What did Paul Revere do when he saw the British crossing the Charles River in Boston?
Paul Revere never saw the British crossing. But here is what he did.
Paul Revere was situated at a location on the shore of the Charles River to see if one lantern or two were placed in the steeple of the North Church. That was done as soon as the spy on the Boston side had ascertained for sure which direction the British were moving.
Revere saw two, so he and his partner Dawes, set out for Lexington to warn John Hancock and company of the British movement. His ride took him through Cambridge, Medford and Menotomy (Arlington) before he reached Lexington. Along the way he informed certain town officials of the British advance. They in turn sent out riders to other towns.
In fact, the British had split their troops sending the infantry via water and their artillery via land, down the "neck of Boston" to Dorchester and then through Watertown to Lexington.