I Think Mr. Filleul's main point, (in the November Journal) somewhat hidden behind some obscure comments about Carnot engines and the like, is that in a by-pass engine with heat exchanger, the heat exchange process is making a definite alteration to the thermodynamic cycle efficiency of the whole engine and not just changing the propulsive efficiency. This may or may not be an important point, depending upon the point of view of the expositor of the subject. In fact, even without heat exchange and when the by-pass compressor and turbine have realistic efficiencies, the by-pass process itself does alter in an unfavourable direction the overall thermodynamic cycle efficiency. This is one reason why in a by-pass compressor with separate jets one does not wish to make for optimum performance the by-pass and jet velocities equal.

The Technical Note by C. H. E. Warren in the September Journal raises a question on the transfer of lift, of an aeroplane in supersonic flight, to the ground. This problem was considered in reference 2. It was shown that, as expected, the resultant of the pressure field produced by the aeroplane is equal to its lift. This apparent inconsistency with the N-wave pattern, predicted by Whitham stems from the fact that most applications of Whitham's theory are based on asymptotic forms for the velocity perturbations. Thus, the formula for the axial velocity perturbation is obtained by allowing (M2— l)l/2r/y to tend to infinity, where M is the flight Mach number, r the radial distance away from the aeroplane, y=x—(M2 — l)1/2, and x is the distance along the aeroplane flight direction. This approximation is valid in the vicinity of the shock waves emanating from the aeroplane.