The fiery eyes were the lanterns in the Twin-Lights towers.

John Brown had been equal to the emergency, and the lamps were lighted.
Seth's anxiety was relieved, but that one glimpse made him even more eager for home.

He rowed on for a short time, and then began edging in toward the invisible left-hand shore.

Judging by the length of time he had been rowing, he must be close to the mouth of the Slough, where, winding through the salt marshes, it emerged into the Back Harbor.
He crept in nearer and nearer, but no shore came in sight.

The fog was now so thick that he could see not more than ten feet from the boat, but if he was in the mouth of the Slough he should have grounded on the marsh bank long before.