﻿578 
  

  

  Messrs. 
  H. 
  E. 
  Roscoe 
  and 
  B. 
  Stewart 
  on 
  [June 
  17, 
  

  

  XIX. 
  " 
  On 
  the 
  Heat 
  of 
  Sunshine 
  at 
  London 
  during 
  the 
  twenty-four 
  

   years 
  1855 
  to 
  1874, 
  as 
  registered 
  by 
  Campbell's 
  Method. 
  5 
  ' 
  By 
  

   H. 
  E. 
  Roscoe, 
  F.R.S., 
  and 
  B. 
  Stewart, 
  F.R.S. 
  Received 
  

   June 
  10, 
  1875. 
  

  

  During 
  the 
  above 
  period 
  Mr. 
  J. 
  F. 
  Campbell 
  observed 
  at 
  London 
  

   the 
  relative 
  heating 
  effect 
  of 
  the 
  sun 
  in 
  the 
  following 
  manner 
  (de- 
  

   scribed 
  in 
  the 
  Report 
  on 
  the 
  Warming 
  and 
  Ventilating 
  of 
  Dwellings, 
  

   printed 
  by 
  order 
  of 
  the 
  House 
  of 
  Commons, 
  25th 
  August, 
  1857, 
  

   p. 
  151). 
  _ 
  

  

  A 
  hemispherical 
  cavity 
  was 
  made 
  in 
  a 
  block 
  of 
  wood, 
  and 
  a 
  spherical 
  

   lens 
  was 
  made 
  to 
  be 
  placed 
  in 
  this 
  cavity 
  in 
  such 
  a 
  position 
  that 
  while 
  

   its 
  centre 
  coincided 
  with 
  the 
  centre 
  of 
  the 
  cavity 
  its 
  chief 
  focus 
  was 
  at 
  

   some 
  point 
  of 
  the 
  hemispherical 
  concave 
  surface, 
  the 
  exact 
  point 
  being 
  

   of 
  course 
  determined 
  by 
  the 
  direction 
  in 
  which 
  the 
  rays 
  struck 
  the 
  

   lens. 
  

  

  Whenever, 
  therefore, 
  the 
  sun 
  shone, 
  a 
  portion 
  of 
  the 
  wood 
  would 
  be 
  

   carbonized 
  or 
  burnt 
  out 
  by 
  his 
  concentrated 
  beams 
  ; 
  and 
  inasmuch 
  as 
  the 
  

   sun 
  continually 
  changes 
  his 
  position 
  not 
  only 
  from 
  hour 
  to 
  hour, 
  but 
  

   from 
  day 
  to 
  day, 
  it 
  follows 
  that 
  different 
  portions 
  of 
  the 
  wood 
  will 
  be 
  

   acted 
  upon 
  not 
  only 
  from 
  one 
  hour 
  to 
  another, 
  but 
  also 
  from 
  one 
  day 
  to 
  

   another. 
  

  

  The 
  blocks 
  were 
  all 
  of 
  mahogany, 
  being 
  as 
  nearly 
  as 
  possible 
  of 
  the 
  

   same 
  quality, 
  and 
  the 
  diameter 
  of 
  the 
  sphere 
  was 
  about 
  5| 
  inches. 
  

  

  When 
  these 
  blocks 
  came 
  into 
  our 
  hands, 
  we 
  considered 
  how 
  it 
  was 
  

   possible 
  best 
  to 
  measure 
  the 
  amount 
  of 
  wood 
  burnt 
  out. 
  At 
  length 
  we 
  

   hit 
  upon 
  the 
  plan 
  of 
  filling 
  up 
  the 
  hollows 
  burnt 
  out 
  with 
  a 
  mixture 
  of 
  

   bees' 
  wax 
  and 
  olive-oil, 
  of 
  such 
  a 
  consistency 
  that 
  we 
  could 
  easily 
  work 
  

   it 
  into 
  the 
  burnt 
  cavities 
  until 
  the 
  whole 
  internal 
  hemisphere 
  should 
  be 
  

   made 
  to 
  present 
  the 
  same 
  smooth 
  surface 
  which 
  it 
  had 
  before 
  it 
  was 
  

   burnt. 
  

  

  A 
  comparison 
  of 
  the 
  weight 
  of 
  the 
  block 
  before 
  and 
  after 
  this 
  process 
  

   was 
  supposed 
  by 
  us 
  to 
  afford 
  an 
  approximately 
  good 
  estimate 
  of 
  the 
  

   extent 
  of 
  the 
  hollows. 
  Mr. 
  J. 
  A. 
  Dodge, 
  a 
  student 
  in 
  Owens 
  College, 
  

   was 
  good 
  enough 
  to 
  assist 
  us 
  in 
  this 
  part 
  of 
  our 
  research. 
  Indeed 
  he 
  

   made 
  two 
  determinations 
  for 
  each 
  block, 
  dissolving 
  out 
  the 
  mixture 
  by 
  

   means 
  of 
  heat 
  and 
  refilling 
  the 
  cavities 
  ; 
  and 
  the 
  near 
  concordance 
  of 
  

   these 
  two 
  determinations 
  gives 
  us 
  reason 
  to 
  believe 
  that 
  the 
  results 
  are 
  

   as 
  accurate 
  as 
  the 
  nature 
  of 
  the 
  experiment 
  requires. 
  We 
  subjoin 
  a 
  

   Table 
  containing 
  these 
  results 
  : 
  — 
  

  

  