the  Descent  of  Glaciers  by  their  Weight  only.  41 
same  way,  the  shearing-pressure  of  10,800  lbs.  per  square  foot 
will  not  be  reached  until  2037  strips  have  been  so  placed,  and  a 
platform  of  ice  has  been  formed  of  that  breadth  in  feet,  resting 
without  friction  or  adhesion  on  the  bottom  of  the  channel,  but 
adhering  by  its  edge  to  the  side.  An  additional  strip  (the  2038th) 
will  cause  this  platform  to  shear  along  its  edge  and  to  descend 
bodily.  Let  now  a  resistance  be  supposed  to  be  applied  to  the 
first  strip  nearest  to  the  side  just  sufficient  to  prevent  it,  and 
therefore  the  platform,  from  descending  by  the  run,  i.  e.  with  an 
accelerated  motion.  Another  strip  (the  2039th)  being  then 
added  at  the  outside  edge  of  the  platform,  the  second  strip  will 
be  made  to  shear  on  the  first ;  and  the  acceleration  of  its  descent 
being  resisted  as  before,  the  addition  of  another  strip  (the  20i0th) 
will  cause  the  third  strip  to  shear  on  the  second;  and  by  conti- 
nuing this  process  of  accretion  at  the  outside  edge  of  the  platform 
and  of  resistance*  at  the  inner  edge,  a  differential  motion  of  the 
strips  at  the  inner  edge  will  be  set  up  (numbering  from  the  first), 
and  there  will  result  from  it  a  bodily  descent  of  2037  feet  of  un- 
broken platform  beyond — the  whole  descending  mass  of  ice 
being  composed  of  a  part  near  the  side  of  the  channel  which  de- 
scends with  a  differential  motion,  and  of  a  part  beyond  it  always 
2037  feet  wide  which  descends  not  with  a  differential  motion 
but  bodily.  Every  addition  made  to  the  outside  edge  of  this 
last-mentioned  part  takes  from  as  much  of  its  inner  edge  its  un- 
broken character  and  adds  it  to  the  part  which  descends  differen- 
tially. Thus  one  half  of  the  platform  of  a  glacier  is  constructed. 
If  the  other  half  be  similarly  constructed,  beginning  from  the 
opposite  side  of  the  channel,  a  complete  platform  will  be  ob- 
tained of  which  the  central  portion  (4074  feet  wide)  descends 
bodily,  t.  e.  without  any  differential  motion,  and  of  which  two 
side  portions,  dependent  for  their  width  on  the  width  of  the 
channel,  descend  with  a  differential  motion. 
Let  now  the  whole  of  this  platform  be  supposed  to  be  frozen 
to  the  bottom.  Its  descent  will  thus  be  arrested;  but  it  will  be 
ready  to  begin  to  descend  again,  as  it  did  before,  if  by  the  ope- 
ration of  any  other  force  the  resistance  to  its  shearing  over  the 
bottom  of  the  channel  shall  be  overcome.  The  thrust  down- 
wards with  which  the  weight  of  each  cubic  foot  of  it  tends  to 
overcome  this  resistance  to  its  shearing  over  the  bottom,  has  been 
shown  to  be  53  lbs.  per  square  foot,  whereas  the  resistance  itself 
*  Such  resistance,  increasing  in  amount  with  the  velocity  of  the  descent, 
must  be  actually  in  operation  in  glaciers  descending  by  tlieir  weight  only, 
or  they  would  descend  by  the  run.  It  will  be  borne  in  mind  that  when 
systems  of  bodies  acted  on  by  given  forces  move  under  the  action  of  those 
forces  with  uniform  velocities,  the  forces  have  the  same  relation  to  one  an- 
other asthev  would  have  if  the  bodies  were  at  rest. 
