1. University of The Ryukyus, Nishihara, Okinawa 903-1, Japan
2. Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560, Japan
3. Tohoku University, Aramaki, Sendai, Miyagi 980, Japan
4. Mechanical Engineering Research Laboratory, Hitachi Ltd., 502 Kandatsu, Tsuchiura, Ibaraki 300, Japan