﻿ACADEMY 
  OF 
  SCIENCES. 
  151 
  

  

  Donations 
  to 
  the 
  Museum 
  : 
  Specimen 
  of 
  a 
  young 
  Octopus 
  ( 
  0. 
  

   punctatus 
  Gabb.), 
  presented 
  by 
  Henry 
  Chapman. 
  Specimen 
  of 
  a 
  

   young 
  " 
  Blue 
  Shark," 
  caught 
  in 
  San 
  Francisco 
  Bay, 
  from 
  S. 
  R. 
  

   Throckmorton. 
  Bottle 
  of 
  Water 
  from 
  the 
  Warm 
  Springs 
  at 
  Point 
  

   Isabel, 
  Sonora 
  (Mexico), 
  presented 
  by 
  Capt. 
  McDonald 
  of 
  the 
  

   steamer 
  " 
  Newborn. 
  " 
  Specimens 
  of 
  Reptiles 
  and 
  Wasp's 
  Nest 
  

   from 
  Vancouver 
  Island, 
  by 
  Henry 
  Edwards 
  ; 
  also, 
  specimen 
  of 
  a 
  

   Snake, 
  presented 
  by 
  Mrs. 
  Henry 
  Edwards. 
  Specimen 
  of 
  White 
  

   Sandstone 
  from 
  Merced 
  Lake, 
  by 
  George 
  Davidson. 
  

  

  Professor 
  Davidson 
  remarked 
  that 
  the 
  sandstone 
  presented 
  by 
  

   him 
  was 
  from 
  an 
  extensive 
  deposit 
  in 
  the 
  neighborhood 
  of 
  Merced 
  

   Lake, 
  which 
  is 
  being 
  worked 
  ; 
  large 
  quantities 
  are 
  taken 
  out 
  and 
  

   shipped 
  to 
  the 
  Eastern 
  States 
  for 
  use 
  as 
  a 
  polish 
  ; 
  it 
  was 
  found 
  

   superior 
  to 
  rotten 
  stone 
  for 
  that 
  purpose. 
  

  

  The 
  following 
  remarks 
  were 
  made 
  : 
  

  

  On 
  an 
  Improved 
  Leveling 
  Rod. 
  

  

  BY 
  GEORGE 
  DAVIDSON. 
  

  

  In 
  the 
  regular 
  work 
  of 
  the 
  Coast 
  Survey 
  upon 
  this 
  Coast, 
  but 
  especially 
  in 
  

   some 
  special 
  examinations 
  connected 
  therewith, 
  it 
  became 
  patent 
  that 
  the 
  ordi- 
  

   nary 
  wooden 
  leveling 
  rod 
  was 
  inadequate 
  to 
  give 
  perfectly 
  reliable 
  results. 
  

   The 
  defects 
  are 
  inherent 
  in 
  the 
  instrument 
  ; 
  the 
  principal 
  ones 
  are 
  : 
  

  

  a. 
  inaccuracy 
  of 
  graduation 
  ; 
  

  

  b. 
  uncertainty 
  of 
  the 
  standard 
  with 
  which 
  it 
  was 
  (if 
  ever) 
  compared 
  ; 
  

  

  c. 
  no 
  rate 
  of 
  expansion 
  of 
  the 
  rod 
  for 
  changes 
  of 
  temperature 
  ; 
  

  

  d. 
  no 
  knowledge 
  of 
  the 
  effect 
  of 
  hygrometric 
  changes 
  ; 
  

  

  e. 
  no 
  means 
  in 
  the 
  rod 
  itself 
  for 
  adjusting 
  the 
  verticality 
  ; 
  

  

  f. 
  no 
  tangent 
  motion. 
  

  

  For 
  all 
  ordinary 
  work 
  of 
  leveling, 
  the 
  best 
  made 
  rods 
  are 
  sufficiently 
  ac- 
  

   curate 
  ; 
  but 
  where 
  investigations 
  are 
  to 
  be 
  made 
  for 
  the 
  coefficient 
  of 
  refrac- 
  

   tion, 
  so 
  important 
  in 
  the 
  hypsometrical 
  of 
  large 
  triangulations, 
  they 
  do 
  not 
  af- 
  

   ford 
  the 
  desired 
  precision. 
  In 
  the 
  reference 
  of 
  the 
  mean 
  level 
  of 
  the 
  sea, 
  at 
  

   any 
  point 
  where 
  years 
  of 
  continuous 
  tidal 
  observations 
  have 
  been 
  made 
  to 
  some 
  

   well-marked 
  bench 
  mark, 
  the 
  utmost 
  precision 
  is 
  demanded, 
  because 
  by 
  such 
  

   observations, 
  through 
  a 
  series 
  of 
  years 
  or 
  of 
  centuries, 
  we 
  shall 
  be 
  able 
  to 
  meas- 
  

   ure 
  the 
  rising 
  or 
  subsidence 
  of 
  the 
  shores 
  of 
  the 
  Continent. 
  When 
  the 
  hypso- 
  

   metrical 
  work 
  of 
  a 
  triangulation 
  is 
  carried 
  across 
  the 
  country 
  with 
  the 
  same 
  

   precision 
  as 
  that 
  of 
  the 
  sides 
  and 
  geographical 
  positions, 
  future 
  observers 
  will 
  

   be 
  able 
  to 
  determine 
  the 
  changes, 
  if 
  any, 
  in 
  the 
  elevation 
  of 
  the 
  interior 
  of 
  the 
  

   Continent. 
  

  

  