1. Mechanical Engineering Imperial College, London SW7 2BX, U.K., Geology Department, Southern Illinois University, Carbondale, Illinois 62901, Geology Department, Royal Holloway, University of London, Egham Hill, Egham, Surrey TW20 0EX, U.K., National Power PLC, 8 Marylebone Passage, London W1N 7HE, U.K., and Department of Materials, Imperial College, London SW7 2BX, U.K.