The problems which arise in the course of a study of aeroplane stability are of considerable complexity as compared with those confronting engineers in other branches of locomotion. Ocean–going vessels provide another instance of the motion of a solid body in a fluid and their stability is clearly one of vital importance. Naval architects have consequently studied the subject and calculations are made for the stability of each new design of ship. The process is so well known and so well founded that it surprises no one that a newly launched vessel remains on an even keel. One instance, not connected with British shipbuilding, serves to remind designers of the serious consequences of faulty calculation. The disaster relates to a ship which was launched fully engined, etc., and which turned turtle and sank within a few minutes of launching.

The type of stability which the naval architect needs to calculate most carefully is that which is concerned with the security of the ship when rolling.

Last year the Aëronautical Society kindly asked me to give my views on Military Aviation. I then tried to show the objects for which military aircraft would be used, the results likely to be gained, and the general lines upon which it was expected that progress would take place—in a word, the military aspect of aviation. Airships have now been handed over to the Naval Wing of the Royal Flying Corps and I propose to–night to deal almost entirely with aeroplane work, in the light of a year's progress. I make no excuse for reiterating the strong links which I feel bind soldier, sailor, designer and aircraft constructor in this matter. Rapidity of progress is essential and it can only be gained by cohesion of effort, lack of friction, and mutual understanding. The Service must remember that the aeroplane designer has to live and is not always merely chasing “X” with a slide rule; the constructor, that the soldier is not only pipe–clay and red tape and that there is sometimes method in his madness.

The material for this paper was collected for a written communication to the Research Committee for publication in The Aëronautical Journal, in the form of a brief annotated account of the work of a number of experimenters on the performance of the screw propeller in air and water; but, at the request of the Council, a month ago, the article has been re–cast in the form of a paper.

As can be readily understood, such a catalogue would be extremely boring to those who have so kindly come here to–night to listen, so that it was considered preferable to give only some general conclusions with sufficient experimental evidence to attempt to justify them.

The short résumé has, however, been retained in the form of an Appendix, for the use of those who desire to refer to it.

It was hoped that, by keeping the paper as short as possible in this manner, there would be plenty of time for the meeting to hear the opinions of the many able members who have had experience of this subject.

On the last occasion I had the honour of attending a meeting of this Society the members were expressing their admiration of the great work done by the late Wilbur Wright. This consisted in first studying the known suggested theories governing the physical laws of flight, and then, after much patient labour, embodying the result of his reflections in the form of an aerial apparatus. Then at great personal risk, with his brother's aid, testing his apparatus as a glider until he was satisfied that a practical machine driven by power could be made; which he successfully accomplished and piloted with results well known to this Society.

To-night we have to consider many theories governing a different kind of law and suggest means by which they can be reduced to practical regulations. We have not the same opportunities as the physicists, no official laboratories or opportunities for making experimental tests, and have to leave this to the cumbersome machinery of the administrative authorities of the governing political powers.

The subject of my paper is the airship of to-morrow, and if I look at it rather from the point of view of military requirements, you must bear in mind that it is these requirements which have made it what it is. Commercial needs and even Naval needs have played a very small part hitherto in its development, though they may be expected to exert considerable influence on its progress in future.

Looking back over the past few years, we find an extraordinary prejudice against airships in this country, fostered in the most amazing manner by the Press, both lay and technical. This did not begin to dissipate until the autumn of 1912, just over a year ago.

As late as October, 1912, the “Times” wrote a leading article on the folly of spending money on airships, taking as its text the accident to “Gamma” after the manœuvres of that year.

Aeronautics is certainly the science of the future and its professors and preachers have the qualities of their science. When you were good enough to ask me to read a paper, by way of opening a discussion this evening, you told me beforehand the subject which I should select. It is a new departure, characteristic of these new times. If you will look at the practice of education in this or any other country you will find that hitherto subjects have been selected, not by those who are to hear, but by those who are to speak. They are the subjects upon which the teacher has already made up his mind. The principle of selection is that the teacher must be able to display superior knowledge and the process of education is a sort of wave of superior knowledge going down from the universities into the most elementary recesses without much regard for what it looks like from the point of view of those hungry for the knowledge of “how it goes,” and still more “why it goes?”

You have provoked this digression by giving me the subject, “Wind Gusts and the Structure of Aerial Disturbances.”

In dealing with the flow of fluids, and the reactions set up on moving surfaces, water and steam turbine engineers have to deal with definite masses of water or steam flowing along channels whose boundary walls are fixed.

Fig. 1 (reproduced from Stodola's Steam Turbines, 3rd edition) shows the nature of the assumption made for the so–called impulse turbine, assumptions which hold to an equal approximation for water turbines, showing nozzle and moving vanes

Reaction turbines present a more complicated state of affairs, which offers a less direct analogy with aeroplanes and airscrews, and which therefore need not be considered here.

In the figure (1) the mass of working fluid delivered per second is the cross section of the nozzle or channels between vanes, at any point, multiplied by the velocity of the fluid relatively to the walls of the channel at that point.