﻿1875.] 
  

  

  Specific 
  Gravity 
  of 
  Sea-water. 
  

  

  303 
  

  

  siderable 
  delicacy 
  is 
  necessary 
  for 
  recording 
  them. 
  As 
  far 
  as 
  I 
  have 
  

   hitherto 
  been 
  able 
  to 
  observe, 
  they 
  lie 
  between 
  the 
  extremes 
  1*02780 
  and 
  

   1*02400 
  ; 
  the 
  results, 
  therefore, 
  to 
  be 
  of 
  any 
  value, 
  must 
  be 
  correct, 
  at 
  

   least 
  to 
  one 
  in 
  the 
  fourth 
  decimal 
  place. 
  In 
  mentioning 
  these 
  extremes, 
  

   it 
  must 
  be 
  observed 
  that 
  they 
  refer 
  to 
  oc^em-waters, 
  and 
  not 
  to 
  the 
  mix- 
  

   tures 
  of 
  fresh 
  and 
  salt 
  water 
  to 
  be 
  found 
  in 
  bays 
  and 
  estuaries, 
  where 
  

   waters 
  of 
  all 
  degrees 
  of 
  saltness 
  may 
  be 
  found. 
  The 
  instrument 
  selected 
  

   was 
  the 
  hydrometer 
  ; 
  and 
  the 
  purpose 
  which 
  it 
  was 
  to 
  serve 
  being 
  of 
  so 
  

   very 
  special 
  a 
  nature, 
  I 
  preferred 
  to 
  have 
  a 
  special 
  instrument 
  made 
  for 
  

   it, 
  to 
  making 
  use 
  of 
  the 
  hydrometer 
  ordinarily 
  supplied 
  by 
  the 
  instrument- 
  

   maker. 
  It 
  has 
  a 
  large 
  body 
  and 
  fine 
  stem, 
  the 
  relation 
  in 
  size 
  of 
  the 
  one 
  

   to 
  the 
  other, 
  and 
  the 
  absolute 
  size 
  of 
  both 
  of 
  them, 
  having 
  been 
  deter- 
  

   mined 
  beforehand 
  by 
  calculation, 
  so 
  as 
  to 
  obtain 
  the 
  requisite 
  delicacy. 
  

   It 
  is 
  evident 
  that, 
  for 
  a 
  hydrometer 
  of 
  given 
  size, 
  in 
  the 
  measure 
  that 
  its 
  

   delicacy 
  is 
  increased 
  its 
  range 
  is 
  diminished. 
  In 
  determining 
  the 
  specific 
  

   gravities 
  of 
  sea-waters 
  both 
  great 
  delicacy 
  and 
  considerable 
  range 
  are 
  

   required 
  ; 
  the 
  latter 
  is 
  secured, 
  without 
  detriment 
  to 
  the 
  former, 
  by 
  the 
  

   application 
  of 
  the 
  principle 
  of 
  Nicholson's 
  hydrometer. 
  In 
  the 
  paper 
  of 
  

   which 
  this 
  is 
  an 
  abstract, 
  the 
  construction, 
  calibration, 
  and 
  method 
  of 
  

   observing 
  the 
  instrument 
  are 
  minutely 
  described 
  and 
  illustrated 
  by 
  a 
  

   drawing. 
  The 
  description 
  of 
  the 
  instrument 
  is 
  briefly 
  as 
  follows 
  : 
  — 
  The 
  

   stem, 
  which 
  carries 
  a 
  millimetre- 
  scale 
  10 
  centimetres 
  long, 
  has 
  an 
  out- 
  

   side 
  diameter 
  of 
  about 
  3 
  millimetres, 
  the 
  external 
  volume 
  of 
  the 
  divided 
  

   portion 
  being 
  accurately 
  0*8607 
  cubic 
  centimetre 
  ; 
  the 
  mean 
  volume 
  of 
  

   the 
  body 
  is 
  160*15 
  cubic 
  centimetres, 
  and 
  the 
  weight 
  of 
  the 
  glass 
  instru- 
  

   ment 
  is 
  160*0405 
  grammes. 
  With 
  this 
  volume 
  and 
  weight 
  it 
  floats 
  in 
  

   distilled 
  water 
  of 
  16° 
  C. 
  at 
  about 
  the 
  lowest 
  division 
  (100) 
  of 
  the 
  scale. 
  

   In 
  order 
  to 
  make 
  it 
  serviceable 
  for 
  heavier 
  waters, 
  a 
  small 
  brass 
  table 
  is 
  

   made 
  to 
  rest 
  on 
  the 
  top 
  of 
  the 
  stem, 
  of 
  such 
  a 
  weight 
  that 
  it 
  depresses 
  

   the 
  instrument 
  in 
  distilled 
  water 
  of 
  16° 
  C. 
  to 
  about 
  the 
  topmost 
  division 
  

   (0) 
  of 
  the 
  scale. 
  By 
  means 
  of 
  a 
  series 
  of 
  six 
  weights, 
  multiples 
  by 
  1, 
  2, 
  

   3, 
  4, 
  5, 
  and 
  6 
  of 
  the 
  weight 
  of 
  the 
  table, 
  specific 
  gravities 
  between 
  1 
  and 
  

   1*03400 
  can 
  be 
  observed. 
  It 
  is 
  not 
  necessary 
  that 
  these 
  weights 
  should 
  

   be 
  accurate 
  multiples 
  of 
  the 
  weight 
  of 
  the 
  table 
  ; 
  it 
  is 
  sufficient 
  if 
  they 
  

   approach 
  it 
  within 
  a 
  centigramme 
  and 
  their 
  actual 
  weight 
  be 
  known 
  

   with 
  accuracy. 
  The 
  weights 
  of 
  the 
  table 
  and 
  weights 
  in 
  actual 
  use 
  

   are 
  : 
  — 
  

  

  Weight 
  of 
  table 
  0*8360 
  gramme. 
  

  

  Weight 
  of 
  weight 
  No. 
  1 
  0*8560 
  

  

  II 
  1*6010 
  

  

  „ 
  III 
  2*4225 
  grammes. 
  

  

  IV 
  3*2145 
  

  

  V 
  4*0710 
  „ 
  

  

  VI 
  4*8245 
  

  

  