When asked to read a paper on the Thermodynamics of Aero Engines I felt very doubtful in the first place whether I ought to attempt such a subject at all, for, though I have posed often as a mechanic, I have not posed before as ascientist, at least not in public. In the second place I felt doubtful, if I attempted anything so rash, how to deal with the subject. Clearly I must adopt one or other of two lines, either play for safety and stick firmly to the fundamental thermo-dynamic laws which are unassailable but also inapplicable to practical working conditions, or take the plunge and endeavour to argue or at least to speculate as to how these fundamental laws must be qualified to meet actual working conditions. In adopting the latter course I realise that I must touch upon many highly controversial questions, and if in places I appear to lay down the law on what at best is little more than mere conjecture, I must ask you to forgive me and to realise from the start that the last thing I intend is to appear didactic.