﻿182 
  

  

  Mr. 
  R. 
  Mallet 
  on 
  the 
  Origin 
  of 
  the 
  [Jan. 
  21, 
  

  

  which 
  tend 
  to 
  cause 
  the 
  outer 
  portions 
  of 
  the 
  prisms 
  to 
  tear 
  asunder 
  at 
  

   intervals 
  in 
  length 
  dependent, 
  like, 
  the 
  diameter 
  of 
  the 
  prisms 
  themselves, 
  

   upon 
  the 
  relation 
  subsisting 
  between 
  the 
  coefficient 
  of 
  contraction 
  and 
  of 
  

   extensibility 
  at 
  rupture 
  of 
  the 
  material. 
  

  

  The 
  prism 
  contracts 
  not 
  only 
  in 
  its 
  length, 
  but 
  in 
  its 
  diameter 
  ; 
  trans- 
  

   verse 
  fracture 
  at 
  its 
  surface, 
  when 
  it 
  occurs, 
  is 
  therefore 
  due 
  to 
  the 
  

   resultant 
  of 
  two 
  orthogonal 
  forces, 
  the 
  one 
  parallel 
  to 
  the 
  axis 
  of 
  the 
  

   prism, 
  as 
  already 
  referred 
  to, 
  and 
  the 
  other 
  in 
  a 
  plane 
  transverse 
  to 
  the 
  

   axis. 
  These 
  two 
  forces 
  are 
  proportionate, 
  the 
  first 
  to 
  the 
  length 
  of 
  the 
  

   prism 
  from 
  a 
  preceding 
  joint 
  or 
  from 
  its 
  extremity, 
  the 
  second 
  approxi- 
  

   mately 
  to 
  the 
  semidiameter 
  of 
  the 
  hexagon 
  or 
  mean 
  radius 
  of 
  the 
  

   coucke 
  ; 
  and 
  the 
  resultant 
  of 
  these 
  two, 
  at 
  any 
  point 
  taken 
  round 
  the 
  

   prism, 
  is 
  oblique 
  to 
  the 
  axis 
  and 
  tending 
  towards 
  it 
  in 
  direction. 
  As 
  

   fracture 
  in 
  a 
  homogeneous 
  solid 
  always 
  takes 
  place 
  transverse 
  to 
  the 
  line 
  

   of 
  strain, 
  so 
  the 
  fracture 
  producing 
  a 
  transverse 
  joint 
  takes 
  place 
  

   oblique 
  to 
  the 
  sides 
  of 
  the 
  prism 
  — 
  the 
  obliquity 
  becoming 
  less 
  as 
  the 
  

   fracture 
  advances 
  towards 
  the 
  axis 
  of 
  the 
  prism, 
  so 
  that 
  when 
  com- 
  

   plete 
  it 
  is 
  cup-shaped, 
  the 
  convex 
  surface 
  of 
  the 
  fracture 
  always 
  point- 
  

   ing 
  in 
  the 
  same 
  direction 
  as 
  that 
  in 
  which 
  the 
  splitting 
  of 
  the 
  prism 
  

   itself 
  is 
  proceeding. 
  

  

  This 
  solution, 
  which 
  is 
  believed 
  to 
  be 
  the 
  first 
  ever 
  presented 
  which, 
  

   resting 
  upon 
  admitted 
  laws, 
  completely 
  accounts 
  for 
  the 
  production 
  of 
  the 
  

   very 
  remarkable 
  cup-shaped 
  joints, 
  is 
  verified 
  and 
  illustrated 
  by 
  several 
  

   diagrams, 
  showing 
  the 
  mode 
  of 
  production 
  of 
  these 
  joints 
  and 
  the 
  

   modifications 
  of 
  their 
  curvature 
  produced 
  by 
  varied 
  conditions 
  in 
  the 
  

   cooling. 
  

  

  It 
  is 
  further 
  shown 
  that 
  the 
  partial 
  or 
  complete 
  detachment 
  of 
  certain 
  

   fragments, 
  frequently 
  observed 
  to 
  be 
  partially 
  or 
  wholly 
  detached 
  from 
  

   the 
  cusps 
  of 
  the 
  concave 
  side 
  of 
  these 
  joints 
  at 
  and 
  near 
  the 
  solid 
  angles 
  

   of 
  the 
  hexagon, 
  is 
  a 
  consequence 
  necessarily 
  resulting 
  from 
  the 
  mode 
  of 
  

   production 
  of 
  the 
  joints 
  themselves. 
  The 
  author 
  then 
  points 
  out 
  that, 
  

   in 
  the 
  case 
  of 
  very 
  slender 
  prisms, 
  other 
  (and 
  mechanical) 
  conditions 
  

   besides 
  those 
  of 
  differential 
  cooling 
  enter 
  into 
  the 
  production 
  of 
  the 
  cross 
  

   joints, 
  which 
  are 
  at 
  more 
  considerable 
  and 
  irregular 
  distances 
  apart, 
  

   and 
  in 
  planes 
  of 
  fracture 
  often 
  nearly 
  transverse 
  to 
  the 
  axis 
  of 
  the 
  

   prism. 
  

  

  He 
  also 
  discusses 
  the 
  modifications 
  produced 
  in 
  the 
  prisms 
  themselves, 
  

   and 
  in 
  their 
  cross 
  joints, 
  by 
  heterogeneity 
  in 
  the 
  mass 
  of 
  basalt 
  itself 
  — 
  

   as, 
  for 
  example, 
  by 
  a 
  more 
  or 
  less 
  previously 
  developed 
  cleavage 
  in 
  the 
  

   basalt 
  in 
  planes 
  transverse 
  to 
  the 
  axis 
  of 
  the 
  prism, 
  or 
  by 
  the 
  presence 
  of 
  

   heterogeneous 
  substances 
  imbedded 
  in 
  the 
  mass. 
  To 
  these 
  latter, 
  and 
  to 
  

   differences 
  in 
  conductivity 
  or 
  in 
  the 
  cooling 
  energy 
  at 
  different 
  points 
  of 
  

   the 
  cooling 
  surface, 
  are 
  chiefly 
  to 
  be 
  ascribed 
  the 
  divergences 
  from 
  the 
  

   normal 
  hexagonal 
  form 
  of 
  the 
  prisms 
  as 
  occasionally 
  observed, 
  the 
  author 
  

   remarking 
  that 
  where 
  such 
  divergences 
  occur 
  they 
  disappear, 
  and 
  the 
  

  

  