I must begin by explaining how I come to be giving this lecture. The experimental work with which I shall deal has, for the most part, been done at the N.P.L. and the R.A.E., under the general direction of the Aeronautical Research Committee. The way in which I come to be connected with the work is that I am a member of this Committee and am Chairman of a small panel that was created, some three years ago, by the Committee, to deal with this and other work relating to control and stability. The experiments that I shall describe and the methods of dealing with the results that I shall employ are, therefore, the results of the combined work of a considerable number of people. I can thus claim no special ownership of any of the ideas that I shall use, except in so far as I belong to the panel that has been working upon them. On the other hand I am giving the lecture as a private person, so that any views I express are personal ones and in no sense official.

The investigation summarised in the two preceding papers formed part of a rapidly carried out research which was made at the commencement of the war in order to determine whether a machine on the helicopter principle could be built, and whether, when built, it was likely to prove of any special value for belligerent purposes.

The manuscript by Leonardo da Vinci, of which a complete English translation is here offered (so far as the writer is aware, for the first time), is known as the “ Codice sul Volo degli Uccelli e Varie Altre Materie ” (Codex on the flight of birds and other matters). It is a small note-book of some 30 pages, and measures- about 8.4 inches by 6 inches. It was written at Florence in the year 1505, between March 14th* and April 15th. t A close'study cf this manuscript has been made by G. Piumati} in a very fine edition containing an excellent facsimile of the original note-book, together with (a) a printed copy folio by folio of the old Italian script in which it was written, (b) a rendering into modern Italian, and (c) a French translation by C. Ravaisson-Mollien.

In the following paper it will be convenient to adopt the definition of inverted flight used by the Accidents Committee in R. & M. 617 (Advisory Committee for Aeronautics), “Reports on the behaviour of aeroplanes when flying inverted, with special reference to some accidents on ‘A,’” which was as follows:—“ In normal straight flight the pressure of the pilot on his seat is equal to his weight. During inverted straight flight he would hang on his belt with an equal force. It is therefore considered useful to call inverted flight such flight only as occurs when the weight of the pilot no longer presses on his seat.”

An Ordinary General Meeting- of the Society was held at the Royal Society of Arts, on Thursday, February ist, 1923, Professor L. Bairstow in the chair.

The Chairman, in opening- the proceedings, said that Mr. G. S. Baker, O.B.E., of the National Physical Laboratory, would deal with flying boats and seaplanes. He would deal with the hull and its design, that part of the seaplane which differentiates it from the aeroplane. That subject had been touched on very lightly by Major Rennie at the previous meeting of the Society, in view of the present paper by Mr. Baker.Mr. Baker had begun work in 1912 on the problems of hull design, at a time when nothing of a definite nature was known; a few individual experiments had been carried out, but there was no systematised knowledge at all at that time. From that state of ignorance a great deal of experimental work had now rescued us. He did not know how far Mr. Baker would stress the point, but it was quite clear, from the investigation of certain accidents to seacraft, that there were fundamental differences in the behaviour of seaplane hulls on the water, differences which had a great deal of effect on the risk of flying-. For instance, if one type of hull was such that when the plane rose in the air it stalled, then all the aerodynamical consequences of stalling- followed, and there was difficulty. On the other hand, it appeared that we had a type of flying- boat which did not make the plane stall on getting into the air, and consequently if it came back to the water it was still controlled. For this type of development, which he believed really dated back to the C.E.i, we were mainly indebted to Mr. Baker and his associates at the National Physical Laboratory, and to the generosity of Sir Alfred Yarrow in placing such a magnificent piece of apparatus as the experimental tank at the disposal of the nation.

Mr. Baker then read his paper on “ Ten Years’ Testing of Model Seaplanes.”

It was suggested by Lanchester some years ago that an aerofoil immersed in a moving fluid gave rise to two trailing vortices of opposite hand, one situated near each of the extremities of span. The existence of the vortex pair has since been demonstrated in a variety of ways, of which perhaps the most striking is that due to Caldwell and Fales, who used the condensation of water vapour in a high-speed wind channel to obtain photographs. The vortex pair constitutes so important a feature in the stream behind most aerofoils that measurements capable of numerical analysis seemed desirable, and an account is given in the present Paper of some work to this end which appears capable of straightforward interpretation.

It may be that the residual motion sufficiently far behind any aerofoil is a vortex pair. Similar experiments to those about to be described, however, which deal with the motion only for a few chords back, have been made with several other aerofoils of a variety of shapes having long, graded wing tips, and with some of these no distinct vortex was found. With others apparently more than one pair could exist in certain circumstances, but opportunity has not occurred to probe deeply into these interesting cases. Attention is confined to two aerofoils representative of considerable variation in a class having uniform section and angle of incidence with blunt, or square-ended, wing tips. The general nature of the results is common even with aerofoils outside this class, but the foregoing remarks will serve to indicate that exceptions are known to exist.

The motives prompting the desirability of human flight from the legendary period to the present time have been very numerous. For the last two hundred years, apart from the war period, we may reasonably assume that the predominant factor has been the rapid transport of personnel or material from one point to another. With this aspect of aviation, aerial navigation is intimately concerned. I want you at the outset to distinguish clearly between the problem of aerial navigation over comparatively short routes—such as from London to Paris— and what is surely the true realm of aircraft, the long distance flights from England to America, India and Australia. In favourable weather the European routes require no more of the pilot than that he should be able to read a map with what, for want of a better term, I will call aeronautical intelligence.

It is very kind of you to invite me again to address the Scottish Branch of the Royal Aeronautical Society, and I am glad to do so at this the opening meeting of what promises to be a highly valuable session.

I will not excuse my temerity in coming here to-night for two reasons. First, since I am no longer in direct touch with aviation I do not claim first hand knowledge of detail, my interest in what we all believe to be one of the greatest of modern developments is still unabated. And secondly, because aeronautical progress must be assisted by open discussion of its difficulties, and it may be of value, whether you agree with them or not, to express some personal ideas from the outside angle of a year's private membership of the House of Commons as a preface to your more technical meetings.

The manuscript by Leonardo da Vinci, of which a complete English translation is here offered (so far as the writer is aware, for the first time), is known as the " Codice sul Volo degli Uccelli e Varie Altre Materie " (Codex on the flight of birds and other matters). It is a small note-book of some 30 pages, and measures about 8.4 inches by 6 inches. It was written at Florence in the year 1505, between March 14th* and April 15th. f A close study of this manuscript has been made by G. Piumati+ in a very fine edition containing an excellent facsimile of the original note-book, together with (a) a printed copy folio by folio of the old „ Italian script in which it was written, (b) a rendering into modern Italian, and (c) a French translation by C. Ravaisson-Mollien.