Very early in his investigations Langley thought he received through watching the anemometer a clue to the mystery of flight. Observations, begun at Pittsburgh in 1887 and continued at Washington in 1893, convinced him that the course of the wind is "a series of complex and little-known phenomena," and that a wind to which we may assign a mean velocity of twenty or thirty miles an hour, even disregarding the question of strata and currents, is far from being a mere mass movement, and consists of pulsations varying both in rate and direction from second to second. If this complexity is revealed by the stationary 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 constructed 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 essential 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 thousand 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. [Illustration: THE FIRST SUCCESSFUL HEAVIER-THAN-AIR FLYING MACHINE A photograph taken at the moment of launching Langley's aerodrome May 6, 1896]