﻿16 
  THE 
  UNFATHOMED 
  UNIVERSE 
  

  

  Yet, 
  as 
  a 
  warning 
  against 
  finality 
  in 
  another 
  direction, 
  we 
  

   must 
  admit 
  the 
  vagueness 
  that 
  is 
  apt 
  to 
  invest 
  our 
  knowledge 
  

   of 
  the 
  past. 
  In 
  the 
  perfectly 
  exact 
  sciences, 
  we 
  can 
  some- 
  

   times 
  work 
  backwards 
  with 
  remarkable 
  certainty 
  to 
  comets, 
  

   eclipses, 
  and 
  the 
  like; 
  but 
  in 
  Biology 
  how 
  watchful 
  we 
  have 
  

   to 
  be 
  lest 
  we 
  get 
  entangled 
  in 
  the 
  vicious 
  circle 
  of 
  inventing 
  

   a 
  past 
  from 
  its 
  continued 
  life 
  in 
  the 
  present, 
  and 
  then 
  in- 
  

   terpreting 
  the 
  present 
  in 
  terms 
  of 
  our 
  invention. 
  

  

  (c) 
  A 
  reasonable 
  humility 
  of 
  mind 
  is 
  also 
  engendered 
  

   by 
  recognising 
  how 
  limited, 
  after 
  all, 
  is 
  our 
  range 
  of 
  exact 
  

   data. 
  The 
  late 
  Professor 
  Rowland, 
  a 
  distinguished 
  physi- 
  

   cist, 
  writes 
  (1899, 
  p. 
  408) 
  : 
  " 
  In 
  time 
  we 
  are 
  limited 
  by 
  a 
  

   few 
  hundred 
  or 
  possibly 
  thousand 
  years. 
  ... 
  In 
  space 
  we 
  

   have 
  exact 
  knowledge 
  limited 
  to 
  portions 
  of 
  our 
  earth's 
  sur- 
  

   face 
  and 
  a 
  mile 
  or 
  so 
  below 
  the 
  surface, 
  together 
  with 
  what 
  

   little 
  we 
  can 
  learn 
  from 
  looking 
  through 
  powerful 
  telescopes 
  

   into 
  the 
  space 
  beyond. 
  In 
  temperature 
  our 
  knowledge 
  ex- 
  

   tends 
  from 
  near 
  the 
  absolute 
  zero 
  to 
  that 
  of 
  the 
  sun, 
  but 
  

   exact 
  knowledge 
  is 
  far 
  more 
  limited. 
  In 
  pressures 
  we 
  go 
  

   from 
  the 
  Crookes 
  vacuum, 
  still 
  containing 
  myriads 
  of 
  flying 
  

   atoms, 
  to 
  pressures 
  limited 
  by 
  the 
  strength 
  of 
  steel, 
  but 
  still 
  

   very 
  minute 
  compared 
  with 
  the 
  pressures 
  at 
  the 
  centre 
  of 
  

   the 
  earth 
  and 
  sun, 
  where 
  the 
  hardest 
  steel 
  would 
  flow 
  like 
  

   the 
  most 
  limpid 
  water. 
  In 
  velocities 
  we 
  are 
  limited 
  to 
  a 
  few 
  

   miles 
  per 
  second. 
  In 
  forces 
  to 
  possibly 
  one 
  hundred 
  tons 
  to 
  

   the 
  square 
  inch. 
  In 
  mechanical 
  rotations 
  to 
  a 
  few 
  hun- 
  

   dred 
  times 
  a 
  second." 
  Perhaps 
  some 
  of 
  these 
  limits 
  have 
  

   been 
  extended 
  since 
  Professor 
  Rowland 
  gave 
  the 
  address 
  

   from 
  which 
  we 
  have 
  quoted, 
  but 
  that 
  would 
  not 
  affect 
  our 
  

   point, 
  the 
  importance 
  of 
  bearing 
  in 
  mind 
  the 
  limits 
  of 
  

   exact 
  knowledge. 
  

  

  (d) 
  Another 
  limitation 
  is 
  involved 
  in 
  the 
  very 
  nature 
  

  

  