5S0  1  DE.  EOBEET  SIEGEE  ON  THE  [Nov.  1 893, 
time  of  Earth  and  Vogel’s,  and  again  of  Rohlfs  and  Nachtigal’s 
visit ;  to  the  variations  of  humidity  in  Southern  Africa  mentioned  by 
Prof.  Fritz  ;  to  the  high  level  of  Lake  Nyassa  about  1875,  and  the 
subsidence  which  followed;  to  the  sinking  of  the  Victoria,  Nyanza, 
inferred  from  Capt.  Speke’s  narrative ;  to  the  rise  of  the  same  lake 
reported  by  Mr.  Wilson  in  1877  and  1878  ;  and  to  the  considerable 
decrease  of  all  the  lakes  of  the  Upper  Nile  region  observed  from 
about  1880  until  1892.  Moreover,  I  would  remind  the  Society  of 
the  observations  on  the  Lower  Nile,  dealt  with  by  Profs.  Fritz  and 
Eriickner  in  their  respective  papers.  Finally,  I  would  especially 
refer  to  so  careful  an  observer  as  Dr.  Franz  Stuhlmann,  who  ex¬ 
plains  the  observations  and  traditions  concerning  changes  of  level 
in  the  Victoria  Nyanza  by  a  44  secular  variation  of  rainfall.” 
The  time  when  Lake  Tanganyika  was  at  its  highest,  and  the 
Lukuga  reopened  its  channel,  being  nearly  coincident  with  the 
maximum  level  in  Lakes  Tsad,  Nyassa,  Albert  and  Victoria  Nyanza, 
etc.,  and  also  with  the"  high  waters  so  frequent  between  1870  and 
1880  in  European  lakes  and  rivers,  even  in  the  Ealtic  Sea,  I  have 
had  no  hesitation  in  regarding  the  oscillations  of  Lake  Tanga¬ 
nyika  as  also  resulting  chiefly  from  the  climatic  changes  enume¬ 
rated  by  Prof.  Bruckner. 
This  rise  and  fall  of  the  lakes,  due  to  meteorological  influences,  is, 
I  believe,  the  main  cause  of  the  changes  in  the  Lukuga ;  these, 
however,  in  their  turn  must  influence  and  modify  the  amplitude  and 
the  particular  mode  of  oscillation  of  level. 
If  the  climatic  conditions  and  the  lake-level  be  constant  or 
‘  normal,’  I  am  unable  to  account  for  agencies  sufficient  for  damming 
up  the  whole  efflux  of  the  lake.  The  stopping-up  has,  indeed,  been 
referred  to  a  periodic  upheaval  of  the  land,  to  the  action  of  earth¬ 
quakes  and  volcanic  forces  (as  visible  in  Lakes  Rudolf  and  Stefanie), 
to  the  famous  grass-bars  or  sindi  and  the  formation  of  a  bar  at  the 
river-entrance.  These  points,  except  the  last  two,  being  discussed 
in  my  papers,  and  Mr.  Carson  insisting  especially  on  the  blocking-up 
of  the  river  by  vegetation  and  its  silting-up  by  sedimentation,  I  do 
not  propose  to  speak  here  of  the  movements  of  the  earth’s  crust,  of 
Mr.  Stanley’s  4  cataclysmal’ theories,  of  Capt.  Storms’s  hypothesis 
attributing  the  riso  of  Lake  Tanganyika  to  an  outburst  of  Lake 
Likwa,  etc.,  but  only  of  the  two  points  referred  to.  Mr.  Carson 
has  himself  seen  the  effects  of  the  sindi  on  the  River  Shire,  and  he 
regards  them  as  inadequate  to  produce,  without  the  help  of  other 
agencies,  a  rise  of  the  lake  to  a  height  of  30  feet.  I  would  add  that 
at  the  very  time  when  these  masses  of  floating  vegetation  blocked 
the  Shire,  Lake  Nyassa  continued  to  sink.  Even  the  4  setts  ’  of 
the  Upper  Nile  (in  connexion  with  which  the  reader  will  doubtless 
remember  the  disaster  that  overtook  Gessi  Pasha  and  his  com¬ 
panions)  had  not  force  enough  to  dam  up  the  Lakes.  The  observa¬ 
tions  show  that  the  maxima  of  level  on  the  Victoria  and  Albert 
Nyanzas  were  nearly  simultaneous  with  the  maxima  on  the  Lower 
Nile.1  Now,  if  the  lake  was  rising,  the  water,  by  its  pressure,  would 
1  See  also  Lugard,  Proc.  Roy.  Geogr.  Soc.  1892,  p.  827. 
