﻿ROTATION 
  OF 
  PILLARS. 
  2lj 
  

  

  stone 
  itself 
  will 
  be 
  solicited 
  to 
  move 
  in 
  the 
  same 
  direction. 
  The 
  measure 
  of 
  force 
  

   with 
  which 
  the 
  movement 
  of 
  the 
  base 
  is 
  capable 
  of 
  affecting 
  the 
  stone 
  or 
  other 
  in- 
  

   cumbent 
  body 
  is 
  equal 
  to 
  the 
  amount 
  of 
  friction 
  of 
  the 
  latter 
  upon 
  its 
  base, 
  a 
  func- 
  

   tion 
  of 
  its 
  weight, 
  which, 
  without 
  the 
  intervention 
  of 
  cement, 
  may 
  be 
  from 
  one- 
  

   fifth 
  to 
  one-tenth 
  of 
  the 
  weight 
  of 
  the 
  body, 
  for 
  cut 
  stone 
  resting 
  on 
  cut 
  stone, 
  but 
  

   may 
  be 
  increased 
  to 
  any 
  amount 
  by 
  the 
  intervention 
  of 
  cement. 
  The 
  stone, 
  how- 
  

   ever, 
  is 
  possessed 
  of 
  weight, 
  and 
  therefore 
  of 
  inertia, 
  that 
  is 
  to 
  say, 
  being 
  at 
  rest, 
  its 
  

   whole 
  mass 
  cannot 
  be 
  instantly 
  brought 
  into 
  motion 
  by 
  the 
  plane 
  on 
  which 
  it 
  

   rests 
  ; 
  and 
  if 
  the 
  amount 
  of 
  adhesion 
  between 
  the 
  stone 
  and 
  its 
  bed 
  be 
  less 
  than 
  the 
  

   inertia 
  due 
  to 
  any 
  given 
  velocity 
  of 
  horizontal 
  movement 
  of 
  the 
  bed, 
  the 
  bed 
  will 
  

   move 
  more 
  or 
  less 
  from 
  under 
  the 
  stone, 
  or 
  the 
  stone 
  will 
  appear 
  to 
  move 
  in 
  a 
  con- 
  

   trary 
  direction 
  to 
  that 
  of 
  the 
  motion 
  of 
  its 
  bed. 
  

  

  Now 
  the 
  inertia 
  of 
  the 
  stone, 
  which 
  is 
  here 
  the 
  resisting 
  force, 
  may 
  be 
  considered 
  

   to 
  act 
  at 
  its 
  centre 
  of 
  gravity. 
  The 
  impelling 
  force 
  is 
  the 
  grasp 
  of 
  the 
  stone 
  which 
  

   its 
  bed 
  holds 
  of 
  it 
  by 
  friction 
  or 
  adhesion, 
  and 
  this 
  may 
  also 
  be 
  referred 
  to 
  some 
  one 
  

   point 
  in 
  the 
  surfaces 
  of 
  contact, 
  which 
  we 
  may 
  call 
  the 
  centre 
  of 
  adherence. 
  

  

  If, 
  then, 
  a 
  stone, 
  or 
  other 
  solid, 
  rest 
  upon 
  a 
  plane 
  which 
  is 
  suddenly 
  moved 
  

   with 
  sufficient 
  velocity 
  to 
  produce 
  motion 
  in 
  the 
  incumbent 
  body, 
  three 
  several 
  

   conditions 
  of 
  motion 
  of 
  the 
  body 
  may 
  occur, 
  according 
  to 
  the 
  respective 
  positions 
  

   of 
  its 
  centre 
  of 
  gravity 
  and 
  of 
  the 
  centre 
  of 
  adherence 
  : 
  — 
  

  

  (i) 
  The 
  centre 
  of 
  gravity 
  of 
  the 
  body 
  may 
  be 
  at 
  such 
  a 
  height 
  above 
  the 
  base 
  

   that 
  it 
  shall 
  upset 
  by 
  its 
  own 
  inertia. 
  This 
  is 
  the 
  case 
  with 
  houses, 
  towers, 
  walls, 
  

   etc., 
  etc., 
  when 
  they 
  fall 
  by 
  earthquakes, 
  accompanied, 
  however, 
  by 
  dislocation 
  of 
  

   their 
  parts. 
  

  

  (2) 
  The 
  centre 
  of 
  adherence 
  may 
  be 
  in 
  a 
  point 
  of 
  the 
  base 
  plumb 
  under 
  the 
  

   centre 
  of 
  gravity 
  of 
  the 
  body, 
  or 
  in 
  a 
  vertical 
  plane 
  passing 
  through 
  its 
  centre 
  of 
  

   gravity, 
  and 
  in 
  the 
  direction 
  of 
  motion 
  of 
  the 
  base. 
  

  

  In 
  this 
  case 
  the 
  stone 
  will 
  appear 
  to 
  move 
  in 
  the 
  opposite 
  direction 
  to 
  that 
  in 
  

   Which 
  the 
  base 
  has 
  moved, 
  that 
  is 
  to 
  say, 
  the 
  body 
  may 
  have 
  acquired 
  more 
  of 
  

   less 
  the 
  direction 
  of 
  motion 
  of 
  the 
  base, 
  according 
  as 
  the 
  motion 
  of 
  the 
  latter 
  has 
  

   been 
  of 
  longer 
  or 
  shorter 
  continuance, 
  or 
  less 
  or 
  more 
  rapid 
  ; 
  but 
  in 
  so 
  far 
  as 
  the 
  

   movement 
  in 
  opposite 
  directions 
  has 
  taken 
  place, 
  the 
  base 
  in 
  reality 
  has 
  slipped 
  

   from 
  under 
  the 
  body. 
  

  

  (3) 
  The 
  centre 
  of 
  adherence 
  may 
  neither 
  be 
  plumb 
  under 
  the 
  centre 
  of 
  gravity 
  

   of 
  the 
  body, 
  nor 
  in 
  the 
  plane 
  of 
  motion 
  passing 
  through 
  its 
  centre 
  of 
  gravity, 
  but 
  

   in 
  some 
  point 
  of 
  the 
  base 
  outside 
  the 
  line 
  of 
  its 
  intersection 
  by 
  the 
  plane 
  ; 
  in 
  which 
  

   case 
  the 
  effect 
  of 
  the 
  rectilinear 
  motion 
  in 
  the 
  plane 
  of 
  the 
  base 
  will 
  be 
  to 
  twist 
  

   the 
  body 
  round 
  upon 
  its 
  bed, 
  or 
  to 
  move 
  it 
  laterally, 
  and 
  twist 
  it 
  at 
  the 
  same 
  time* 
  

   thus 
  converting 
  the 
  rectilinear 
  into 
  a 
  curvilinear 
  motion 
  in 
  space. 
  The 
  relative 
  

   amount 
  of 
  the 
  two 
  compounded 
  motions 
  being 
  dependent 
  upon 
  the 
  velocity 
  and 
  

   time 
  of 
  movement 
  of 
  the 
  base, 
  and 
  upon 
  the 
  perpendicular 
  distance 
  measured 
  

   horizontally 
  at 
  the 
  surface 
  of 
  adherence 
  between 
  the 
  centre 
  of 
  adherence 
  and 
  the 
  

   centre 
  of 
  gravity 
  of 
  the 
  body. 
  

  

  This 
  latter 
  case 
  is 
  that 
  which 
  appears 
  to 
  have 
  twisted 
  the 
  atones 
  of 
  Calabria, 
  

   South 
  America 
  and 
  Greece, 
  and 
  affords, 
  as 
  I 
  feel 
  assured, 
  the 
  true 
  explanation 
  of 
  

   the 
  phenomenon." 
  

  

  ( 
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  ; 
  

  

  