from  Lightning  at  Purfleet.  &6i 
In  obferving  this  laft  rule,  proper  allowance  was  made 
for  the  different  ffates  of  the  air,  the  ftate  of  the  glafs 
/ 
itfelf,  and  that  of  the  cufhion  or  rubber,  which  excited 
the  fluid  by  a proper  preflure  upon  the  glafs,  joined  to 
other  circumftances.  The  molt  material  of  thefe  circum- 
ftances  confifted  in  the  cufhion’s  having  a free  communi- 
cation with  the  earth  and  glafs  cylinder,  in  that  part  where 
the  fridtion  was  applied ; and  alfo  a fimilar  communication 
between  the  great  cylinder  and  the  oppofite  fide  of  the 
glafs.  Now,  becaufe  three  pointed  wires  conducted  the 
fluid  readily  from  the  glafs  to  the  great  cylinder,  I ap- 
prehended it  was  proper  to  have  three  fuch  points  within 
the  cufhion  itfelf  to  condudt  the  fluid  as  readily  from  the 
earth  to  the  glafs.  For  thofe  points  communicated  by  a 
wire  with  the  well. 
It  will  appear,  that  the  effedts  produced  by  this  artifi- 
cial method  of  charging  the  cylinder  muft  be  very  dif- 
ferent from  thofe  produced  in  nature. 
Suppofe  a thunder-cloud  coming  already  charged,  the 
lightning  from  this  cloud  ftrikes  at  Purfleet ; the  fame 
cloud,  palling  afterwards  over  other  places,  will  ftrike 
the  earth  again  and  again,  without  any  apparent  dimi- 
nution of  the  quantity  of  lightning  contained  therein,  as 
hath  been  frequently  obferved.  But  the  cafe  is  not  the 
fame  with  the  great  artificial  apparatus,  becaufe  we  not 
L 1 2,  only 
