Mr. Glauert's paper “Theoretical Relationships for the Lilt and Drag of an Aerofoil Structure ” strikes a bold note in asking us to discuss the fundamental principles of Prandtl's aerofoil theory rather than its detailed application. His suggestion is a particularly good one, however difficult the task. Justification of the geeneral truth of the suggestions brought forward by Lanchester, and so ably developed bv Prandtl and his colleagues and others, have frequently been afforded by the application of the theory, and similar support only awaits the seeking. But, to achieve its greatest utility, the theory should enable us to extrapolate experimental knowledge with confidence; an example is provided by the question as to what limit, if any, exists to the camber of wings on economic grounds and apart from burble considerations. An approximate tally—rough, in some cases—of the theory with observation over a limited range is inadequate to establish it sufficiently for such a purpose. The theory may indicate with truth the existence of, say, a limit to a certain development, but its numerical calculation of the limit may yet be widely wrong.