Aircraft engineering has been described as “ordinary engineering made more difficult.” Probably most of you will agree that our difficulties are often greater than those that confront other branches of the engineering profession, and that the chief source of them is our inability to forecast with entire confidence what air will do under any given circumstances. Our position in this matter is much better than it was, for instance, at the end of the war. During the last thirteen years we have seen the theories associated with the name of Professor Prandtl receive general acceptance, with a profound effect upon our attitude towards some of the most important practical problems of air-flow with which we have to deal, namely, those associated with the production of lift. The boundary layer, then little more than a scientific curiosity, has become a matter of common concern. It achieved a striking practical success in the rationalisation of the international tests of airship models, which led to a marked advance in our attitude towards the whole problem of scale effect. But in many respects we are still greatly hampered. In so far as there has been any advance in the pure hydrodynamical theory of the flow of viscous fluids, it is hardly too much to say that its practical effect has been negligible. What then are we to do?