236 THE HISTORY OF SCIENCE 



anemometer which may register a momentary calm 

 in the midst of a gale how great a diversity of 

 pressure must exist in a large extent of atmosphere. 

 This internal work of the wind will lift the soaring 

 bird at times to higher levels, from which without 

 special movement of the wings it may descend in 

 the very face of the wind's general course. 



From the beginning, however, of his experiments 

 Langley had sought to devise a successful flying 

 machine. In 1887 and the following years he con- 

 structed about forty rubber-driven models, all of 

 which were submitted to trial and modification. 

 From these tests he felt that he learned much about 

 the conditions of flight in free air which could not 

 be learned from the more definitely controlled tests 

 with simple planes on the whirling table. His essen- 

 tial object was, of course, to reduce the principles of 

 equilibrium to practice. Besides different forms and 

 sizes he tried various materials of construction, and 

 ultimately various means of propulsion. Before he 

 could test his larger steam-driven models, made for 

 the most part of steel and weighing about one thou- 

 sand times as much as the air displaced, Langley 

 spent many months contriving and constructing 

 suitable launching apparatus. The solution of the 

 problem of safe descent after flight he in a sense 

 postponed, conducting his experiments from a house- 

 boat on the Potomac, where the model might come 

 down without serious damage. 



It was on May 6, 1896 (the anniversary of which 

 date is now celebrated as Langley Day), that the 

 success was achieved which all who witnessed it con- 

 sidered decisive of the future of mechanical flight. 



