﻿118 
  NEW 
  YORK 
  STATE 
  MUSEUM 
  

  

  The 
  constant 
  0.844 
  is 
  equal 
  to 
  the 
  number 
  of 
  feet 
  in 
  a 
  square 
  

   mile 
  divided 
  by 
  the 
  seconds 
  in 
  a 
  year. 
  

  

  In 
  these 
  formulas 
  the 
  sliding 
  coefficient 
  is 
  also 
  recognized. 
  The 
  

   results, 
  however, 
  are 
  based 
  on 
  averages, 
  although 
  it 
  seems 
  clear 
  

   enough 
  that, 
  in 
  either 
  power 
  or 
  water 
  supply 
  works, 
  what 
  is 
  

   wanted 
  is 
  the 
  minimum 
  runoff 
  for 
  a 
  year 
  or 
  a 
  series 
  of 
  years. 
  

   For 
  instance, 
  the 
  minimum 
  rainfall 
  at 
  Lake 
  Minnetonka 
  in 
  1889 
  

   was 
  only 
  18.36 
  inches, 
  while 
  the 
  maximum 
  in 
  1892 
  was 
  37.90 
  

   inches, 
  or 
  a 
  little 
  more 
  than 
  double 
  the 
  minimum. 
  It 
  is 
  evident 
  

   enough 
  to 
  any 
  person 
  who 
  has 
  gaged 
  streams 
  extensively 
  that 
  the 
  

   runoff 
  in 
  1889 
  must 
  have 
  been 
  very 
  much 
  less 
  than 
  in 
  1892. 
  In 
  

   the 
  absence 
  of 
  statements 
  as 
  to 
  the 
  amount 
  of 
  runoff 
  in 
  1889, 
  the 
  

   writer 
  can 
  only 
  estimate 
  it, 
  but 
  he 
  doubts 
  if 
  it 
  were 
  over 
  10 
  per 
  

   cent 
  to 
  12 
  per 
  cent 
  of 
  the 
  rainfall. 
  Probably 
  about 
  2 
  inches 
  is 
  

   not 
  far 
  from 
  the 
  mark. 
  What 
  is 
  wanted, 
  therefore, 
  is 
  a 
  concise 
  

   statement, 
  not 
  only 
  in 
  this 
  case 
  but 
  in 
  every 
  other, 
  of 
  the 
  runoff 
  

   of 
  the 
  year 
  or 
  series 
  of 
  years 
  of 
  minimum 
  rainfall. 
  

  

  Danger 
  of 
  using 
  averages. 
  The 
  writer 
  has 
  dwelt 
  upon 
  the 
  fore- 
  

   going 
  point 
  somewhat 
  because 
  only 
  a 
  few 
  of 
  the 
  more 
  advanced 
  

   students 
  of 
  hydrology 
  have 
  thus 
  far 
  fully 
  appreciated 
  its 
  import- 
  

   ance. 
  A 
  very 
  large 
  proportion 
  of 
  all 
  the 
  papers 
  and 
  reports 
  pre- 
  

   pared 
  in 
  the 
  last 
  ten 
  years 
  have 
  proceeded 
  on 
  the 
  supposition 
  that 
  

   safe 
  deductions 
  could 
  be 
  made 
  from 
  an 
  average 
  runoff. 
  It 
  is 
  

   needless 
  to 
  say 
  that 
  all 
  such 
  are, 
  without 
  exception, 
  erroneous. 
  

   What 
  is 
  wanted 
  is 
  a 
  clear 
  statement 
  of 
  the 
  minimum, 
  together 
  

   with 
  the 
  longest 
  period 
  which 
  such 
  minimum 
  may 
  be 
  expected 
  

   to 
  occupy. 
  A 
  study 
  of 
  the 
  meteorological 
  records 
  of 
  the 
  State 
  of 
  

   New 
  York 
  shows 
  that 
  the 
  minimum 
  period 
  may 
  be 
  expected 
  fre- 
  

   quently 
  to 
  extend 
  over 
  three 
  years. 
  In 
  the 
  writer's 
  report 
  to 
  the 
  

   United 
  States 
  Board 
  of 
  Engineers 
  on 
  Deep 
  Waterways, 
  in 
  the 
  

   chapter 
  on 
  the 
  Meteorology 
  of 
  New 
  York 
  and 
  the 
  relation 
  of 
  pre- 
  

   cipitation 
  to 
  runoff, 
  a 
  large 
  number 
  of 
  specific 
  cases 
  are 
  cited, 
  

   but 
  space 
  will 
  not 
  be 
  taken 
  here 
  to 
  discuss 
  them. 
  This 
  proposi- 
  

   tion 
  is 
  true 
  for 
  other 
  regions 
  than 
  the 
  State 
  of 
  New 
  York. 
  

  

  Danger 
  of 
  using 
  percentages. 
  A 
  much 
  greater 
  danger 
  arises 
  

   from 
  the 
  use 
  of 
  percentage 
  of 
  rainfall 
  appearing 
  in 
  runoff. 
  In 
  

  

  