Pictogram demonstrating how the device functions
Device consisted of a clear tube half-filled with styrofoam balls.  A pointer was attached to the top-most ball.  The tube was held vertically in a water bottle that was used to collect rain water.  As the collector would fill with water the balls would float up and the pointer would trace the green copper stain on the facade of the building
 

Photograph of the device in front of the
MacDonald Engineering Building, McGill University