We figured that Lilienthal in five years of time had spent only about five hours in actual gliding through the air. The wonder was not that he had done so little, but that he had accomplished so much.

Speech to the Western Society of Engineers (18 September 1901); published in the Journal of the Western Society of Engineers (December 1901); republished with revisions by the author for the Annual Report of the Board of Regents of the Smithsonian Institution (1902)