from  Lightning  at  Purfleet. 
■exp.  xi.  The  effect,  however,  was  different,  when  a 
pointed  conductor  of  the  fame  length  as  in  the  fourth 
experiment  Was  made  ufe  of;  for  then  the  fhort  fpout 
was  charged  nearly  in  the  fame  manner  as  in  the  firft 
experiment,  and  the  ffream  of  light  at  the  top  of  the 
bent  fpout  difappeared  the  inftant  a communication  was 
made  from  the  fhort  fpout  to  the  earth. 
fourth  observation.  The  two  pofts  of  wood  upon 
the  ground,  which  fupported  the  ciftern  at  the  bottom  of 
the  fliort  fpout,  were  therefore  the  true  caufe  of  thefe 
effects  taking  place  in  the  fliort  fpout,  by  preventing  a 
communication  with  the  earth,  and  hindering  the  fluid, 
that  was  conftantly  charging  the  fliort  fpout,  from  difl- 
charging  itfelf  properly  into  the  earth. 
It  may  now  be  proper  to  take  notice  of  other  expert 
ments,  and  alfo  of  two  other  circumftances  that  leem  to 
be  of  confiderable  confequence  in  this  inquiry. 
One  of  there  circumftances  refpedts  the  motion  of  the 
model,  inftead  of  that  of  the  cloud  ; and  the  other,  the 
quantity  of  the  fluid  contained  in  the  great  cylinder 
when  properly  charged. 
As  to  the  former  of  thefe,  it  appears  from  obferva- 
tions,  that  clouds,  in  a very  high  ftorm,  frequently  move 
at  the  rate  of  eighty  miles  in  an  hour;  and,  with  a mo- 
derate wind,  about  twenty  miles^.  Now  when  the  clouds 
(a)  In  the  Phil.  Tranf.  there  is  an  account  of  a ftorm  that  was  computed  to 
move  at  the  rate  of  three  miles  in  a minute. 
Vol.  LXVIII.  L 1 
move 
