from  Lightning  at  Purfleet.  273 
trouble  to  adjuft  this  diftance  between  the  two  fubftitutes. 
Now  the  diftance  between  them  will  depend  upon  the 
quantity  of  the  charge  given,  and  the  method  to  deter- 
mine that  diftance  may  be  found  by  removing  one  fub- 
ftitute  from  the  other  fo  far  as  not  to  caufe  any  previous 
or  partial  explofions  before  the  great  ftroke  happens; 
and  When  it  does  happen,  it  muft  not  only  ftrike  between 
the  fubftitutes,  but,  at  the  fame  inftant,  between  the  re- 
mote end  of  the  lefs  fubftitute  and  the  object  oppofed 
to  it. 
exp.  xviii.  To  each  end  of  a {lender  fubfiitute  made 
of  wood,  about  eleven  feet  in  length,  and  fomething  lefs 
than  one  inch  in  diameter,  was  fixed  a ball  of  the  fame 
matter.  The  larger  of  thefe  meafured  three  inches  in 
diameter,-  and  the  lefler  one  inch  nine-tenths.  The  exa£t 
meafure  of  thefe  balls  was  attended  to  the  more,  in  fome 
of  the  experiments,  becaufe  Mr.  nairne  has  given  a de- 
fcription  of  an  apparatus  in  the  Phil.  Tranf.  vol.  LXIV. 
part  1.  p.  87  and  88  . to  which  this  is  nearly  fimilar.  Hav- 
ing covered  the  whole  of  this  {lender  fubftitute. with  tin- 
foil,  it  was  then  fupported  near  the  center  by  a {lender 
frame  of  wood  upon  a pillar  of  glafs,  to  adjuft  it  to  the 
height  and  diftance  required.  The  larger  ball  was  then 
brought  within  one  inch  and  a quarter  (and  fometimes 
at  a greater  diftance)  of  another -ball,  one  inch  nine-  - 
tenths^ 
