IT is a common experience, and by no means confined to engineers, to fail to understand the general theory of non‐rectilinear flow of air. An investigation culminates, of course, in the well ‐ known equations of viscous motion. But, as Dr. Prescott remarked recently when introducing his new proof, many derivations of these are cither unsatisfying or else difficult to follow. Indeed, the equations are usually taken for granted. Yet the disadvantage is obvious: even the mathematician can do very little with them in the end, and fundamental ideas on which the proof rests, though likely to be of general service to engineers, lie buried. An example is perhaps worth while. Consider the problem of determining the viscosity of an oil from measurement of the torque required slowly to revolve a circular cylinder in a tank or pot of it. The equations of motion can be solved in this case, but it is not necessary to go so far; there is a well‐known treatment much more to the heart of an engineer. On the other hand, the usual theory of rectilinear viscous flow is without avail, and if the engineer can solve this simple problem without the equations then he understands the bulk of the proof of them.