WATER  CISTERNS  IN  VENICE. 
79 
WATER  CISTERNS  IN  VENICE. 
The  French  Academy  of  Sciences  has  received  a  communica- 
tion from  M.  G.  Grimaud,  on  the  manner  in  which  the  Vene- 
tians construct  their  cisterns,  a  plan  which  he  thinks  might  be 
advantageously  introduced  on  the  heights  which  overlook  Paris, 
and  are  occupied  by  large  establishments  and  a  numerous  popu- 
lation, and  which  would  greatly  benefit  by  them.    Venice  occu- 
pies a  surface  of  5,200,000  square  metres  (1300  acres),  exclu- 
sive of  all  the  great  and  small  canals  which  intersect  it.  The 
annual  average  of  rain  is  31  inches,  the  greater  part  of  which 
is  collected  in  2077  cisterns,  177  of  which  are  public.    The  rain 
is  sufficiently  abundant  to  fill  the  cisterns  five  times  in  the  course 
of  the  year,  so  that  the  distribution  of  water  is  at  the  rate  of 
16  litres  (3j  gallons)  per  head.     To  construct  a  cistern  after 
the  Venetion  fashion,  a  large  hole  is  dug  in  the  ground  to  the 
depth  of  about  9  feet,  the  infiltration  of  the  lagoons  preventing 
their  going  any  deeper.    The  sides  of  the  excavation  are  sup- 
ported by  a  frame-work  made  of  good  oak  timber,  and  the  cis- 
tern thus  has  the  appearance  of  a  square  truncated  pyramid 
with  the  wider  base  turned  upwards.    A  coating  of  pure  and 
compact  clay,  1  foot  thick,  is  now  applied  on  the  wooden  frame 
with  great  care ;  this  opposes  an  invincible  obstacle  to  the 
progress  of  the  roots  of  any  plants  growing  in  the  vicinity,  and 
also  to  the  pressure  of  the  water  in  contact  with  it.    No  crevices 
are  left  which  might  allow  the  air  to  penetrate.    This  prelimi- 
nary work  being  done,  a  large  circular  stone,  partly  hollowed 
out  like  the  bottom  of  a  kettle,  is  deposited  in  the  pyramid  with 
the  cavity  upwards ;  and  on  the  foundation  a  cylinder  of  well 
baked  bricks  is  constructed,  having  no  interstices  whatever,  ex- 
cept a  number  of  conical  holes  in  the  bottom  row.    The  large 
vacant  space  remaining  between  the  sides  of  the  pyramid  and 
cylinder  is  filled  with  well  scoured  sea  sand.    At  the  four  cor- 
ners of  the  pyramid,  they  place  a  kind  of  stone  trough  covered 
with  a  stone  lid  pierced  with  holes.    These  troughs  communicate 
with  each  other  by  means  of  a  small  rill,  made  of  bricks,  and 
resting  on  the  sand,  and  the  whole  is  then  paved  over.  The 
