It  is  evident  that  the  influence  of  viscosity  on  large  waves  is  small,      1
since  they  persist  for  a  long  time  after  the  wind  ceases  and  run
for  long  distances  with  little  change;  and  yet  viscosity  has  an  appre-      I
ciable  effect,  since  the  waves  finally  come  to  rest*     The  length  is  mote
persistent  than  the  height;    indeed,  it  is  not  easy  to  see  how  the
length  is  affected  by  viscosity*  ^|

Observations  are  reported  by  Lieut,  A.   Piris*  on  the  same      •
system  of  waves  at  two  places  350  miles  apart,  as  follows:


First  Place.  Second  Place.
Speed,  meters  per  second 15,3  15

Length     *' ^..I43  I45

Height      **       ...... - 4.5  2,25


278


NAyAL  ARCHITECTURE,


The  second  observation  gives  a  very  good  agreement  with  the
theory  when  the  speed  is  computed  by  equation  (46),  page  275;  the
first  shows  a  greater  speed  than  is  indicated  by  that  equation;  in
both  cases  the  height  is  too  small  to  introduce  any  appreciable
difference  between  the  results  by  equations  (45)  and  (46).
Waves  have  been  observed  with  periods  of  22  to  24  seconds
and  lengths  of  2500  to  3000  feet.  The  greatest  measured  height  of
waves  is  45  feet;  waves  40  feet  high  are  rare,  and  30  feet  is  an
uncommon  height.  The  ratio  of  height  to  length  tends  to  diminish
as  the  length  increases.  The  common  value  of  the  ratio  of  height
to  length  is  from  0.05  to  o.i;  very  long  waves  have  about  0.02  for
that  ratio.
The  following  table  gives  the  dimensions  and  other  interesting
information  concerning  waves  subject  to  regular  winds  in  several
well-known  seas,  quoted  from  the  memoir  by  Lieut.  P4ris  already
mentioned:


PROPERTIES  OF   WAVES  IN   VARIOUS  SEAS.
Region.
"u.Sa
o»c  0

1
1^

Speed,  C.
