A new industry calls for new methods—autrea jours, autres mceurs—and the aeroplane industry is therefore not exceptional in demanding many departures from recognised woodworking practice. In the first place the number of machines required to–day is so large that their construction should be described as a manufacturing rather than a building operation. That is to say, they should preferably be almost entirely put together by the aid of accurate machinery instead of being dependent to such an extent upon the skill of the individual workman, in trying and fitting one part to another until the whole is built up. In this connection it may be remarked that the design of the metal fittings for aeroplanes is not such as to call for any radical departure from previous high class small metal working practice. It is the woodworking side of the business which presents the new scope for initiative. It is this latter aspect, therefore, which will be considered here.

Sir David Brewster in 1816 discovered that an isotropic transparent solid, such as glass, behaves as a temporary crystal under stress, and when examined in the polariscope between crossed Nicols exhibits the characteristic colours of crystalline plates.

Since Brewster's day this property has been studied by a number of observers. Clerk Maxwell showed that isinglass jelly would show the colours under stress when in its soft state, and retained them, more or less permanently, when allowed to harden under strain. To him is due the first attempt to apply this effect to the exploration of the stress system in the plane of a strained plate; experimental investigations of this nature were afterwards carried out by Carus Wilson, by Major Filon, and by Prof. E. G. Coker, F.R.S.

Neither glass, however, nor isinglass jelly forms a suitable material for the investigation by this means of stresses in engineering structures. Glass does not lend itself readily to the construction of such models as regards shape and jointing.

The entire science of aerial fighting has developed, as everyone knows, since the beginning of this war, in other words in the short space of four years.

To–night I am going to attempt to give you an outline of the various stages through which aerial fighting has progressed to its present state of efficiency, though it would be foolish to claim that we have even begun to realise the methods and tactics which will be employed in the future.

All through the history of aerial fighting we find two main governing factors, and I would ask you to notice how these affect each step. These two factors are:—

1. (1) Field of fire.

2. (2) Field of view.

In talking of the latter, I shall be constantly mentioning blind spots. By a blind spot is meant anywhere in the sky which is hidden by the planes, tail or fuselage, from the pilot as he sits in his cockpit.

The failure of struts under end load is due to two causes, crushing, due to direct compression, and excessive bending, due to elastic instability. In a general way we may say that the end portions of the strut tend to fail by crushing and the middle portion tends to fail by bending, but we cannot make a rule to decide where the middle portion ends and the end portions begin, because both causes are really operating simultaneously all along the strut, and we can only form a vague impression of their relative importance at various sections of the strut. A parallel strut, for instance, unless it is very short, will fail by bending at or near the middle, and the end portions will show no signs of crushing; so we may fairly infer that weight can be saved in a solid wooden strut without much loss of strength by tapering the ends; and that on the whole the efficiency of a strut, as measured by the ratio of crippling end load to weight or (resistance + m weight) where m* is a fraction, can be improved by tapering.

During my visit to Great Britain I have been surprised to find how many factories have sprung up during the war, which whilst producing munitions of a highly complicated character lack a staff, the members of which as a whole possess that long and careful experience in technical production always considered necessary in the qualifications of men for executive or responsible positions.

Yet the fact remains that these concerns are producing, and the product in many cases is of a high quality. We are, however, living in exceptional times, when competition—in the true sense of the word—is absent; but if the best is to be obtained now and in the future, when these factories will have to strain every effort for their existence, it will be necessary that production on a strictly economical basis be considered.

My experience shows that in a number of cases there exists a misconception of what a proper system of control means, and the advantage accruing there from is unknown.

The average orthodox mathematician who writes or reads books about hydrodynamics is so keen about getting a peg on which he can hang what he calls examples that he quite forgets to apply his formulae to compressible fluids. It appeared to me on reading Colonel De Villamil's letter that a useful purpose would be served by writing the present note; it contains nothing but what anyone could write out for himself, but it was only recently that I made the attempt myself and I should be surprised if many readers have seen the same thing before. We know that in the irrotational motion of an incompressible fluid the velocity potential satisfies Laplace's equation, the present object is to find the corresponding equation for a compressible fluid, to grind it out at full length, and (what is more important than grinding it out) to interpret the result.