Why does Longfellow compare each part of the British ship to a prison bar?

Longfellow compared the part of British ship to a prison bar because he wants to show that ship is very dangerous.

Explanation:

In 'Paul Revere's Ride' by 'Henry Wadsworth Longfellow', author wants to state that even one action can make a difference. He glorifies Paul Revere's action and how it determined nation's destiny. In the lines where he says that phantom British ship is arriving which can be seen across the moon. He is alerting the colonists that people should be ready and try to win their independence from the British. The gigantic British ship is like a prison bar.

Longfellow says that if people stay united they can win from the British who are evil.