﻿ROTATION 
  OF 
  PILLARS. 
  2if 
  

  

  be 
  any 
  revolution. 
  If 
  we 
  divide 
  our 
  section 
  A, 
  B, 
  C, 
  D 
  up 
  into 
  eight 
  divisions 
  by 
  

   lines 
  running 
  through 
  these 
  directions, 
  we 
  shall 
  see 
  that 
  any 
  shock 
  the 
  direction 
  

   of 
  which 
  passes 
  through 
  any 
  of 
  the 
  octants 
  which 
  are 
  shaded, 
  will 
  cause 
  a 
  positive 
  

   revolution 
  in 
  the 
  body; 
  that 
  is 
  to 
  say, 
  a 
  revolution 
  corresponding 
  in 
  its 
  direction 
  

   to 
  that'of 
  the 
  movements 
  of 
  the 
  hands 
  of 
  a 
  watch; 
  whilst 
  if 
  its 
  direction 
  passes 
  

   through 
  any 
  of 
  the 
  remaining 
  octants, 
  the 
  revolutions 
  will 
  be 
  negative 
  or 
  opposite 
  

   to 
  that 
  of 
  the 
  hands 
  of 
  a 
  watch. 
  

  

  From 
  the 
  direction 
  in 
  which 
  any 
  given 
  stone 
  has 
  turned, 
  we 
  can, 
  therefore, 
  give 
  

   two 
  sets 
  of 
  limits 
  between 
  one 
  of 
  which 
  the 
  shock 
  must 
  have 
  come. 
  

  

  Further, 
  it 
  will 
  be 
  observed 
  that 
  the 
  tendency 
  of 
  the 
  turning 
  is 
  to 
  bring 
  a 
  

   stone, 
  like 
  the 
  one 
  we 
  are 
  discussing, 
  broadside 
  on 
  to 
  the 
  shock 
  ; 
  therefore, 
  if 
  a 
  

   stone 
  with 
  a 
  rectangular 
  cross 
  section 
  has 
  turned 
  sufficiently 
  the 
  direction 
  of 
  a 
  

   shock 
  will 
  be 
  parallel 
  to 
  one 
  of 
  its 
  faces, 
  but 
  if 
  it 
  has 
  not 
  turned 
  sufficiently 
  it 
  will 
  

   be 
  more 
  nearly 
  parallel 
  to 
  its 
  faces 
  in 
  their 
  new 
  position 
  than 
  it 
  was 
  to 
  its 
  faces 
  

   when 
  in 
  their 
  original 
  position. 
  

  

  If 
  a 
  stone 
  receives 
  a 
  shock 
  nearly 
  parallel 
  with 
  its 
  diagonal 
  on 
  account 
  of 
  its 
  

   unstability, 
  it 
  may 
  turn 
  either 
  positively 
  or 
  negatively. 
  

  

  Similarly, 
  if 
  a 
  stone 
  receives 
  a 
  shock 
  parallel 
  to 
  one 
  of 
  its 
  faces, 
  the 
  twisting 
  may 
  

   be 
  either 
  positive 
  or 
  negative, 
  but 
  the 
  probability 
  is 
  that 
  it 
  would 
  only 
  turn 
  slightly, 
  

   whereas 
  in 
  the 
  former 
  case, 
  where 
  the 
  shock 
  was 
  nearly 
  parallel 
  to 
  a 
  diagonal, 
  

   the 
  turning 
  would 
  probably 
  be 
  great. 
  " 
  

  

  We 
  have 
  then 
  four 
  alternative 
  hypothesis, 
  for 
  the 
  suggestion 
  of 
  

   Mr. 
  Darwin 
  is 
  too 
  vague 
  and 
  indefinite 
  to 
  rank 
  as 
  an 
  hypothesis 
  or 
  

   explanation. 
  These 
  are 
  : 
  — 
  

  

  . 
  (i) 
  The 
  theory 
  which 
  is 
  implied 
  in 
  the 
  term 
  vorticose, 
  and 
  

   attributes 
  the 
  rotation 
  to 
  a 
  rotary 
  or 
  circular 
  motion 
  

   cf 
  the 
  ground 
  during 
  the 
  earthquake. 
  

  

  (2) 
  Mallet's 
  theory, 
  which 
  attributes 
  the 
  rotation 
  to 
  the 
  centre 
  

  

  of 
  resistance 
  and 
  the 
  centre 
  of 
  gravity 
  not 
  being 
  in 
  the 
  

   same 
  vertical 
  plane 
  with 
  the 
  direction 
  of 
  the 
  wave 
  

   motion, 
  and 
  to 
  the 
  stone 
  being 
  consequently 
  dragged 
  

   round. 
  

  

  (3) 
  Mallet's 
  second 
  theory, 
  which 
  attributes 
  the 
  twisting 
  to 
  a 
  

  

  second 
  shock, 
  affecting 
  the 
  object 
  twisted 
  while 
  it 
  is 
  

   tilted 
  on 
  one 
  edge 
  or 
  corner 
  by 
  the 
  first. 
  The 
  second 
  

   shock 
  being 
  oblique 
  to 
  the 
  first, 
  causes 
  the 
  object 
  to 
  

   rotate 
  on 
  the 
  edge 
  on 
  which 
  it 
  is 
  tilted. 
  

  

  (4) 
  Gray's 
  theory, 
  which 
  attributes 
  the 
  rotation 
  to 
  the 
  centre 
  

  

  ( 
  217 
  ) 
  

  

  