To analyze the author's purpose in the poem "Paul Revere's Ride," we can complete the statement as follows:
The author's purpose in comparing each "mast and spar" to a "ship", is to illustrate the consequences of losing the war to the British.
(Note: In this case, the completion is a hypothetical example, as the specific excerpt from the poem is not provided in your request. Please ensure that the phrase used aligns correctly with the text from the poem.)