594 
THE DEVELOPMENT OF ARMOUR. 
In®estimating probable effect, not only the perforation due to the 
energy of the projectile must be considered, but also the structural 
strength of the projectile to stand up to its work. With shells this is 
principal consideration. It generally becomes a very simple matter, 
because if the energy is sufficient to break up the shell at all ranges 
the perforation becomes a constant, the shell's strength always being 
the limit of its power. The following may serve as a very rough guide 
to replace the rules given in 1893. 
Rough 
rules for 
structural 
strength of 
projectiles. 
Common cast iron shell may perforate % calibre of wrought iron. 
Common steel shells with nose fuzes may perforate from i to f calibre of steel 
(unhardened). 
Common steel shells pointed, may perforate slightly more than the above. 
A.P. steel and Palliser shells may perforate over 1 calibre of ordinary steel. 
( may perforate from 2 to 3 calibres of wrought iron* 
Steel shot < „ „ li to 2 calibres of ordinary steel. 
„ „ 1 calibre of Krupp or Harveved steel. 
It is specially important for the attack of ship 3 s Q.F. batteries to know 
that a 6-in. common shell will perforate 4 inches of ordinary steel up to 
the glancing angle , that is about 45°. 
One point deserves special notice with regard to common shell 
attack, namely that a hard face which defeats the boring attack of 
sharp pointed armour piercers by fracturing the point is probably of 
little or of no use in resisting the punching attack of common shell 
which drive out a dise of metal. Consequently if thin hardened faced 
plates are substituted for thicker steel plates of an ordinary kind, it 
may follow that while resistance to armour piercing shot has been 
maintained or increased, the armour is less able to resist such guns 
as are sufficiently powerful to attack with common shell. This is 
important with regard to the lighter armour on the upper structure of 
ships. Consequently, the increased toughness of Krupp process 
armour has a special value. On this question, however, experiments 
are needed to enable any definite statement to be made. 
