Within the Enemy's Lines by Oliver Optic

As Captain Carboneer blew his whistle, a mile below the moorings of the Bellevite, an occasional response came from the shore. Everything was remarkably quiet on the river, though at long intervals a steamer passed on its way up or down the stream. The signals made by the naval officer were not loud, and the replies, made without the aid of any instrument, were quite feeble. One might have taken them for some frolic on the part of the boys.

"I don't quite understand this business," said Major Pierson, after he had listened a while to the signals. "I suppose from the answers you get, that your men are all along the river, and the woods seem to be full of them."