1. C. A. Brock, J. C. Wilson, H. H. Jonsson, Department of Engineering, University of Denver, Denver, CO 80208, USA.
2. P. Hamill, Department of Physics, California State University, San Jose, CA 95192, USA.
3. K. R. Chan, Ames Research Center, National Aeronautics and Space Administration, Moffett Field, CA 94035, USA.