the  Descent  of  Glaciers  by  their  Weight  only,  45 
does  so.  But  it  is  in  a  high  degree  improbable  that  the  glacier 
is  carried  beyond  the  limits  of  its  elasticity  whose  bend  is  not 
the  fiftieth  part  of  that  of  the  ice-plank.  And  this  high  impro- 
bability becomes,  I  think,  an  impossibility  when  it  is  considered 
that  each  infinitely  thin  horizontal  stratum  of  the  glacier  must 
(to  descend  as  an  actual  glacier  does)  bend  further  thau  the  one 
below  it,  and  in  doing  so  overcome  the  resistance  to  shearing  of 
the  subjacent  stratum  to  which  it  is  frozen — so  that  to  the  resist- 
ance to  its  bending  bodily  must  be  added  the  resistances  to  the 
differential  bendiugs  of  its  superimposed  strata. 
Thus,  from  whatever  point  of  view  the  question  of  the  descent 
of  glaciers  is  looked  at,  in  the  light  of  exact  science,  it  is  seen  to 
be  impossible  they  should  descend  by  their  weights  only.  As, 
however,  it  is  possible  that  Mr.  Mathews,  taking  his  stand  on 
the  strong  ground  of  popular  science,  may  refuse  to  be  con- 
vinced'by  mathematical  reasoning  even  so  elementary  as  that 
which  I  have  here  been  using,  I  will  submit  to  him  the  following 
considerations. 
He  argues  that  because  an  ice-plank  bends  vertically  flatwise 
by  its  own  weight,  and  takes  a  set  when  placed  between  two  sup- 
ports, therefore  the  Mer  de  Glace,  when  placed  on  a  slope  of 
4°  52',  bends  lengthwise  by  its  own  weight.  He  has  nothing, 
so  far  as  appears  from  his  papers,  but  that  fact  to  bring  him  to 
that  conclusion — nothing  to  bridge  over  the  intervening  space. 
Now  if  a  plank  of  lead,  long  enough  in  relation  to  its  thick- 
ness, were  placed  flatwise  between  two  supports  it  would  certainly 
bend  and  take  a  set.  That  is  a  fact  strictly  analogous  to  Mr. 
Mathews's  fact  of  the  ice-plank.  According  to  him,  therefore, 
if  the  Mer  de  Glace  were  of  lead  instead  of  ice  it  would  descend 
by  its  weight  as  a  glacier  does. 
But  if  the  argument  is  good  for  ice  and  also  for  lead  it  is 
good  also  for  iron.  For  if  a  plank  of  wrought  iron  thin  enough, 
with  reference  to  its  length,  wrere  placed  flatwise  between  two 
supports,  it  would  bend  by  its  weight  and  take  a  set.     It  follows, 
moduli  of  elasticity  and  equal  specific  gravities, 
Dx-  sm  t. 
c- 
Now  iu  the  supposed  glacier  «  =  1398  feet,  c=22,600  feet,  t  =  4°  52'. 
Io  Mr.  Mathews's  ice-plank,  a=6feet,  c  =  Q\9J<)  feet,  t=90°. 
If  therefore  Dt  be  the  deflection  of  the  glacier,  and  D2  that  of  the  ice- 
plank, 
13381    .     ,0  -of 
sm  4    o2 
£=~ =.01!M2o. 
