Also,  as  in  the  case  of  water  waves,  all  waves  have  a  definite
wave  length  X,  which  is  the  distance  between  successive  crests  or
successive  hollows.  If  we  use  the  term  phase  to  mean  the  position
at  any  time  of  a  point  on  the  wave  outline,  we  can  say  that  in  general
the  wave  length  is  the  distance  between  two  successive  points  in
the  same  phase.
Furthermore,  waves  of  all  kinds  travel  with  a  definite  velocity.
If  one  looks  at  a  fixed  point  on  the  surface  of  a  pool  over  which
ripples  are  moving,  he  can  see  that  a  crest  appears  at  that  point  a

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224  RADIO  COMMUNICATION.
definite  number  of  times  in  any  given  interval  of  time.  The  number
so  appearing  in  one  second  is  called  the  frequency,  /.  If  now  the
frequency  is  multiplied  by  the  length  of  one  wave,  we  get  the
distance  moved  by  the  waves  in  one  second,  which  is  the  velocity,  c,
or,  in  symbols,  c=X/.  The  velocity  of  waves  of  different  kinds  is
very  different  in  amount.  Thus  ripples  on  water  travel  with
velocities  of  from  10  to  100  cm.  per  second;  for  sound  waves  in  air
the  velocity  is  330  meters  per  second ;  and  the  velocity  of  light  and
electric  waves  is  300,000,000  meters  per  second.1

The  more  the  surface  of  the  water  is  displaced  by  the  waves  from
its  position  of  rest,  the  greater  is  the  amount  of  energy  being  passed
along.  The  amount  of  energy  transmitted  by  water  waves  is  con-
nected with  the  height  of  the  crests  and  the  depth  of  the  troughs.
