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  PROCEEDINGS 
  OF 
  THE 
  CALIFORNIA 
  

  

  any 
  angle 
  from 
  naught 
  to 
  one 
  hundred 
  and 
  eighty 
  degrees, 
  while 
  reflecting 
  but 
  

   one 
  object 
  ; 
  but 
  in 
  measuring 
  the 
  angles 
  from 
  about 
  one 
  hundred 
  and 
  forty 
  to 
  

   one 
  hundred 
  and 
  eighty 
  degrees, 
  the 
  sextant 
  must 
  be 
  inverted, 
  and 
  the 
  manipu- 
  

   lation 
  is 
  therefore 
  embarrassing. 
  Their 
  sextant 
  is 
  too 
  heavy 
  for 
  practical 
  use, 
  

   and 
  has 
  not 
  been 
  largely 
  adopted. 
  

  

  The 
  improvements 
  to 
  be 
  referred 
  to 
  are 
  very 
  important, 
  making 
  the 
  sextant 
  

   better 
  adapted 
  to 
  hydrographic 
  work 
  than 
  it 
  has 
  heretofore 
  been. 
  Mr. 
  Lowry 
  

   has 
  solved 
  the* 
  problems 
  very 
  ingeniously, 
  and 
  I 
  have 
  had 
  models 
  made 
  to 
  exhibit 
  

   their 
  practical 
  application. 
  In 
  studying 
  the 
  subject, 
  several 
  other 
  problems 
  

   which 
  occur 
  to 
  the 
  practical 
  hydrographer 
  have 
  been 
  very 
  well 
  solved. 
  In 
  the 
  

   work 
  of 
  the 
  U. 
  S. 
  Coast 
  Survey, 
  and 
  in 
  the 
  hydrographic 
  surveys 
  conducted 
  

   by 
  the 
  navies 
  of 
  all 
  countries, 
  demands 
  daily 
  arise 
  for 
  such 
  instrumental 
  aid 
  to 
  

   solve 
  these 
  problems; 
  and 
  it 
  is 
  believed 
  that 
  most 
  of 
  these 
  devices 
  of 
  Mr. 
  

   Lowry 
  are 
  novel. 
  Very 
  few 
  can 
  thoroughly 
  appreciate 
  their 
  value, 
  unless 
  they 
  

   be 
  practical 
  hydrographers. 
  

  

  Mr. 
  Lowry 
  has 
  given 
  the 
  following 
  statement 
  of 
  the 
  requirements 
  of 
  the 
  

   hydrographer 
  in 
  his 
  work, 
  and 
  the 
  solutions 
  of 
  them 
  : 
  

  

  Problem 
  I. 
  To 
  measure 
  with 
  a 
  sextant, 
  any 
  angle 
  from 
  naught 
  degrees 
  to 
  one 
  

   hundred 
  and 
  eighty 
  degrees, 
  without 
  inverting 
  the 
  instrument, 
  and 
  while 
  reflect- 
  

   ing 
  but 
  one 
  object. 
  

  

  This 
  may 
  be 
  solved 
  in 
  the 
  following 
  manner 
  : 
  The 
  front 
  and 
  back 
  faces 
  of 
  

   index 
  glass 
  (of 
  ordinary 
  sextant) 
  are 
  made 
  reflectors, 
  and 
  a 
  second 
  horizon 
  

   glass 
  placed 
  on 
  the 
  line 
  of 
  sight, 
  (behind 
  the 
  index 
  glass) 
  and 
  at 
  such 
  an 
  angle 
  

   as 
  to 
  reflect 
  the 
  rays, 
  which 
  are 
  reflected 
  first 
  from 
  the 
  back 
  face 
  of 
  the 
  index 
  

   glass, 
  parallel 
  to 
  the 
  line 
  of 
  sight. 
  Fig. 
  1 
  illustrates 
  this 
  form. 
  

  

  We 
  here 
  have 
  an 
  arc 
  of 
  only 
  sixty 
  degrees, 
  graduated 
  as 
  usual 
  from 
  naught 
  

   degrees 
  to 
  one 
  hundred 
  and 
  twenty 
  degrees, 
  with 
  a 
  second 
  reading 
  of 
  the 
  same 
  

   arc, 
  commencing 
  at 
  naught 
  degrees 
  as 
  one 
  hundred 
  and 
  twenty 
  degrees, 
  and 
  

   numbered 
  to 
  two 
  hundred 
  and 
  forty 
  degrees 
  at 
  one 
  hundred 
  and 
  twenty 
  degrees. 
  

   This 
  adaptation 
  may 
  be 
  designated 
  as 
  the 
  " 
  double 
  reflecting 
  index 
  glass." 
  

  

  Problem 
  II. 
  To 
  make 
  the 
  sextant 
  capable 
  of 
  measuring 
  two 
  angles, 
  one 
  to 
  

   the 
  right 
  and 
  the 
  other 
  to 
  the 
  left 
  of 
  an 
  object, 
  at 
  the 
  same 
  instant 
  ; 
  either 
  

   angle 
  being 
  any 
  number 
  of 
  degrees 
  from 
  degrees 
  to 
  120 
  degrees. 
  

  

  In 
  practice, 
  the 
  hydrographer 
  sets 
  his 
  index 
  arm 
  so 
  that 
  the 
  reflected 
  and 
  

   direct 
  images 
  of 
  the 
  objects 
  (say 
  left 
  hand 
  and 
  middle) 
  of 
  one 
  of 
  the 
  angles 
  

   which 
  he 
  is 
  to 
  measure, 
  are 
  not 
  coincident, 
  yet 
  approaching 
  on 
  account 
  of 
  the 
  

   progress 
  of 
  the 
  boat 
  ; 
  then 
  with 
  the 
  second 
  index 
  glass 
  he 
  makes 
  the 
  direct 
  and 
  

   reflected 
  images 
  of 
  the 
  middle 
  and 
  right 
  hand 
  objects 
  coincident, 
  and 
  keeps 
  

   them 
  coincident 
  with 
  tangent 
  screw 
  until 
  the 
  first 
  two 
  objects 
  are 
  coincident. 
  

  

  Problem 
  II 
  may 
  be 
  solved 
  in 
  four 
  different 
  forms 
  of 
  the 
  sextant. 
  In 
  Fig. 
  

   2, 
  there 
  are 
  one 
  horizon 
  glass 
  and 
  two 
  index 
  glasses 
  ; 
  one 
  of 
  the 
  latter 
  

   mounted 
  directly 
  over 
  the 
  other 
  ; 
  but 
  each 
  attached 
  to 
  distinct 
  index 
  arms, 
  

   which 
  have 
  a 
  common 
  center 
  of 
  motion. 
  The 
  arc 
  is 
  120 
  degrees 
  instead 
  of 
  60 
  

   degrees, 
  as 
  in 
  the 
  ordinary 
  forms, 
  and 
  the 
  graduation 
  is 
  numbered 
  in 
  opposite 
  

   directions, 
  from 
  degrees 
  to 
  120 
  degrees 
  (actually 
  at 
  60 
  degrees 
  from 
  each 
  

   zero). 
  

  

  