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 OEVILLE WRIGHT 573 They approached the subject and carried on their studies and experiments in a scientific way. Their discovery can well be called one of the most scientific of the world's achievementa. They worked out the formula with great care and precision. While they owe much to the enthusiasm of Moullard and Lilienthal, they obtained a good understanding of the prob- lem of flying from Chanute's Progress in Flying Machines ^ Langley's Experiments in Aerodynamics^ the Aeronautical Annals of 1905, 1906, and 1907, as well as several pamphlets published by the Smithsonian Institution. Gliding flights enlisted their attention at the first, chiefly because of the greater expense necessary for experimenting with power machines. At the very outset they met the great- est problem of aviation — that of equilibrium. They discov- ered that one who has not actually navigated the air cannot appreciate the difficulty. They learned how false is the com- mon belief that the atmosphere runs in comparatively regular currents, called winds. Whoever attempted a gliding flight under such a theory would find himself thrown about, rising or falling ten, twenty, or even thirty feet in a few seconds. The air along the surface of the earth is continually churn- ing. It is thrown upward from every irregularity, like sea breakers on a coast line; every hill and tree and building sends up a wave of slanting current. The currents move, not directly back and forth, but in whirling rotary masses rising in some instances to hundreds of yards. In a fairly strong wind the air near the earth is more strongly disturbed than the whirlpools of Niagara. Their opinion was that, to work intelligently, one needed to know the effects of the multitude of variations that could be incorporated in the surfaces of flying-machines. The pres- sures on squares are different from those on rectangles, cir- cles, triangles, or ellipses ; arched surfaces differ from planes, and vary among themselves according to the depth of curva- ture; true arcs differ from parabolas, and the latter differ among themselves ; thick surfaces differ from thin, and sur- faces thicker in one place than another vary in pressure when the positions of maximum thickness are different; some sur-