Based on the provided context from the poem "Paul Revere's Ride" by Henry Wadsworth Longfellow, the author's purpose in comparing each "mast and spar" to a "HUGE black hulk" is to show the need for strength and power when fighting the British.
The phrase "Prison-bar" emphasizes the gravity and seriousness of the conflict and loss faced by the colonists, which further illustrates the persistent threat posed by the British.
To summarize:
The author's purpose in comparing each "mast and spar" to a "HUGE black hulk" is to show the need for strength and power when fighting the British.
The phrase "Prison-bar" would then likely relate to the consequences of losing the war, but based on the initial sentence, it does not fit as cleanly into the author's direct comparison.