▁was ▁in ▁action ▁night ▁after ▁night ▁from ▁ 7 ▁October ▁ 1 9 4 0 ▁to ▁the ▁end ▁of ▁May ▁ 1 9 4 1 ▁as ▁the ▁bom ber ▁streams ▁approached ▁the ▁London ▁Inner ▁Art illery ▁Zone ▁( IA Z ). ▁British ▁night ▁air ▁def ences ▁were ▁in ▁a ▁poor ▁state . ▁F ew ▁guns ▁had ▁fire - control ▁systems , ▁and ▁the ▁under power ed ▁search light s ▁were ▁usually ▁in effect ive ▁against ▁aircraft ▁at ▁alt itudes ▁above ▁. ▁In ▁July ▁ 1 9 4 0 , ▁only ▁ 1 , 2 0 0 ▁heavy ▁and ▁ 5 4 9 ▁light ▁guns ▁were ▁deployed ▁in ▁the ▁whole ▁of ▁Britain . ▁Of ▁the ▁" he av ies ", ▁some ▁ 2 0 0 ▁were ▁of ▁the ▁ob sol es cent ▁ ▁type ; ▁the ▁remainder ▁were ▁the ▁effective ▁ ▁and ▁ ▁guns , ▁with ▁a ▁theoretical ▁" ce iling " ' ▁of ▁over ▁ ▁but ▁a ▁practical ▁limit ▁of ▁ ▁because ▁the ▁predict or ▁in ▁use ▁could ▁not ▁accept ▁greater ▁height s . ▁The ▁light ▁guns , ▁about ▁half ▁of ▁which ▁were ▁of ▁the ▁excellent ▁B of ors ▁ 4 0 ▁mm , ▁de alt ▁with ▁aircraft ▁only ▁up ▁to ▁. ▁Although ▁the ▁use ▁of ▁the ▁guns ▁improved ▁civil ian ▁mor ale , ▁with ▁the ▁knowledge ▁the ▁German ▁bom ber ▁cre ws ▁were ▁facing ▁the ▁bar rage , ▁it ▁is ▁now ▁believed ▁that ▁the ▁anti - air craft ▁guns ▁achieved ▁little ▁and ▁in ▁fact ▁the ▁falling ▁shell ▁fragments ▁caused ▁more ▁British ▁cas