﻿124 
  

  

  Mr. 
  J. 
  Y. 
  Buchanan 
  on 
  the 
  

  

  [Dec. 
  17, 
  

  

  the 
  surface-water 
  has, 
  however, 
  had 
  another 
  effect, 
  besides 
  increasing 
  

   its 
  volume 
  ; 
  it 
  has, 
  by 
  evaporation, 
  rendered 
  it 
  denser 
  than 
  it 
  was 
  before, 
  

   at 
  the 
  same 
  temperature. 
  Keeping 
  in 
  view 
  this 
  double 
  effect 
  of 
  the 
  

   summer 
  heat 
  upon 
  the 
  surface-water, 
  let 
  us 
  consider 
  the 
  effect 
  of 
  the 
  

   winter 
  cold 
  upon 
  it. 
  The 
  superficial 
  water 
  having 
  assumed 
  the 
  atmo- 
  

   spheric 
  temperature 
  of, 
  say, 
  60° 
  F., 
  will 
  sink 
  through 
  the 
  warmer 
  water 
  

   below 
  it, 
  until 
  it 
  reaches 
  the 
  stratum 
  of 
  water 
  having 
  the 
  same 
  tempera- 
  

   ture 
  as 
  itself. 
  Arrived 
  here, 
  however, 
  although 
  it 
  has 
  the 
  same 
  tem- 
  

   perature 
  as 
  the 
  surrounding 
  water, 
  the 
  two 
  are 
  no 
  longer 
  in 
  equilibrium, 
  

   for 
  the 
  water, 
  which 
  has 
  come 
  from 
  the 
  surface, 
  has 
  a 
  greater 
  density 
  

   than 
  that 
  below 
  at 
  the 
  same 
  temperature. 
  It 
  will 
  therefore 
  not 
  be 
  

   arrested 
  at 
  the 
  stratum 
  of 
  the 
  same 
  temperature, 
  as 
  would 
  have 
  been 
  

   the 
  case 
  with 
  fresh 
  water 
  ; 
  but 
  it 
  will 
  continue 
  to 
  sink, 
  carrying 
  of 
  course 
  

   its 
  higher 
  temperature 
  with 
  it, 
  and 
  distributing 
  it 
  among 
  the 
  lower 
  

   layers 
  of 
  colder 
  water. 
  At 
  the 
  end 
  of 
  the 
  winter, 
  therefore, 
  and 
  just 
  

   before 
  the 
  summer 
  heating 
  recommences, 
  we 
  shall 
  have 
  at 
  the 
  surface 
  

   a 
  more 
  or 
  less 
  thick 
  stratum 
  of 
  water 
  having 
  a 
  nearly 
  uniform 
  tempera- 
  

   ture 
  of 
  60° 
  F., 
  and 
  below 
  this 
  the 
  temperature 
  decreasing 
  at 
  a 
  consider- 
  

   able 
  but 
  less 
  rapid 
  rate 
  than 
  at 
  the 
  termination 
  of 
  the 
  summer 
  heating. 
  

   If 
  we 
  distinguish 
  between 
  surface-water, 
  'the 
  temperature 
  of 
  which 
  rises 
  

   with 
  the 
  atmospheric 
  temperature 
  (following 
  thus, 
  in 
  direction 
  at 
  least, 
  

   the 
  variation 
  of 
  the 
  seasons), 
  and 
  subsurface-w&tev, 
  or 
  the 
  stratum 
  imme- 
  

   diately 
  below 
  it, 
  we 
  have 
  for 
  the 
  latter 
  the, 
  at 
  first 
  sight, 
  paradoxical 
  effect 
  

   of 
  summer 
  cooling 
  and 
  winter 
  heating. 
  The 
  effect 
  of 
  this 
  agency 
  is 
  to 
  

   diffuse 
  the 
  same 
  heat 
  to 
  a 
  greater 
  depth 
  in 
  the 
  ocean, 
  the 
  greater 
  the 
  

   yearly 
  range 
  of 
  atmospheric 
  temperature 
  at 
  the 
  surface. 
  This 
  effect 
  is 
  

   well 
  shown 
  in 
  the 
  chart 
  of 
  isothermals, 
  on 
  a 
  vertical 
  section, 
  between 
  

   Madeira 
  and 
  a 
  position 
  in 
  lat. 
  3° 
  8' 
  N., 
  long. 
  14° 
  49' 
  W. 
  The 
  isothermal 
  

   line 
  for 
  45° 
  F. 
  rises 
  from 
  a 
  depth 
  of 
  740 
  fathoms 
  at 
  Madeira 
  to 
  240 
  

   fathoms 
  at 
  the 
  above-mentioned 
  position*. 
  In 
  equatorial 
  regions 
  there 
  

   is 
  hardly 
  any 
  variation 
  in 
  the 
  surface-temperature 
  of 
  the 
  sea 
  ; 
  conse- 
  

   quently 
  we 
  find 
  cold 
  water 
  very 
  close 
  to 
  the 
  surface 
  all 
  along 
  the 
  line. 
  

   On 
  referring 
  to 
  the 
  temperature 
  section 
  between 
  the 
  position 
  lat. 
  3° 
  8' 
  N., 
  

   long. 
  14° 
  49' 
  W., 
  and 
  St. 
  Paul's 
  rocks, 
  it 
  will 
  be 
  seen 
  that, 
  with 
  a 
  sur- 
  

   face-temperature 
  of 
  from 
  75° 
  F. 
  to 
  79° 
  F., 
  water 
  at 
  55° 
  F. 
  is 
  reached 
  at 
  

   distances 
  of 
  less 
  than 
  100 
  fathoms 
  from 
  the 
  surface. 
  Midway 
  between 
  

   the 
  Azores 
  and 
  Bermuda, 
  with 
  a 
  surface-temperature 
  of 
  70° 
  F., 
  it 
  is 
  

   only 
  at 
  a 
  depth 
  of 
  400 
  fathoms 
  that 
  we 
  reach 
  water 
  of 
  55° 
  F. 
  

  

  The 
  above 
  theory 
  of 
  vertical 
  diffusion 
  of 
  temperature 
  in 
  the 
  ocean, 
  

   owing 
  to 
  convection 
  brought 
  about 
  by 
  the 
  yearly 
  range 
  of 
  temperature 
  at 
  

   the 
  surface, 
  presupposes 
  that 
  (at 
  least 
  in 
  regions 
  where 
  the 
  range 
  is 
  

   considerable, 
  and 
  where 
  the 
  great 
  vertical 
  diffusion 
  of 
  heat 
  in 
  question 
  is 
  

  

  * 
  There 
  will, 
  I 
  think, 
  be 
  no 
  violence 
  in 
  assuming 
  an 
  acquaintance 
  with 
  these 
  charts, 
  

   at 
  least 
  among 
  the 
  scientific 
  public, 
  as 
  they 
  have 
  lately 
  formed 
  the 
  subject 
  of 
  lectures 
  

   by 
  Dr. 
  Carpenter, 
  and 
  will, 
  no 
  doubt, 
  have 
  been 
  published 
  before 
  this 
  reaches 
  England. 
  

  

  