46  Canon  Moseley  on  the  Descent  of  Glaciers. 
therefore,  by  Mr.  Mathews's  argument,  that  the  Mer  de  Glace 
would  descend  by  its  weight  only,  if  it  were  of  iron  instead  of  ice, 
and  that  it  would  descend  with  a  differential  motion. 
Every  body  knows  how  much  more  easily  a  rod  is  bent  than 
sheared  across.  The  question  "  why  this  happens  "- — what  are 
those  molecular  displacements  lengthwise  and  crosswise  which 
accompany  this  change  of  form,  and  what  are  the  forces  which 
oppose  themselves  to  it — is  one  of  the  most  subtle  in  physics. 
But  little  is  to  be  understood  of  it  by  the  light  of  the  highest  ma- 
thematics, and  nothing  without  it.  I  will  not  invite  Mr.  Ma- 
thews to  accompany  me  into  that  region.  The  question  what 
would  happen  if  a  glacier  bent  itself  down  its  channel  does  not 
indeed  touch  my  argument,  because  I  have  shown  that  a  glacier 
does  not  bend  itself  down.  There  is,  however,  an  inference  Mr. 
Mathews  has  drawn  from  the  bending  of  his  ice-plank  which 
does  touch  my  argument.  The  cross  section  of  his  ice-plank 
measured,  he  says,  12  square  inches.  If  it  were  sheared  across 
as  ice  was  sheared  in  my  experiments,  it  would,  at  75  lbs.  per 
square  inch  shearing-resistance,  require  700  lbs.  to  shear  it. 
But  the  entire  weight  of  the  ice-plank  was  only  32  lbs.,  and  yet 
it  deflected  with  this  weight  and  therefore  also  sheared.  Mr. 
Mathews  is  curious  to  see  how  I  shall  explain  this  fact  to  him. 
His  explanation  is,  that  instead  of  the  unit  of  shear  in  ice  being 
not  less  than  75  lb.  per  square  inch,  as  I  found  it  to  be  by 
direct  experiment,  it  is  shown  indirectly  by  this  experiment  of 
his  to  be  less  than  1J  lb.,  probably  much  less. 
Now  I  will  suggest  to  Mr.  Mathews  a  parallel  experiment  and 
a  parallel  explanation.  If  a  bar  of  wrought  iron  1  inch  square 
and  20  feet  long  were  supported  at  its  extremities,  it  would  bend 
by  its  weight  alone,  and  would  therefore  shear.  Now  the  weight 
of  such  a  rod  would  be  about  67  lbs.  According  to  Mr.  Mathews's 
explanation  in  the  case  of  the  ice-plank,  the  unit  of  shear  in 
wrought  iron  should  therefore  be  67  lbs.  per  square  inch.  It  is 
actually  50,000  lbs. 
I  think  I  have  not  left  unanswered  any  question  raised  by 
Mr.  Mathews's  first  paper. 
With  reference  to  his  second  paper,  I  have  only  to  bear 
testimony  to  the  value  which  I  attach  to  his  observations  and 
those  of  Mr.  Reilly  on  the  Surface-motions  of  Glaciers.  The 
exceedingly  small  differences  of  velocity  of  points  exceedingly 
near  to  one  another  of  which  he  speaks,  following  Mr.  Ball,  is 
no  argument  against  the  results  at  which  I  have  arrived.  Those 
results  suppose  infinitely  small  differences  of  velocity  at  points 
infinitely  near  to  one  another,  and  they  aggregate  the  work  done 
under  those  infinitesimally  small  differences  of  motion.  The 
aggregate  of  an  infinite  number  of  infinitely  small  things  may 
