﻿1875.] 
  

  

  On 
  the 
  Transit 
  of 
  Venus 
  across 
  the 
  Sun. 
  

  

  379 
  

  

  I. 
  " 
  Some 
  Particulars 
  of 
  the 
  Transit 
  of 
  Venus 
  across 
  the 
  Sun, 
  

   1874, 
  Dec. 
  9, 
  observed 
  on 
  the 
  Himalaya 
  Mountains, 
  Mus- 
  

   soorie, 
  at 
  Mary 
  Villa/'' 
  — 
  Note 
  II., 
  with 
  Appendix. 
  By 
  J. 
  B. 
  

   N. 
  Hennessey, 
  F.R.A.S. 
  Communicated 
  by 
  Prof. 
  Stokes, 
  

   Sec. 
  U.S. 
  Received 
  January 
  11, 
  1875. 
  

  

  1. 
  The 
  instruments 
  used 
  were 
  the 
  following 
  : 
  — 
  

  

  An 
  altazimuth 
  by 
  Trough 
  ton 
  and 
  Simms, 
  with 
  an 
  azimuth 
  circle 
  of 
  8 
  

   inches 
  diameter 
  read 
  by 
  three 
  verniers, 
  and 
  a 
  complete 
  vertical 
  circle, 
  also 
  

   of 
  8 
  inches, 
  read 
  by 
  two 
  verniers. 
  The 
  circles 
  are 
  divided 
  to 
  every 
  10' 
  of 
  

   arc, 
  and 
  the 
  verniers 
  afford 
  readings 
  to 
  10", 
  or 
  by 
  estimation 
  to 
  at 
  least 
  

   5". 
  The 
  instrument 
  is 
  well 
  provided 
  with 
  spirit-levels 
  surmounted 
  by 
  

   scales. 
  The 
  altazimuth 
  was 
  used 
  for 
  fixing 
  the 
  station 
  of 
  observation 
  

   and 
  for 
  determining 
  time 
  . 
  

  

  A 
  mountain 
  barometer. 
  

  

  A 
  thermometer. 
  

  

  Tour 
  chronometers, 
  viz. 
  : 
  — 
  Dent 
  2047, 
  used 
  as 
  a 
  journeyman 
  chrono- 
  

   meter, 
  and 
  packed 
  in 
  wool 
  in 
  a 
  wooden 
  box 
  by 
  itself 
  ; 
  Barraud 
  885, 
  Dent 
  

   2775, 
  and 
  Arnold 
  and 
  Dent 
  758, 
  were 
  the 
  three 
  fixed 
  chronometers, 
  and 
  

   were 
  placed 
  in 
  the 
  same 
  case 
  within 
  a 
  room 
  where 
  the 
  temperature 
  had 
  

   only 
  a 
  moderate 
  diurnal 
  range 
  (probably 
  under 
  8°). 
  The 
  latter 
  case 
  was 
  

   also 
  well 
  padded 
  with 
  wool 
  and 
  hemp, 
  and 
  these 
  chronometers 
  were 
  

   never 
  moved 
  between 
  4th 
  and 
  10th 
  December. 
  

  

  The 
  equatoreal 
  has 
  already 
  been 
  sufficiently 
  described 
  in 
  Note 
  No. 
  I. 
  

  

  2. 
  The 
  point 
  over 
  which 
  the 
  equatoreal 
  stood 
  is 
  called 
  Venus 
  Station. 
  

   Its 
  coordinates 
  are 
  as 
  follows, 
  and 
  were 
  determined 
  by 
  angles 
  from 
  two 
  

   known 
  points 
  fixed 
  by 
  the 
  Great 
  Trigonometrical 
  Survey 
  of 
  India, 
  viz. 
  

   Camel's 
  Back 
  and 
  Vincent's 
  Hill 
  Stations 
  : 
  — 
  - 
  

  

  3. 
  Time 
  was 
  determined 
  from 
  the 
  zenith-distances 
  of 
  a 
  Tauri 
  (east) 
  

   and 
  a 
  AquilaD 
  (west), 
  when 
  these 
  stars 
  were 
  (nearly) 
  on 
  the 
  prime 
  ver- 
  

   tical. 
  As 
  a 
  rule, 
  four 
  pairs 
  of 
  zenith-distances 
  were 
  taken 
  to 
  each 
  star 
  — 
  a 
  

   pair 
  consisting 
  of 
  one 
  observation-instrument 
  face 
  east 
  and 
  another 
  face 
  

   west, 
  taken 
  in 
  rapid 
  succession. 
  The 
  chronometer-time 
  for 
  each 
  obser- 
  

   vation 
  was 
  obtained 
  from 
  transits 
  over 
  five 
  horizontal 
  wires 
  ; 
  the 
  result- 
  

   ing 
  chronometer-error 
  by 
  each 
  pair 
  was 
  computed. 
  The 
  journeyman 
  

   chronometer 
  was 
  compared 
  with 
  the 
  three 
  fixed 
  chronometers 
  before 
  and 
  

   after 
  observation, 
  and 
  the 
  errors 
  and 
  rates 
  of 
  the 
  latter 
  were 
  thus 
  deter- 
  

  

  Venus 
  Station. 
  

  

  Longitude 
  E. 
  of 
  Greenwich 
  

   Height 
  above 
  sea 
  , 
  

  

  Latitude 
  N, 
  

  

  30° 
  27' 
  36"-3 
  

   78° 
  3' 
  3"-2 
  

  

  5 
  h 
  12 
  m 
  12 
  s. 
  2 
  

  

  6765 
  feet. 
  

  

  