in Thoreau’s Journal:
How nicely is nature adjusted. The least disturbance of her equilibrium is betrayed and corrects itself. As I looked down on the surface of the brook, I was surprised to see a leaf floating, as I thought, up stream, but I was mistaken.
The motion of a particle of dust on the surface of any brook far inland shows which way the earth declines toward the sea, which way lies the constantly descending route, and the only one.