Melville’s note to “The Swamp Angel” reads:

The great Parrott gun, planted in the marshes of James Island, and employed in the prolonged, though at times intermitted bombardment of Charleston, was known among our soldiers as the Swamp Angel.

A poem with the same title by T. N. J. is printed in Volume VIII, P. 3, and is prefaced by this remark: “The large Parrott gun used in bombarding Charleston from the marshes of James Island is called the Swamp Angel.—Soldier’s Letter.”
The similar notes and the identical titles indicate that Melville’s poem was inspired by T. N. J.’s. The only similarities in phrasing occur in Melville’s references to “a scream that screams up to the zenith” and “their crumbling walls,” echoes of “scream unlike her tropical sisters” and “crumbling wall” in T. N. J.’s poem.