In April of 1775, patriots learned that a British military action against Lexington was being planned using troops quartered in Boston. They identified two likely avenues for the British movement out of Boston: one by marching southwest ("by land") along the isthmus connecting it to the mainland, and one by taking boats northwest ("by sea") across the Charles River. By April 18, the patriots had learned that the British troop movement to Lexington was imminent, but did not yet know which way they would depart from Boston. Dr. Joseph Warren asked Paul Revere to ride to Lexington late that night to warn the patriots. Revere had arranged for a visual signal from the steeple of Christ Church (popularly known as Old North Church) to confirm the movement route. He paused for a moment on the far shore of the Charles River. Seeing the light from two lanterns, he wheeled and rode off into history.