﻿126 
  JOURNAL 
  OF 
  THE 
  WASHINGTON 
  ACADEMY 
  OF 
  SCIENCES 
  VOL. 
  12, 
  NO. 
  5 
  

  

  Tides. 
  — 
  Of 
  all 
  the 
  phenomena 
  of 
  the 
  sea, 
  the 
  tides 
  seem 
  to 
  have 
  been 
  

   among 
  the 
  first 
  to 
  attract 
  man's 
  attention, 
  although 
  there 
  is 
  an 
  in- 
  

   terval 
  of 
  at 
  least 
  two 
  thousand 
  years 
  between 
  the 
  earliest 
  mention 
  

   of 
  the 
  tides 
  that 
  has 
  come 
  down 
  to 
  us, 
  and 
  the 
  time 
  when 
  Jeremiah 
  

   Harrox 
  first 
  undertook 
  a 
  series 
  of 
  three 
  months' 
  continuous 
  tidal 
  

   observations 
  in 
  the 
  year 
  1640. 
  

  

  Our 
  knowledge 
  of 
  the 
  tides 
  is 
  based 
  almost 
  entirely 
  on 
  observa- 
  

   tions 
  confined 
  to 
  the 
  immediate 
  vicinity 
  of 
  the 
  coast. 
  Observations 
  

   have 
  not 
  yet 
  been 
  made 
  in 
  the 
  wide 
  stretches 
  of 
  the 
  open 
  ocean; 
  

   and 
  even 
  close 
  inshore 
  where 
  the 
  difficulties 
  of 
  measuring 
  the 
  rise 
  and 
  

   fall 
  of 
  the 
  tide 
  are 
  much 
  less, 
  little 
  has 
  been 
  done, 
  and 
  there 
  are 
  nu- 
  

   merous 
  localities 
  along 
  the 
  coasts 
  where 
  accurate 
  information 
  is 
  still 
  

   wanting. 
  There 
  is 
  need 
  of 
  investigations 
  to 
  determine 
  for 
  each 
  lo- 
  

   cality 
  the 
  changes 
  from 
  the 
  normal 
  heights 
  of 
  the 
  tide, 
  due 
  to 
  wind 
  

   and 
  changes 
  in 
  atmospheric 
  pressure. 
  

  

  The 
  tide-producing 
  forces 
  are 
  definitely 
  known, 
  but 
  the 
  problem 
  of 
  

   correlating 
  these 
  forces 
  with 
  the 
  time 
  and 
  height 
  of 
  the 
  tide 
  at 
  all 
  

   places 
  on 
  the 
  earth 
  by 
  means 
  of 
  a 
  general 
  formula 
  is 
  yet 
  to 
  be 
  solved 
  

   and 
  much 
  more 
  of 
  observation 
  is 
  needed 
  where 
  none 
  as 
  yet 
  exists, 
  

   especially 
  in 
  the 
  open 
  sea, 
  for 
  the 
  settlement 
  of 
  the 
  question 
  of 
  the 
  

   general 
  tidal 
  theory. 
  

  

  Mean 
  sea 
  level. 
  — 
  Closely 
  related 
  to 
  the 
  subject 
  of 
  tides 
  is 
  the 
  ques- 
  

   tion 
  of 
  mean 
  sea 
  level 
  and 
  its 
  relation 
  to 
  elevation 
  of 
  the 
  land 
  of 
  the 
  

   sea 
  coasts. 
  By 
  mean 
  sea 
  level, 
  I 
  mean 
  that 
  resulting 
  from 
  continu- 
  

   ous 
  tidal 
  observations 
  over 
  a 
  period 
  of 
  at 
  least 
  one 
  complete 
  lunar 
  

   cycle 
  of 
  approximately 
  nineteen 
  years' 
  duration. 
  

  

  I 
  do 
  not 
  see 
  just 
  how 
  we 
  can 
  know 
  anything 
  definite 
  about 
  the 
  

   variations 
  of 
  mean 
  sea 
  level. 
  The 
  sea 
  areas 
  are 
  so 
  much 
  greater 
  

   than 
  the 
  land 
  areas 
  of 
  the 
  globe 
  that 
  I 
  think 
  we 
  must 
  assume 
  for 
  all 
  

   practical 
  purposes 
  that 
  mean 
  sea 
  level 
  remains 
  practically 
  constant 
  

   and 
  that 
  any 
  changes 
  in 
  vertical 
  distance 
  between 
  reference 
  points 
  on 
  . 
  

   the 
  sea 
  coast 
  and 
  the 
  mean 
  ocean 
  surface 
  are 
  due 
  to 
  the 
  movements 
  of 
  

   the 
  land, 
  and 
  not 
  to 
  variations 
  of 
  mean 
  sea 
  level. 
  The 
  problem 
  there- 
  

   fore 
  becomes 
  one 
  of 
  determining 
  the 
  elevations 
  of 
  the 
  coasts 
  with 
  ref- 
  

   erence 
  to 
  the 
  mean 
  level 
  of 
  the 
  sea. 
  This 
  problem 
  is 
  of 
  much 
  impor- 
  

   tance 
  to 
  the 
  engineer 
  and 
  the 
  scientist, 
  especially 
  the 
  geologist 
  and 
  

   geophysicist. 
  Measurements 
  on 
  land 
  and 
  sea 
  are 
  all 
  referred 
  to 
  

   sea 
  level 
  wherever 
  a 
  vertical 
  reference 
  point 
  is 
  required. 
  Many 
  of 
  

   our 
  industrial 
  works 
  are 
  based 
  on 
  this 
  reference 
  plane, 
  and 
  on 
  account 
  of 
  

   its 
  invariability 
  and 
  readiness 
  of 
  reproduction, 
  it 
  is 
  a 
  natural 
  standard. 
  

  

  