Velocities  of  Cannon  Balls , See.  79 
of  thefe  three  caufes,  as  that  of  the  fecond  will  amount 
to  very  little ; becaufe  that  the  effeCt  arifing^  from  the 
greater  velocity  with  which  the  fluid  efcapes  at  the  vent 
and  windage,  is  partly  balanced  by  the  fhorter  time  in 
which  it  a<£ts. 
From  the  above  reflections  we  may  alfo  perceive,  how 
fmall  the  quantity  of  powder  is  which  is  blown  out  un- 
fired in  any  of  thefe  cafes,  and  the  amazing  quicknefs 
with  which  it  fires  in  all  cafes : for  although  the  time  in 
which  the  ball  pafled  through  the  barrel,  when  impelled 
by  the  eight  ounces  of  powder,  was  not  greatly  different 
from  the  half  only  of  the  time  in  which  it  was  impelled 
by  the  two  ounces,  it  is  evident  that  in  half  the  time 
there  was  nearly  four  times  the  quantity  of  powder  fired. 
The  fifth  or  laft  courfe  was  on  the  2 1 ft  of  September, 
1775,  fine  clear  weather,  but  a little  windy. 
The  machinery  and  the  balls  were  of  iron,  but  pow- 
der the  fame  as  in  the  laft  courfe,  and  the  dimenfions  as 
follows : 
p = 553  pounds,  the  weight  of  the  pendulum. 
r — 1 o 1 inches,  its  length. 
g—  7 8|  inches,  the  diftance  of  the  center  of  gravity. 
b — 84.775  inches  = 7.065  feet,  that  of  the  center  of 
ofcillation,  the  pendulum  making  68  vibrations  in 
100  feconds. 
Number 
