During the past few years particular attention has been paid to the protection of metal parts of engineering structures of all kinds against corrosion. The problem of protecting metal structures against deterioration is now recognised as of primary concern both to users and to manufacturers. Corrosion prevention is an economic problem and its cost must be based on the cost of preventative measures compared with the loss directly and indirectly due to deterioration. In those sections of industry in which marked advances have occurred in corrosion prevention, the advances can be seen to be the result of close co-operation between the laboratory, manufacturers and users.

A noticeable peculiarity in the flight of a certain number of birds is the way their wing-tip feathers separate, both in flapping and in gliding flight. So wide do the gaps between the feathers become, at times, that the outer parts of the wings take on the appearance of hands with their fingers spread out. One's first thought about the matter is that there is probably nothing in it; that the feathers separate simply because they are feathers, and, as such, cannot help themselves; but, on investigation, this turns out to be one of the most interesting of the many aspects of the flight of birds; interesting, not only because it brings to light the infinite care and cunning that have been bestowed on the construction of their wings, but also because it demonstrates the possibility of applying some of the lessons that birds can teach us, to the design of flying machines—gliders in particular. When considering such questions we should always humbly remember that birds are the outcome of the law of the survival of the fittest through countless ages of flying, while we have been at it only for about thirty years.

An interesting item in the above essay, published in the Journal for December, 1931, is the solution, given in Appendix II, of the determinantal equation

ΔN=0,

But before this equation is discussed a dynamical problem will be formulated in the solution of which a similar determinantal equation arises.