Get out the microscope, because we’re going through this poem line-by-line.
Meanwhile, his friend, through alley and street,
Wanders and watches with eager ears,
Till in the silence around him he hears
The muster of men at the barrack door
- Meanwhile, back in Boston, Paul's friend (we never do get his name) has his ears open for news about the British army.
- Finally, the quiet is broken by the sound of the gathering (the "muster") of soldiers in their camps. That lets him know that something is up, but not exactly what the British are planning to do.
The sound of arms, and the tramp of feet,
And the measured tread of the grenadiers,
Marching down to their boats on the shore.
- Now Paul's friend gets the info he needs: he hears the British soldiers gathering their weapons and marching toward the water. We know what that means: two lanterns!
- When Longfellow mentions "grenadiers," he's just talking about a particularly tough and scary kind of British soldier.