Answer:
i was going to say helmet but since it's not there then reflector or horn
Explanation:
The metaphor in this sentence likens the exchange of ideas (and specifically the arrival of new, radical ideas) as a battle.
The "radical thought" coming from the northeast is defended, not by believers or by thinkers, but by "shots" from newspapers and is "reinforced by regiments." This metaphor makes the arrival of new ideas a dangerous, violent -- possibly even bloody -- situation.
