For many years there have been used in aircraft construction aluminium alloys to a steadily increasing extent, and particularly alloys of Al.-Cu.-, Mg. and Al.-Cu. capable of being improved by heat treatment. Among the different methods known for permanent connection of structural parts, the rivet is still to-day the most reliable and safest connecting element. According to the special quality of this light metal group, due to their natural material characteristics and owing to the fact that in most cases the connections of aircraft structural parts are very complicated and highly stressed, the rivets are always driven cold. Due to these conditions, the tools, machines and working methods for strengthening of riveting diverge essentially from the commonly known riveting methods of general machine construction. In the course of my lecture I shall give further details regarding these questions.

In this paper I have endeavoured to give an account of the various methods which have been employed in the past ten years to alter—for better or worse— the conditions in the boundary layer of a body in turbulent flow. I have refrained from speaking of “ control ” in reference to the-boundary layer, since in the minds of most people, this implies an amelioration of the flow and a reduction of drag. As will appear in what follows, some of the devices in which I and other experimenters have been concerned have just the opposite effect. Nevertheless, following the principles current in clinical research in medicine, I think we should find both what conditions will cause “ deterioration ”—if one may use the term—as well as those which will cause “ improvement ” of the flow round a body, if we are to progress in our knowledge of the state of affairs in the boundary layer. Working rather on my own in aerodynamics and not being committed to any rigid programme of research, I have followed my bent to explore unorthodox paths of experiment, the results of which I present with some diffidence in the hope that engineers may pursue them further with better equipment. I am afraid I have not gone deeply into the vexed question of the efficiency—in the engineering sense—of the devices I am going to describe to you, since, as a physicist that—nevertheless, important—aspect of the question does not interest me.