They  either  decay,  wear  out,  or  are  destroyed  by  floods  in  a
limited  number  of  years.  Such  dams  have  plaj-ed  an  important
part  in  the  development  of  industry  hitherto,  and  will  continue
to  do  so  in  future ;  but  there  is  a  constantly  increasing  demand
for  permanence  in  this  class  of  structures,  and  we  may  look
with  confidence,  in  the  future,  for  a  constantly  increasing  pro-
portion of  masonry  dams.  This  expectation  is  founded  upon
these  considerations:

1.  The  increasing  cost  of  timber.
2.  The  diminishing  cost  of  masonry.
3.  The  greatly  diminished  rate  of  interest.
The  increase  in  cost  of  timber,  though  very  decided,  is  not
so  great  as  might  have  been  expected.  Accessible  forest  lands
have  been  stripped  of  timber,  or  have  greatly  increased  in  price,
and  resort  is  had  to  distant  lands ;  but  these  tendencies  are
largely  counteracted  by  the  great  improvements  that  have
taken  place  in  means  of  manufacture  and  transportation.
Moreover,  the  stripping  of  timber  from  forest  lands  does  not
destroy  its  power  of  reproduction.  Timber,  although  a  crop
of  slow  growth,  is  none  the  less  a  usufruct  of  land,  which  comes
to  maturity  in  due  time,  and  the  increased  attention  now  being
directed  to  the  care  and  preservation  of  forests  wiJl,  it  is  to  be
hoped,  insure  reasonable  supplies  in  future.
119


120  DAMS   OF  MASONRY.
