﻿HYDROLOGY 
  OF 
  NEW 
  YORK 
  133 
  

  

  are 
  extremely 
  unsatisfactory. 
  In 
  one 
  case 
  of 
  a 
  flat-crested 
  weir, 
  

   the 
  flow 
  at 
  a 
  given 
  depth 
  is 
  only 
  75 
  per 
  cent 
  of 
  what 
  it 
  is 
  over 
  a 
  

   sharp-crested 
  weir. 
  Variations 
  of 
  from 
  5 
  per 
  cent 
  to 
  20 
  per 
  cent 
  

   are 
  common, 
  as 
  may 
  be 
  easily 
  observed 
  by 
  examining 
  the 
  tables 
  in 
  

   the 
  paper 
  on 
  the 
  flow 
  of 
  water 
  over 
  dams. 
  1 
  

  

  In 
  view 
  of 
  the 
  importance 
  which 
  gagings 
  are 
  now 
  shown 
  to 
  bear 
  

   in 
  estimating 
  the 
  value 
  of 
  a 
  stream 
  for 
  water 
  power 
  or 
  city 
  

   water 
  supply, 
  in 
  future 
  every 
  statement 
  of 
  stream 
  flow 
  should 
  

   be 
  accompanied 
  by 
  a 
  concise 
  statement 
  of 
  the 
  method 
  of 
  gaging 
  

   used, 
  thus 
  permitting 
  hyd'rologists 
  to 
  judge 
  of 
  the 
  general 
  relia- 
  

   bility 
  of 
  the 
  method. 
  Had 
  this 
  been 
  done 
  in 
  the 
  past, 
  some 
  of 
  

   the 
  uncertainty 
  which 
  now 
  attaches 
  to 
  many 
  gaging 
  records 
  

   would 
  undoubtedly 
  be 
  removed. 
  

  

  Genesee 
  and 
  Hudson 
  gagings 
  reduced 
  to 
  sharp-crested 
  weir 
  

   measurements. 
  The 
  writer 
  has 
  shown 
  in 
  another 
  place 
  that 
  Gen- 
  

   esee 
  river 
  gagings 
  have 
  been 
  reduced 
  to 
  sharp-crested 
  weir 
  meas- 
  

   urements. 
  As 
  to 
  the 
  Hudson 
  gagings, 
  pi. 
  CXXVII 
  in 
  the 
  Report 
  

   to 
  the 
  United 
  States 
  Board 
  of 
  Engineers 
  on 
  Deep 
  Waterways, 
  may 
  

   be 
  cited. 
  This 
  plate 
  is 
  a 
  comparison 
  of 
  the 
  discharge 
  over 
  weirs 
  

   by 
  different 
  formulas, 
  and 
  it 
  appears 
  from 
  it 
  that 
  Mullins's 
  for- 
  

   mula 
  for 
  a 
  flat-crested 
  weir, 
  which 
  has 
  been 
  used 
  for 
  the 
  Upper 
  

   Hudson 
  gagings, 
  at 
  a 
  depth 
  of 
  4 
  feet 
  gives 
  results 
  less 
  than 
  Fran- 
  

   cis's 
  formula 
  for 
  a 
  sharp-crested 
  weir 
  by 
  about 
  10 
  per 
  cent. 
  How- 
  

   ever, 
  in 
  order 
  to 
  simplify 
  the 
  computation 
  and 
  to 
  avoid 
  velocity 
  

   of 
  approach, 
  the 
  width 
  of 
  the 
  crest 
  was 
  taken 
  at 
  5 
  feet. 
  Again, 
  

   the 
  crest 
  at 
  Mechanicville 
  is 
  not 
  flat, 
  but 
  is 
  slightly 
  sloping 
  back- 
  

   ward. 
  The 
  sloping 
  front 
  probably 
  affects 
  the 
  flow 
  to 
  increase 
  it 
  

   somewhat. 
  There 
  are 
  also 
  flashboards 
  used 
  during 
  low 
  water, 
  

   which 
  are 
  properly 
  computed 
  by 
  Francis's 
  formula 
  for 
  a 
  sharp- 
  

   crested 
  weir. 
  These 
  several 
  elements 
  undoubtedly 
  make 
  the 
  

   problem 
  somewhat 
  complicated, 
  but 
  taking 
  everything 
  into 
  

   account 
  it 
  is 
  probable 
  that 
  the 
  results 
  as 
  computed 
  are 
  not 
  far 
  

   from 
  right. 
  They 
  may, 
  however, 
  be 
  in 
  error 
  as 
  much 
  as 
  2 
  inches 
  

   per 
  year. 
  2 
  

  

  x 
  On 
  the 
  Flow 
  of 
  Waters 
  over 
  Dams; 
  loc. 
  cit. 
  

  

  2 
  See 
  the 
  diagrams 
  of 
  Hudson 
  and 
  Genesee 
  rivers 
  on 
  this 
  point. 
  

  

  