1907J 
Current-Measurements. 
17 
The  course  of  the  tidal  current  is  very  well  seen  from  the 
vector-diagrams,  Plate  II.  In  the  open  sea  the  maxima  of  velocity 
appear  at  extreme  high  or  low  water.  In  the  upper  strata  at  Ling 
Bank  these  maxima  -were  met  with  on  the  7th  August  at  9 p.m. 
and  on  the  8th  August  at  3.30  a.m.  The  first  maximum  coincided 
with  a direction  from  S.,  while  the  second  maximum  was  fouud  in 
a current  from  N.  Now  the  tidal  wave  at  this  locality  is  procee- 
ding  from  N.  to  S.  We  may  then  conclude,  that  ebb-tide  occurred 
on  Aug.  7 at  9 p.m.,  and  high-water  on  Aug.  8 at  3.30  a.m.  The 
meridian  passage  of  the  moon  (lower)  took  place  at  3.28  p.m.,  on 
the  7th  August.  Thus  the  establishment  of  this  place  was  12  hours, 
and  the  co-tidal  line  of  12  passed  over  the  Ling  Bank.  The  same 
co-tidal  line  is  found  at  Kinnaird  Head,  S.  of  the  Moray  Firth,  Scot- 
land,  and  at  Lindesnæs  in  Southern  Norway.^)  The  true  course  of 
the  co-tidal  lines  has  not  been  previously  known  as  far  as  the  open 
sea  is  coneerned,  but  we  may  find  them  from  an  analysis  as  shown 
here.  Our  observatious  seem  to  indicate  that  the  co-tidal  line  of 
1 2 follows  the  bottom-line  o f 50  fathoms,  and  bends  towards  N. 
in  the  central  part  of  the  North  Sea.  It  thus  would  appear  to  follow 
a track  quite  in  accordance  with  the  wave-theory.  It  will  be  most 
important  for  our  studies  of  the  tides  to  have  the  cotidal  lines 
traced  all  over  the  North  Sea,  and  the  measurements  referred  to 
here  seem  to  prove  that  this  may  be  done  in  a trustworthy  mauner 
by  observatious  of  the  currents. 
The  velocity-curves  for  50  and  75  meters  show  maximum  and 
minimum  an  hour  earlier  than  the  curves  for  the  upper  layers. 
This  relation  between  the  upper  aud  bottom  layers  in  all  probability 
indicates  the  phenomenon,  which  is  known  on  the  coast,  that  the 
tidal  flow  ceases  earlier  uear  the  land  than  in  the  open  waters  owing 
to  the  turn  of  the  current  taking  place  on  the  shore  itself  at  extreme 
high  and  low  water  and  not  at  midwater.  It  is  however  not  impos- 
sible, that  a different  tidal  impulse  is  to  be  found  in  the  bottom 
layer  from  the  impulse  at  the  surface.  It  is  worth  noticing  that  the 
Principal  axis  of  the  ellipses  represented  on  Plate  II  has  not  exactly 
the  same  course  for  50  and  75  meters  (NNE  to  SSW)  which 
it  has  for  the  upper]  strata  (N  to  S).  This  may  perhaps  be  due 
to  a tidal  wave,  reflected  from  the  Western  Coast  of  Norway  and 
proceeding  into  the  North  Sea  in  the  deeper  strata.  The  bottom- 
Cf.  BoGUSLAWSKY-KRttiiMEL : Hanclbucli  der  Oceanographie,  Vol.  II,  p.  189. 
42 
