410  River  Conservancy , and  Cause  and  Prevention  of  Floods. 
in  this  country  ; but  the  high  rents  which  the  water  meadows 
command  in  Devonshire,  the  value  of  irrigated  lands  in  Italy, 
Spain,  and  even  in  Norway,  and  the  rich  meadows  which  are 
periodically  covered  with  water  from  the  overflowing  of  some 
rivers,  clearly  prove  that  water  has  a fertilizing  power  inde- 
pendent of  climate,  and  beyond  that  of  the  moisture  supplied  to 
the  growing  vegetation. 
As  a source  of  power,  water  is  most  valuable,  and  might  be 
used  far  more  extensively  than  it  is  as  an  aid  to  the  working 
of  farms.  Threshing,  grinding,  chaff-cutting,  &c.,  could  as 
well  be  done  by  a water-wheel  or  a turbine  as  by  a steam- 
engine,  and  at  far  less  cost  ; but  to  make  the  rivers  available 
for  this  purpose,  it  is  necessary  to  hold  the  water  up  for  the 
purpose  of  obtaining  the  required  head  and  supply. 
For  all  these  reasons  great  caution  requires  to  be  exer- 
cised before  the  weirs  and  other  works  of  abandoned  navi- 
gations are  removed,  or  the  rights  of  millers  purchased  with 
a view  to  improving  the  rivers.  Attention  should  be  more 
directed  to  obtaining  a complete  control  over  these  weirs,  and 
having  them  placed  in  the  hands  of  the  body  responsible  for 
the  proper  conservancy  of  the  river,  so  as  to  regulate  their 
discharge,  not  with  the  view  of  any  private  interest,  but  for  the 
good  of  the  whole  watershed.  It  is  an  error  to  suppose  that 
weirs  are  necessarily  detrimental  to  the  discharge  of  a river  in 
flood  time.  Weirs,  if  properly  constructed,  and  of  sufficient 
capacity,  are  aids  rather  than  hindrances  to  the  free  flow  of  the 
water.  The  still  pond  of  water  which  they  hold  up  converts  the 
action  of  the  downward  current  into  something  of  the  nature  of  a 
wave,  and  saves  the  friction  caused  by  the  water  having  to  over- 
come the  obstacles  in  the  bed  of  a stream,  at  one  time  dry  and  at 
another  time  a torrent,  washing  the  stones  and  soil  on  its  bed  into 
alternate  shoals  and  deep  cavities,  and  destroying  its  character 
as  an  efficiemt  water-carrier.  The  wrater  coming  into  a river 
having  a weir,  at  the  first  of  a flood,  instead  of  entering  a 
channel  almost  dry  and  rushing  along  it  in  a raging  torrent,, 
encounters  instead  a quiet  pool  of  water,  gently  raises  its  level, 
and  passes  on  in  a steady  gradually  deepening  stream,  moun- 
ting over  the  crest  of  the  weir,  if  it  be  a fixed  one,  or  otherwise 
passing  freely  through  its  openings. 
Where  water  is  thus  permanently  held  up,  the  drainage  of  the 
adjacent  country  can  be  provided  for  by  carrying  the  water, 
either  by  side  cuts  running  parallel  with  the  main  stream  or 
other  arterial  drains,  to  a point  below  the  pond,  and  thus 
obviating  any  objection  that  could  be  urged  against  this  system 
on  the  score  of  drainage. 
In  all  works  for  the  improvement  of  rivers,  water  should  be 
