"If the British march
By land or sea from the town to-night,
Hang a lantern aloft in the belfry arch
Of the North Church tower, as a signal light, –
One, if by land, and two, if by sea;
And I on the opposite shore will be,
Ready to ride and spread the alarm
Through every Middlesex village and farm,
For the country-folk to be up and to arm."
IT IS THE SIGNAL TO RIDE AND SPREAD THE WARNING.
A different perspective: