The law, to which I wish to call attention first in this article, can hardly be called unknown, but it has certainly remained unnoticed by the great majority of designers of aeroplanes and other flying machines, and has been generally ignored by scientific writers in aerodynamics, although some of the latter have recognised its existence without apparently seeing its importance ; or, perhaps, although cognizant of its existence, they have been unable to reconcile it with “Langley's law,” and have therefore refrained from giving it the emphasis it undoubtedly deserves.