Government scientists who want to find out how fast and far a chemical attack could move through a city will release colorless, harmless gases in subways, an office building and some of Manhattan's most crowded streets to see which way the wind blows them. Ultimately scientists hope they can produce a computerized model of air flow patterns that could help authorities decide where to evacuate people and in which direction after a bioterrorist attack. ``You can use those models to say, `What if something happened here?''' said James Allwine, an engineer with the Pacific Northwest National Laboratory in Richland, Wash....