cup  and  turns  right  and  left  in-
side of  the  cup.  The  change  of  direction  transfers  the
energy  to  the  wheel.
76.  Wasted  Water  Power. — Very  few  people  realize  the
vast  amount  of  water  energy  that  goes  to  waste  every  year.
Every  particle  of  falling  or  running  water  represents  energy,
the  amount  of  which  depends  upon  the  quantity  and  the
depth  of  the  fall.     This  energy  can  be  harnessed  in  the  same
way  as  the  energy  obtained  from  the  coal  we  burn.     Water
that  possesses  energy,   that  is,   water  that  falls,  is  often
spoken  of  as  " white  coal."     Power  plants  that  transform
the  energy  of  falling  water  into  electrical  energy,  oftentimes


MECHANICS  OF  LIQUIDS


77


are  the  centers  of  distributing  systems  that  cover  hundreds
of  miles  of  territory,  and  give  electrical  service  to  many
towns.
77.  Measurement  of  Flowing  Water. —Oftentimes,  as  when
water  is  sold  to  a  corporation  or  city,  it  is  necessary  to  know  the
quantity  of  water  coming
down  a  stream.  To  mea-
sure this  a  device  called  a
weir  (Fig.  49)  is  constructed
at  the  sides  of  the  stream  so
as  to  form  either  a  rec-
tangular or  angular  opening
through  which  the  water
flows.  Where  a  large  quanti-
ty of  water  is  measured  the
opening  is  usually  shaped  in
this  way  I ] ;  where


FIG.
49. — Measuring     Water
Weir.
with


the  quantity    is    small  the

opening  is  V-shaped,  as  the  flow  of  water  may  then  be  measured

with  greater  accuracy.
