Correspondence; lecture notes; patents; proposals; reports; research notebooks and notes; unpublished articles; drafts of proposed publications; a diary. This collection provides a record of Clapp's wide ranging career. While employed at the MIT Radiation Laboratory (1942-1946) Clapp worked on microwave scanning linear ray antennas, and made pioneering contributions to the theoretical and experimental understanding of radar reflections from the terrain. The material reflecting this work comprises correspondence, patents (describing microwave antenna inventions), and a report. The lecture notes are from lectures given at Harvard and MIT by Percy Williams Bridgman (thermodynamics), David H. Frisch (scattering elementary particles), Bruno Rossi (cosmic ray phenomena), Julian Schwinger (nuclear physics, wave fields, and quantum electrodynamics), and Victor Frederick Weisskopf (nuclear reactors). Notebooks on gravitation, proposals, peer reviews, correspondence, and unpublished articles extensively document Clapp's research and theories. Topics covered in the collection include gravitation, elementary particle theory, biophysics, cell wall molecular operation, nerve cell electrical communication, mobius theory of electron excitation, non-impact printing-electrophoresis, electron orbits, philosophy, and plant growth. Correspondents include: John A. Behnke (as editor of BioScience), Albert Einstein, Richard P. Feynman, David Finkelstein (as editor of International Journal of Theoretical Physics), Thomas Kuhn, Elihu Lubkin, and John A. Wheeler.