supporting member, or organ of sustentation of undefined form. Thus a plane aerofoil is an aeroplane, or a pterygoid aerojoil is an aerofoil of wing-like form.
There are cogent reasons why the aeroplane should take foremost place in the matter of experimental study. It is recognised as essential to the inductive mode of investigation that, whenever possible, one of the conditions of experiment, and one only, should be changed at a time, and it is primarily on this ground that the aeroplane recommends itself. The aeroplane is possessed of a geometrical definiteness that admits of no ambiguity; a specified contour form, making a definite angle with, and presenting a definite aspect to the line of flight, constitutes (for any given velocity) the whole of the factors by which the conditions of experiment are defined. Beyond this there is (skin-friction apart) a certain obvious relationship between the pressure components about the co-ordinate axes, and the angle of flight, that forms a valuable and instructive link in the interpretation of experimental results.
§ 129. Historical.—Our knowledge of the aeroplane to-day is the result of the work of a number of investigators. The exact date at which the study of the subject was seriously taken in hand is in doubt; a certain amount of experimental work on the resistance of bodies in the air is known to have been done early in the eighteenth century, notably by Sir Isaac Newton (1710), and Dr. Desaguliers (1719), whose observations are, however, believed to have been confined to the motion of spherical bodies. Newton also extended his researches to the theoretical study of bodies of different forms in a hypothetical medium (ref. § 2), and showed that the theoretical and experimental results are not altogether out of harmony in spite of the unreal nature of his hypothesis. Newton further attempted to solve the problem of the normal plane in an incompressible continuous medium (Principia, prop, xxxvii. and cors. 7 and 8, prop, xxxvi). These propositions, resting as they do on the supposition of the
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