PRINCIPLES OE GUNNERY. 
451 
The mean resistance, B, is the total force acting on the projectile in 
libs.; and knowing.^, the resistance in lbs. can be calculated either the resist- 
per inch of circumference or per square inch of sectional area. For projectile. 6 
instance : suppose R, is the resistance per square inch of projectile's 
sectional area, and R n the resistance per inch of projectile's circum¬ 
ference, then 
R = Rfird^ = R n 7cd , 
and R n — \dR t ; 
where d is the diameter of the projectile in inches. 
So that if the energy per square inch of projectile's sectional area 
be required, so as just to perforate a resisting medium, we have 
W 
.( 6 ) 
or if the energy per inch of projectile's circumference be required, 
Wvf 
*9 
(7) 
The question then arises, which method most nearly agrees with the 
results of experiment ? For thin plates, which can be easily per¬ 
forated, experiment seems to show that the projectile acts as a punch, 
cracking the plate circumferentially , as in Fig. 2, at the same time 
slightly bulging the plate, which commences to crack on the inside 
during perforation, as in Fig. 1. In this case the method of esti¬ 
mating the resistance per inch of projectile's circumference gives the 
Fig. l. 
Fig. 2. 
