text:  The equivalent strengths of the different armor plates was as follows : 15 in ( 381 mm ) of wrought iron was equivalent to 12 in ( 305 mm ) of either plain steel or compound iron and steel armor , and to 7 @.@ 75 in ( 197 mm ) of Harvey armor or 5 @.@ 75 in ( 146 mm ) of <unk> armor . 
  