﻿304 
  

  

  Mr. 
  J. 
  Y. 
  Buchanan 
  on 
  the 
  

  

  [Mar. 
  4, 
  

  

  For 
  oceanic 
  waters 
  the 
  hydrometer 
  is 
  always 
  used 
  with 
  the 
  table 
  and 
  

   either 
  No. 
  IV. 
  or 
  No. 
  V. 
  weight. 
  

  

  When 
  the 
  mechanical 
  part 
  of 
  the 
  construction 
  of 
  the 
  instrument 
  was 
  

   finished, 
  with 
  the 
  exception 
  of 
  the 
  closing 
  of 
  the 
  top 
  of 
  the 
  stem 
  (which, 
  

   instead, 
  was 
  widened 
  into 
  a 
  funnel- 
  shape 
  large 
  enough 
  to 
  receive 
  the 
  

   ordinary 
  decigramme 
  weights), 
  the 
  calibration 
  of 
  the 
  stem 
  was 
  effected 
  by 
  

   loading 
  the 
  stem 
  with 
  successive 
  weights, 
  and 
  observing 
  the 
  consequent 
  

   depressions 
  in 
  distilled 
  water 
  of 
  known 
  temperature. 
  This 
  done, 
  the 
  top 
  

   was 
  sealed 
  up 
  and 
  the 
  instrument 
  carefully 
  weighed. 
  The 
  expansion 
  

   of 
  the 
  body 
  with 
  temperature 
  was 
  determined 
  in 
  a 
  similar 
  manner 
  by 
  

   reading 
  the 
  instrument 
  in 
  distilled 
  water 
  of 
  various 
  temperatures. 
  The 
  

   coefficient 
  of 
  expansion 
  of 
  the 
  glass 
  was 
  then 
  found 
  to 
  be 
  0-000029 
  per 
  

   degree 
  Centigrade. 
  

  

  For 
  using 
  this 
  instrument 
  at 
  sea 
  about 
  900 
  cubic 
  centimetres 
  of 
  water 
  

   are 
  taken, 
  and 
  the 
  containing 
  cylinder 
  placed 
  on 
  a 
  swinging 
  table, 
  in 
  a 
  

   position 
  as 
  near 
  the 
  centre 
  of 
  the 
  ship 
  as 
  possible. 
  The 
  observation 
  with 
  

   the 
  hydrometer, 
  loaded 
  with 
  the 
  necessary 
  table 
  and 
  weight, 
  is 
  then 
  

   effected 
  in 
  the 
  ordinary 
  way, 
  the 
  accuracy 
  of 
  the 
  readings 
  being 
  but 
  little 
  

   affected 
  by 
  rolling 
  ; 
  pitching, 
  however, 
  is 
  found 
  to 
  have 
  a 
  distinctly 
  dis- 
  

   turbing 
  effect, 
  and, 
  when 
  it 
  is 
  in 
  any 
  way 
  violent, 
  it 
  is 
  advisable 
  to 
  store 
  

   the 
  specimen 
  of 
  water 
  till 
  the 
  weather 
  improves. 
  The 
  precautions 
  to 
  be 
  

   observed 
  in 
  making 
  these 
  observations 
  at 
  sea 
  must 
  be 
  sought 
  for 
  in 
  the 
  

   paper. 
  

  

  The 
  temperature 
  of 
  the 
  water, 
  at 
  the 
  time 
  of 
  observation, 
  is 
  determined 
  

   by 
  one 
  of 
  Geissler's 
  " 
  normal 
  " 
  or 
  standard 
  thermometers, 
  graduated 
  into 
  

   tenths 
  of 
  a 
  degree 
  Centigrade 
  ; 
  and 
  it 
  is 
  essential 
  for 
  the 
  accuracy 
  of 
  the 
  

   results 
  that 
  the 
  water, 
  during 
  the 
  observation 
  of 
  the 
  hydrometer, 
  should 
  

   be 
  sensibly 
  at 
  the 
  same 
  temperature 
  as 
  the 
  atmosphere, 
  otherwise 
  the 
  

   changing 
  temperature 
  of 
  the 
  water 
  makes 
  the 
  readings 
  of 
  both 
  the 
  hydro- 
  

   meter 
  and 
  the 
  thermometer 
  uncertain. 
  At 
  low 
  temperatures 
  (below 
  10° 
  

   or 
  12° 
  C.) 
  a 
  tenth 
  of 
  a 
  degree 
  makes 
  no 
  sensible 
  difference 
  in 
  the 
  result- 
  

   ing 
  specific 
  gravity 
  ; 
  but 
  at 
  the 
  high 
  temperatures 
  always 
  found 
  at 
  the 
  

   surface 
  of 
  tropical 
  seas, 
  rising 
  sometimes 
  to 
  30° 
  C, 
  the 
  same 
  difference 
  

   of 
  temperature 
  may 
  make 
  a 
  difference 
  of 
  3 
  to 
  4 
  in 
  the 
  fifth 
  decimal 
  place 
  

   of 
  the 
  resulting 
  specific 
  gravity. 
  

  

  Having 
  obtained 
  the 
  specific 
  gravity 
  of 
  the 
  water 
  in 
  question 
  at 
  a 
  tem- 
  

   perature 
  which 
  depends 
  upon 
  that 
  of 
  the 
  air 
  at 
  the 
  time, 
  it 
  is 
  necessary, 
  

   in 
  order 
  that 
  the 
  results 
  may 
  be 
  comparable, 
  to 
  reduce 
  them 
  to 
  their 
  value 
  

   at 
  one 
  common 
  temperature. 
  For 
  this 
  purpose 
  a 
  knowledge 
  of 
  the 
  law 
  

   of 
  expansion 
  of 
  sea-water 
  with 
  temperature 
  is 
  necessary. 
  This 
  had 
  been 
  

   determined 
  with 
  sufficient 
  accuracy 
  for 
  low 
  temperatures 
  by 
  Despretz 
  and 
  

   others 
  ; 
  but 
  as 
  the 
  temperatures 
  at 
  which 
  specific-gravity 
  observations 
  are 
  

   usually 
  made 
  are 
  comparatively 
  high, 
  their 
  results 
  were 
  of 
  but 
  little 
  use, 
  

   directed 
  as 
  they 
  were 
  chiefly 
  to 
  the 
  determination 
  of 
  the 
  freezing- 
  and 
  

   maximum-density 
  points. 
  "When 
  the 
  late 
  Captain 
  Maury 
  was 
  developing 
  

  

  