﻿126 
  On 
  the 
  Distribution 
  of 
  Temperature 
  in 
  the 
  Ocean. 
  [Dec. 
  17, 
  

  

  From 
  the 
  figures 
  in 
  the 
  Table 
  it 
  will 
  be 
  seen 
  that 
  in 
  that 
  part 
  of 
  the 
  

   ocean 
  the 
  specific 
  gravity 
  of 
  the 
  water 
  in 
  summer 
  decreases 
  from 
  the 
  

   surface 
  downwards. 
  As 
  a 
  rule 
  it 
  attains 
  an 
  inferior 
  limit 
  at 
  a 
  depth 
  of 
  

   from 
  400 
  to 
  500 
  fathoms, 
  which 
  it 
  preserves 
  to 
  the 
  bottom. 
  In 
  those 
  

   latitudes, 
  therefore, 
  the 
  stratum 
  of 
  intermixture 
  extends 
  down 
  to 
  500 
  

   fathoms 
  ) 
  and 
  this 
  niay 
  be 
  said 
  also 
  to 
  be 
  the 
  depth 
  to 
  which 
  the 
  sun's 
  

   influence 
  at 
  the 
  surface 
  penetrates. 
  The 
  results 
  in 
  column 
  III. 
  show 
  the 
  

   curious 
  phenomenon 
  of 
  the 
  surface-water 
  being 
  specifically 
  lighter 
  than 
  

   any 
  water 
  below 
  it, 
  and 
  that 
  under 
  an 
  equatorial 
  sun. 
  The 
  position 
  of 
  

   this 
  sounding 
  was 
  peculiar, 
  inasmuch 
  as 
  it 
  was 
  within 
  line 
  of 
  separation 
  

   between 
  the 
  Guinea 
  and 
  the 
  equatorial 
  currents. 
  All 
  aloug 
  the 
  equa- 
  

   torial 
  section 
  the 
  water 
  at 
  50 
  and 
  100 
  fathoms 
  was 
  found 
  to 
  be 
  speci- 
  

   fically 
  heavier 
  than 
  either 
  at 
  the 
  surface 
  or 
  that 
  at 
  greater 
  depths. 
  All 
  

   along 
  the 
  equator, 
  however, 
  a 
  current 
  runs 
  with 
  great 
  velocity 
  ; 
  and 
  I 
  have 
  

   invariably 
  observed 
  that 
  strong 
  surface-currents 
  introduce 
  considerable 
  

   irregularities 
  into 
  the 
  specific 
  gravity 
  of 
  the 
  water 
  near 
  the 
  surface. 
  The 
  

   effect 
  of 
  the 
  greater 
  specific 
  gravity 
  at 
  100 
  fathoms 
  conspires, 
  of 
  course 
  

   within 
  the 
  small 
  yearly 
  range 
  of 
  temperature, 
  in 
  preventing 
  vertical 
  

   diffusion 
  in 
  the 
  above 
  described 
  manner. 
  Column 
  IV. 
  shows 
  a 
  return 
  

   in 
  the 
  southern 
  hemisphere 
  to 
  a 
  state 
  of 
  things 
  similar 
  to 
  that 
  which 
  

   obtains 
  in 
  the 
  North 
  Atlantic. 
  

  

  We 
  have 
  seen 
  that 
  the 
  effect 
  of 
  climate 
  in 
  equatorial 
  regions 
  is 
  to 
  

   render 
  the 
  subsurface-water 
  much 
  colder 
  than 
  it 
  is 
  in 
  temperate 
  regions 
  ; 
  

   let 
  us 
  consider 
  what 
  would 
  be 
  the 
  effect 
  of 
  a 
  polar 
  climate 
  on 
  the 
  sea- 
  

   water. 
  It 
  must 
  be 
  observed 
  that 
  the 
  effect 
  of 
  the 
  atmospheric 
  tempera- 
  

   ture 
  on 
  the 
  sea 
  is 
  determined 
  by 
  the 
  temperature 
  assumed 
  by 
  the 
  surface- 
  

   water 
  ; 
  now 
  the 
  lowest 
  temperature 
  which 
  surface-water 
  can 
  attain 
  is 
  its 
  

   freezing-point. 
  As 
  the 
  temperature 
  of 
  the 
  air 
  when 
  the 
  ' 
  Challenger 
  ' 
  

   was 
  beyond 
  the 
  60th 
  parallel 
  was 
  almost 
  constantly 
  below 
  32° 
  freezing 
  

   must 
  go 
  on 
  to 
  a 
  very 
  great 
  extent 
  in 
  winter 
  ; 
  and 
  the 
  effect 
  of 
  freezing 
  

   such 
  water 
  is, 
  in 
  the 
  end, 
  similar 
  to 
  that 
  of 
  evaporating 
  it 
  ; 
  it 
  is 
  sepa- 
  

   rated 
  into 
  lighter 
  ice 
  and 
  denser 
  mother-liquor, 
  which 
  sinks, 
  leaving 
  the 
  

   ice 
  on 
  the 
  surface. 
  This 
  ice 
  I 
  found 
  to 
  be 
  a 
  mixture 
  ; 
  and 
  on 
  determining 
  

   the 
  melting-point 
  of 
  some 
  in 
  crystals, 
  which 
  had 
  formed 
  in 
  a 
  bucketful 
  

   of 
  sea-water, 
  I 
  found 
  it 
  began 
  to 
  melt 
  at 
  29 
  0, 
  5 
  I\, 
  the 
  water 
  produced 
  by 
  

   it 
  being 
  almost 
  fresh 
  in 
  comparison 
  with 
  sea-water. 
  The 
  lowest 
  tem- 
  

   perature 
  of 
  surface-water 
  registered 
  was 
  27° 
  F. 
  ; 
  this 
  happened 
  on 
  two 
  

   occasions, 
  but 
  was 
  quite 
  exceptional, 
  the 
  usual 
  surface-temperature 
  vary- 
  

   ing 
  from 
  32° 
  to 
  34° 
  E. 
  At 
  this 
  temperature 
  a 
  sensible 
  quantity 
  of 
  ice 
  

   would 
  melt, 
  giving 
  very 
  light 
  surface-water. 
  On 
  two 
  occasions 
  the 
  

   specific 
  gravity 
  of 
  the 
  surface-water 
  was 
  found 
  between 
  1*02400 
  and 
  

   1*02410. 
  The 
  specific 
  gravity 
  increased 
  rapidly 
  up 
  to 
  a 
  depth 
  of 
  100 
  

   fathoms, 
  when 
  it 
  remained 
  pretty 
  uniform 
  to 
  the 
  bottom. 
  Here, 
  as 
  at 
  the 
  

   equator, 
  it 
  is 
  in 
  winter 
  that 
  the 
  subsurface-water 
  perceives 
  the 
  effect 
  of 
  

   the 
  change 
  of 
  season, 
  the 
  mother-liquor 
  of 
  the 
  forming 
  ice 
  diffusing 
  in 
  

   its 
  descent 
  the 
  temperature 
  of 
  its 
  formation. 
  

  

  