And they had breastplates, as it were breastplates of iron; and the sound of their wings was as the sound of chariots of many horses running to battle. They had breastplates - of iron - They seemed to be invulnerable, for no force availed against them.
The sound of their wings - Their hanging weapons and military trappings, with the clang of their shields and swords when they make their fierce onsets. This simile is borrowed from Joel 2:5-7.