FIG. 1. Pattern of the panel that forms the back of the array of contiguous settlement chambers in the experiment. Cross-sectional pro®les of the 24 3-cm highÖ3-cm wideÖ15.5-cm long chambers are seen. Chambers on the left were dark and those on the right were lighted. Within each of the three levels (high, middle, low), each chamber in the lighted side had a unique ¯ow velocity through it determined by the size of the downstream opening (symbolized by the size of the circle within each square). Four elongate bounding surfaces de®ned each elongate chamber: a downward-facing ceiling, two lateral walls and an upward-facing ¯oor. Distance within the chambers is not portrayed in the ®gure, but each of the four elongate walls of chambers was examined and scored as three equal segments: the portion closest to the entrance (upstream end), the middle portion and the portion closest to the back panel (downstream end).