  gives 
  a 
  similar 
  area 
  when 
  there 
  is 
  a 
  bottom 
  slip 
  of 
  

   50 
  per 
  cent. 
  When 
  the 
  hypothetical 
  depth 
  is 
  66'6 
  per 
  cent. 
  

   of 
  the 
  actual 
  depth 
  there 
  is 
  no 
  slip, 
  and 
  when, 
  for 
  example, 
  

   the 
  hypothetical 
  depth 
  is 
  83*5 
  per 
  cent., 
  the 
  slip 
  is 
  50 
  per 
  

   cent. 
  This 
  is 
  the 
  case 
  when 
  the 
  vertical 
  curve 
  of 
  velocity 
  is 
  

   a 
  parabola. 
  

  

  But 
  we 
  have 
  given 
  reasons 
  for 
  believing 
  that 
  the 
  mean- 
  

   speed 
  curve 
  is 
  not 
  a 
  parabola 
  near 
  the 
  surface, 
  owing 
  to 
  the 
  

   effect 
  of 
  the 
  sun's 
  heat 
  upon 
  the 
  upper 
  portion 
  of 
  the 
  glacier. 
  

   The 
  conditions 
  are 
  probably 
  more 
  nearly 
  as 
  shown 
  in 
  fig. 
  9. 
  

   Here 
  the 
  mean 
  annual 
  curve 
  of 
  velocity 
  is 
  of 
  the 
  form 
  A'B 
  and 
  

   the 
  winter 
  curve 
  the 
  parabola 
  CD, 
  for 
  we 
  have 
  shown 
  that 
  

   the 
  winter 
  velocity 
  is 
  probably 
  12*3 
  per 
  cent, 
  less 
  than 
  the 
  

   mean 
  annual 
  velocity. 
  With 
  such 
  a 
  mean 
  annual 
  curve 
  

   the 
  hypothetical 
  depth 
  would 
  be 
  about 
  60 
  per 
  cent, 
  for 
  no 
  

   slip 
  to 
  take 
  place. 
  But 
  the 
  hypothetical 
  depth, 
  according 
  to 
  

   Blumcke 
  and 
  Hess, 
  is 
  77 
  per 
  cent. 
  Under 
  these 
  circum- 
  

   stances 
  the 
  mean 
  annual 
  curve 
  must 
  be 
  AC 
  (fig. 
  9) 
  and 
  the 
  

   slip 
  about 
  43 
  per 
  cent, 
  of 
  the 
  mean 
  annual 
  velocity, 
  or 
  

   20'64 
  metres 
  per 
  annum. 
  In 
  the 
  winter 
  the 
  differential 
  

   motion 
  would 
  be 
  42'1 
  "-20*64 
  = 
  21*46 
  metres 
  per 
  annum. 
  

   We 
  thus 
  have 
  a 
  winter 
  slip 
  of 
  51 
  per 
  cent. 
  

  

  The 
  necessity 
  for 
  getting 
  the 
  slip 
  accurately 
  arises 
  from 
  

   the 
  fact 
  that 
  to 
  calculate 
  the 
  viscosity 
  it 
  is 
  the 
  difference 
  

  

  