Final answer:
The soldier, a Federal scout, gave Farquhar detailed information about the bridge as part of a trap he was setting. Farquhar, eager to serve the Confederacy, took the bait, which led to his own doom.
Step-by-step explanation:
The soldier who visited Farquhar gave him such detailed information about the bridge to set a trap for Farquhar. The soldier, who was actually a Federal scout, purposefully provided this information knowing that Farquhar, a civilian who was ardently devoted to the Southern cause, might be tempted to act upon it. This conversation was a calculated move to exploit Farquhar's eagerness to aid the Confederacy by giving him just enough information to spark an ill-fated plan.
As we later find out, Peyton Farquhar's longing to have an impact on the war led him to fall into the trap that the scout laid. The discussion of the bridge's strategic importance and the driftwood that could easily catch fire served as a bait. Farquhar's subsequent actions, motivated by the dishonestly obtained information, led to his capture and execution by the very forces he hoped to undermine.