NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the committee responsible for the report were chosen for their special competence and with regard for appropriate balance.

This report has been reviewed by a group other than the authors according to procedures approved by a Report Review Committee consisting of members of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine.

Support for this project was provided by the U.S. Office of Naval Research Contract No. N00014-92-J-1560/R.

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Cover art by Ellen Hill-Godfrey. Ms. Hill-Godfrey received her Master of Fine Arts degree from the University of North Carolina-Chapel Hill. Her paintings and prints have been exhibited in the Washington, DC, area and throughout the Mid-Atlantic and Southern regions. She has done illustrations for the University of Georgia Press and the University of North Carolina's Endeavors magazine. She lives in Germantown, MD, and teaches at The Barnesville School.