40         Canon  Moseley  on  the  Mechanical  Impossibility  o 
argument.  To  follow  it  out  completely,  a  somewhat  difficult  ap- 
plication of  the  principles  of  dynamics  is  required.  The  question 
of  the  descent  of  glaciers  is  indeed  essentially  one  of  mechanical 
philosophy,  and,  as  such,  cannot  be  argued  with  precision  other- 
wise than  by  mathematical  reasoning.  It  is  a  question  which  has 
nevertheless  long  been  in  possession  of  that  kind  of  scientific 
opinion  (as  distinguished  from  exact  science)  which  refuses  to 
take  account  of  mathematical  reasoning.  Unless  I  discard  all 
but  the  most  elementary  forms  of  this  kind  of  reasoning,  I  can- 
not therefore  expect  to  be  listened  to  by  those  who  chiefly  take 
an  interest  in  the  discussion.  I  will  therefore  attempt,  although 
I  confess  with  no  very  sanguine  hope  of  success,  to  conduct  my 
argument  subject  to  this  condition.  Mr.  Mathews  will  under- 
stand at  what  a  disadvantage  I  thus  place  myself. 
I  will  ask  him  to  imagine  a  rectangular  channel  of  ice  to  be 
cut  out  of  my  imaginary  glacier  of  the  same  form  and  nearly  of 
the  same  dimensions  as  the  glacier  itself,  so  that  only  a  compa- 
ratively small  thickness  of  ice,  forming  the  sides  and  bottom  of 
this  ice-channel,  shall  lie  between  it  and  the  rock.     Before  the 
ice  was  taken  out  of  this  ice-channel  it  descended  with  a  differ- 
ential motion.     To  fix  ideas,  let  the  channel  be  dug  a  mile  long 
and  open  at  both  ends,  and  let  the  ice  be  imagined  to  be  then 
replaced  and  the  glacier  reconstructed  as  follows.     Let  a  strip  of 
ice  a  foot  square  in  section  and  a  mile  long  be  conceived  to  be 
placed  at  the  bottom  corner  of  the  channel  at  its  left-hand  side 
looking  down  the  glacier,  and  to  be  frozen  by  its  side  against 
the  side  of  the  channel,  but  not  by  its  base  against  the  bottom ; 
so  that,  to  be  made  to  slide  down,  this  ice- strip  must  be  made  to 
shear  by  its  side  only.     Now  the  weight  of  each  foot  of  the 
length  of  this  ice-strip  is  62^  lbs.  nearly,  being  the  weight  of  a 
cubic  foot  of  water;  and  the  glacier  being  inclined  at  an  angle 
of  4°  52'  to  the  horizon  (being  the  inclination  of  the   Mer  de 
Glace),  this  62J  lbs.  pressure  of  each  foot  in  the  length  of  the 
strip  vertically  produces  a  pressure  down  the  glacier  of  62*5  lbs. 
x  sin  4°  52',  or  of  62*5  lbs.  x  -084837  =  5-3  lbs.  nearly.      But 
the  shearing-force  per  square  inch  being  75  lbs.,  each  foot  in  the 
length  of  the  strip  being  frozen  to  the  side  of  the  channel  re- 
quires 144  x  75  lbs.,  or  10,800  lbs.  to  shear  it.    The  strip,  there- 
fore, will  not  descend.     Let  now  another  strip  of  the  same  size 
be  placed  on  the  bottom  of  the  glacier  beside  the  first  and  frozen 
to  its  side,  but  not  to  the  bottom,  on  which  it  is  to  be  supposed 
free  to  slide  without  friction.    The  two  strips  thus  frozen  together 
will  produce  a  shearing-force  on  the  side  of  the  channel  of  5"3  lbs. 
X  2  per  square  foot,  whereas  the  resistance  to  shearing  is  10,800 
lbs.  per  square  foot.     The  two  strips  will  still  therefore  remain 
fixed  to  it;  and  if  strip  after  strip  be  added  side  by  side  in  the 
