﻿Geological 
  Society. 
  135 
  

  

  moment 
  when 
  the 
  measurement 
  is 
  made 
  is 
  extremely 
  small 
  

   and 
  quite 
  beyond 
  reach 
  of 
  any 
  thermo-electric 
  ammeter. 
  

   The 
  writer 
  is 
  therefore 
  o£ 
  opinion 
  that 
  an 
  exact 
  experimental 
  

   determination 
  of 
  the 
  telephone 
  impedance 
  under 
  actual 
  

   working 
  conditions 
  is 
  a 
  matter 
  of 
  higher 
  order 
  of 
  difficult 
  v 
  

   than 
  the 
  measurement 
  of 
  received 
  antenna-current 
  itself. 
  

  

  It 
  must 
  further 
  be 
  borne 
  in 
  mind 
  that 
  in 
  any 
  case 
  the 
  

   shunt 
  value 
  which 
  quenches 
  the 
  telephone 
  sound 
  is 
  difficult 
  

   to 
  determine 
  in 
  practice 
  with 
  any 
  but 
  a 
  rough 
  approxima- 
  

   tion. 
  In 
  a 
  very 
  quiet 
  room 
  it 
  may 
  perhaps 
  be 
  determined 
  to 
  

   within 
  5 
  or 
  10 
  per 
  cent., 
  but 
  in 
  a 
  wireless 
  station 
  or 
  on 
  board 
  

   ship 
  perhaps 
  not 
  within 
  30 
  or 
  40 
  per 
  cent. 
  

  

  Xo 
  assumptions 
  as 
  that 
  made 
  by 
  Prof. 
  Howe 
  that 
  the 
  true 
  

   impedance 
  of 
  the 
  telephone 
  under 
  actual 
  working- 
  conditions 
  

   is 
  equal 
  to 
  four 
  times 
  the 
  steady 
  resistance 
  has 
  been 
  justified 
  

   by 
  any 
  experiments. 
  Hence, 
  to 
  avoid 
  suppositions 
  not 
  based 
  

   on 
  experiment, 
  the 
  value 
  taken 
  for 
  the 
  calculation 
  of 
  the 
  

   audibility 
  factor 
  in 
  the 
  case 
  of 
  my 
  experiments 
  was 
  the 
  

   steady 
  resistance, 
  although 
  1 
  was 
  perfectly 
  well 
  aware 
  that 
  

   this 
  was 
  not 
  identical 
  with 
  the 
  true 
  impedance 
  for 
  the 
  wave- 
  

   form 
  and 
  frequencies 
  used. 
  

  

  Having 
  regard 
  to 
  the 
  uncertainty 
  attending 
  the 
  constants 
  

   employed 
  by 
  Austin 
  and 
  Hogan, 
  and 
  the 
  difficulty 
  of 
  deter- 
  

   mining 
  exact 
  valnes, 
  it 
  seemed 
  better 
  to 
  base 
  the 
  reduction 
  

   of 
  the 
  observations 
  on 
  known 
  measurements 
  rather 
  than 
  on 
  

   assumptions 
  as 
  to 
  the 
  ratio 
  of 
  impedance 
  to 
  resistance. 
  

  

  XIV. 
  Proceedings 
  of 
  Learned 
  Societies. 
  

  

  GEOLOGICAL 
  SOCIETY. 
  

  

  [Continued 
  from 
  toI. 
  xxxiv. 
  p. 
  528.] 
  

  

  June 
  20th, 
  1917.— 
  Dr. 
  Alfred 
  Harker, 
  F.R.S., 
  President, 
  

   in 
  the 
  Chair. 
  

   r 
  PHE 
  following 
  communications 
  were 
  read 
  : 
  — 
  

  

  1. 
  ' 
  The 
  Pre- 
  Cambrian 
  and 
  Associated 
  Eocks 
  of 
  the 
  District 
  

   of 
  Mozambique.' 
  By 
  Arthur 
  Holmes, 
  A.R.C.S., 
  D.I.C., 
  B.Sc, 
  

   F.G.S. 
  

  

  Beyond 
  the 
  coastal 
  and 
  volcanic 
  beds 
  of 
  Mozambique 
  (described 
  

   in 
  a 
  previous 
  contribution 
  — 
  Abs. 
  Proc. 
  Geol. 
  Soc. 
  1916, 
  No. 
  994, 
  

   p. 
  72 
  j 
  the 
  country 
  assumes 
  the 
  form 
  of 
  a 
  gently 
  undulating 
  plateau, 
  

   gradually 
  rising 
  towards 
  the 
  west 
  and 
  diversified 
  by 
  innumerable 
  

   inselberg 
  peaks 
  and 
  abruptly-rising 
  clusters 
  of 
  hills. 
  The 
  

   dominant 
  rock 
  throughout 
  is 
  a 
  grey 
  biotite-gneiss. 
  Interfoliated 
  

   with 
  this 
  are 
  occasional 
  lenticular 
  masses 
  of 
  hornblende-gneiss 
  and 
  

  

  