﻿HYDROLOGY 
  OF 
  NEW 
  YORK 
  141 
  

  

  Professor 
  Coffin's 
  results 
  were 
  obtained 
  by 
  observing 
  the 
  varia- 
  

   tions 
  of 
  weight 
  of 
  a 
  dish 
  of 
  water 
  of 
  the 
  same 
  size 
  as 
  the 
  rain 
  

   gage, 
  with 
  which 
  the 
  evaporation 
  determinations 
  are 
  compared. 
  

   This 
  method 
  would 
  probably 
  give 
  results 
  considerably 
  in 
  excess 
  of 
  

   the 
  truth. 
  Moreover, 
  a 
  single 
  year 
  is 
  too 
  short 
  a 
  period 
  for 
  safe 
  

   results. 
  

  

  Croton 
  Water 
  Department 
  evaporation 
  records. 
  Table 
  No. 
  29 
  

   presents 
  the 
  results 
  of 
  evaporation 
  for 
  certain 
  indicated 
  months 
  

   (1) 
  for 
  the 
  years 
  1867-1870, 
  at 
  Boyd's 
  Corners 
  storage 
  reservoir, 
  

   Putnam 
  county, 
  as 
  determined 
  by 
  J. 
  J. 
  R. 
  Croes, 
  1 
  from 
  a 
  wooden 
  

   tank 
  sunk 
  in 
  the 
  earth; 
  (2), 
  for 
  certain 
  indicated 
  months, 
  1864- 
  

   1869, 
  inclusive, 
  from 
  a 
  wooden 
  tank 
  sunk 
  in 
  the 
  earth 
  at 
  the 
  

   receiving 
  reservoir 
  in 
  the 
  City 
  of 
  New 
  York; 
  (3), 
  from 
  a 
  wooden 
  

   tank 
  in 
  a 
  batteau 
  at 
  the 
  receiving 
  reservoir 
  in 
  New 
  York, 
  and 
  

   (4), 
  from 
  a 
  tin 
  box 
  in 
  a 
  batteau 
  at 
  the 
  receiving 
  reservoir 
  in 
  New 
  

   York. 
  

  

  The 
  foregoing 
  evaporation 
  experiments 
  are 
  referred 
  to 
  in 
  a 
  

   paper 
  by 
  Mr 
  Croes 
  on 
  the 
  flow 
  of 
  the 
  West 
  Branch 
  of 
  Croton 
  river. 
  

   The 
  gage 
  used 
  was 
  a 
  tight 
  wooden 
  tank 
  4x4x3 
  feet, 
  sunk 
  in 
  the 
  

   earth 
  in 
  an 
  exposed 
  situation 
  and 
  filled 
  with 
  water. 
  As 
  indi- 
  

   cated, 
  the 
  mean 
  evaporation 
  at 
  Boyd's 
  Corners 
  for 
  the 
  

   indicated 
  months 
  ivas 
  24.47 
  inches, 
  while 
  at 
  a 
  similar 
  

   tank 
  at 
  the 
  reservoir 
  in 
  New 
  York 
  city 
  it 
  was 
  34.06 
  

   inches. 
  Mr 
  Croes 
  attributes 
  the 
  difference 
  in 
  these 
  results 
  

   to 
  the 
  different 
  methods 
  of 
  observation 
  and 
  measurement, 
  

   and 
  states 
  that 
  the 
  Boyd's 
  Corners 
  observations 
  were 
  made 
  twice 
  

   a 
  day, 
  and 
  any 
  discrepancies 
  that 
  might 
  have 
  occurred 
  were 
  thus 
  

   found 
  and 
  corrected 
  at 
  once, 
  Avhile 
  the 
  observations 
  at 
  the 
  reser- 
  

   voir 
  in 
  New 
  York 
  city 
  were 
  made 
  only 
  once 
  a 
  month, 
  the 
  differ- 
  

   ence 
  between 
  the 
  reading 
  of 
  the 
  gage 
  on 
  the 
  tank 
  and 
  the 
  ob- 
  

   served 
  rainfall 
  being 
  taken 
  as 
  evaporation. 
  Mr 
  Croes 
  therefore 
  

   considers 
  the 
  work 
  done 
  at 
  Boyd's 
  Corners 
  reservoir 
  as 
  more 
  

   reliable. 
  

  

  "Trans. 
  Am. 
  Soc. 
  C. 
  E., 
  Vol. 
  Ill, 
  1874. 
  

  

  