833.  Measures  against  inundations. — Many  ways  of  over-
coming floods,  and  preventing  the  disasters  that  arise  there-
ironi,  have  been  proposed.  None,  however,  have  been  entirely
successful,  and,  in  the  present  state  of  science,  the  question
feems  to  have  no  certain  solution.  The  various  systems  pro-
posed are  as  follows:  1st,  to  build  reservoirs  for  holding
back  the  hurtful  part  of  floods  and  checking  the  flow  of  the
water ;  2d,  levees  or  embankments  raised  along  the  sides  of
the  river;  3d,  cut-off  canals  and  horizontal  ditches,  weirs;
4th,  re-wooding  and  resodding  hills  and -mountains.
1st.  Reservoirs. — Damage  by  floods  arises  from  two  causes,
the  volume  of  the  water  and  its  velocity.  The  former  acts  by
submerging  buildings,  crops,  etc.  ;  the  latter  by  cutting  away
the  land,  carrying  away  structures  of  various  kinds.  Of  these
the  velocity  is  probably  the  more  destructive,  and  the  remedy
is  to  lessen  the  velocity.  This  the  partisans  of  the  reservoir
system  claim  can  be  best  accomplished  by  means  of  storing
the  greater  part  of  the  flood,  and,  when  it  subsides,  letting  off
gradually  the  accumulated  water.  The  opponents  of  the  sys-
tem claim  that  it  would  be  impossible  to  build  reservoirs  of
sufficient  capacity  to  hold  back  any  appreciable  part  of  the
falling  water.  Furthermore,  the  water  held  in  check  on  one
tributary  might  be  let  loose  just  in  time  to  meet  that  coming
out  of  another,  and  thus  make  the  second  flood  much  worse.
