The ship, the HMS Somerset, described in the lines twenty to twenty-three of Henry Wadsworth Longfellow's "Paul Revere's Ride" refers to the embellished power of the British Empire, that is no longer a protection for the Americans. Longfellow calls the ship "phantom." By dictionary definition "phantom" means something that is apparent to sense but has no substantial existence. Longfellow implies that this powerful Empire, the ship, is no longer a protector and a friend to Americans; it is now the enemy. It doesn't exist as an ally for America although it appears to be. He also calls that the ship reminds a "prison bar." In other words, the British Empire is now acting as a jailer, trying to repress Americans. Then, he describes it as a big "black hulk," a heavy old ship, which is "magnified"ÃÂ¦ By its own reflection in the tide." He is putting forward that the Britain's power, which has very often been represented by their navy, in this poem by a ship, is overestimated.
In other words, he wants to say that what appears to be huge, may not be so. Its "own reflections," the Empire's own extravagant insinuations of its power reverberated by its numerous invasions and glorious victories in many parts of the world is glorified by many. However, this old Empire, the "hulk," doesn't really have the mythical power the way it's perceived by many.