The lecturer has been concerned with commercial air survey operations since their inception after the war, more on the organisation, finance and business side, rather than on the technical. It is therefore from the angle seen from the directing side that this paper is submitted. As certain technical aspects will be of interest to the Institution, they are referred to, in order that attention may be drawn to them. Through the courtesy of the firms responsible for manufacture, certain apparatus is exhibited which includes an air camera, the automatic pilot, and the contouring stereoscope. Certain technical experts have kindly promised to be available to answer questions or to demonstrate apparatus, and certain individuals and firms have been good enough to lend photographs and slides. Acknowledgment for such assistance will be made in the reply to the discussion.

At quite short notice I was asked to deliver this paper and told at the same time that I must make it popular and not adhere too much to technicalities. This makes it very difficult for me.

Although the problem of the stability of a fuselage that is subjected totorsional force is important in connection with the case of light metalconstruction of an aeroplane in flight, the theoretical side of the problemdoes not seem to have received much attention. This is probably because ofthe difficulty of obtaining its mathematical solution even in the case wherethe fuselage is assumed to be a circular hollow cylinder.Thus, I studied the problem in assuming that the fuselage is a cylindricalshell for the first approximation. Southwell and Skan have dealt with thestability of a plane elastic strip due to edge shearing forces, but theirresult cannot be applied to the problem of the cylindrical shell unless itslength is very short. Schwerin seems to be the only one who has written onthe torsional stability of a cylindrical shell. Although his method ofconstructing the differential equations of the equilibrium of a cylindricalshell has been chiefly derived from Love's text book, and appears correct inthe main, yet owing to the certain apparent particularities on his part hissolutions of equations are open to grave doubts.

The great interest being taken at present in the use of aeroplanes in connection with mining, shown in the reception given to a recent paper before the Institution, has suggested to the author that a paper may be welcomed which deals with aerial survey as an aid to the geologist and to the prospector, and which attempts to put before members information on the subject, much of which is available but not altogether accessible.

On the assumption that for winged flight a man would have to rely on the phasic activity of his muscles (that is to say on an activity induced by nerve impulses from the brain at a rate of 50 per second, or thereabouts) it is easy to believe that for man such flight is impossible.

During the last few years the use of the Diesel engine for aircraft has become important; it has advantages and some disadvantages when compared with the petrol engine, and what we are directly concerned with is overcoming, or at least reducing, the disadvantages. The most obvious of these is the relatively high weight-power ratio of the engine, but it is to be hoped that this will be overcome as rapidly as it was in the development of the petrol engine. In any case, the unfavourable weight-power ratio of the compression-ignition engine is, for long flights, offset by the weight of fuel required to be carried.

The high-speed internal combustion engine presents many problems arising from dynamic effects. Amongst these is the phenomenon known as “ surging ” in the helical springs used for the operation of the valves.

If a helical spring is held at both ends, any disturbance in the spring passes up and down as a wave, being reflected at each end in turn. This to and fro movement continues until it is damped out by friction and air resistance. With most springs the speed of propagation of the disturbance is considerable and only a confused flutter of the coils is apparent to an observer. A disturbance of this type is caused by any movement of the end of the spring. The more abrupt the movement of the end, the more pronounced will the disturbance be. An instance of the type of movement producing a pronounced surge is that due to impact between the tappet and the valve when the valve commences to open.

The work described below deals, in detail, with the methods whereby the motion of an airship can be calculated. The possible motions of an airship are affected by such factors as its stability, motion of controls, release of hydrogen, dropping of ballast, engine power available, and gustiness of the air. The methods to be described are capable of dealing with these effects either separately or combined in any arbitrary proportions.

Let us first consider the oscillations, in still air, of a monoplane wing whose aileron is supposed locked to the wing in such a way that it behaves as though it were an integral part of the wing structure. When the wing is displaced from its position of equilibrium and released it will, in general, vibrate both in flexure and torsion. The initial displacement may be purely flexural, but if the inertial forces called into play, over any wing section, produce a twisting moment about the centre of twist (i.e., the centre about which the wing section twists on the application of a pure torque at that section) torsional as well as flexural oscillations will be set up. Inertia, in general, robs the two kinds of oscillation of their independence, and, when they are interdependent, we may conveniently speak of “inertial couplings” between the two motions. In still air these vibrations must, of necessity, die down. One part of the wing may gain energy at the expense of another, but the store of elastic energy given to the wing by the initial displacement must grow progressively less as the wing does work against the viscous air damping and structural hysteresis forces.

Until recently it was believed that Sir George Cayley's note-books had been lost. In 1927 Mr. J. E. Hodgson, the Honorary Librarian of the Society, learnt that a quantity of miscellaneous manuscript papers were in existence in the home of the Cayley family, and permission was obtained to sort out from such papers matter bearing on Cayley's aeronautical and other engineering or technical labours. Among them was the Note-book, which will shortly be published in full by the Newcomen Society, by kind permission of Sir Kenelm Cayley.

In considering the design of marine aircraft, viz., flying boats andseaplanes, an entirely separate and additional subject is brought in forinvestigation, and upon which there would appear to be much reserve as todefinite formulations and data, etc. This subject, which I prefer to entitle“Hydrodynamics,” can become, besides interesting and absorbing, certainlymost deep in its complexity, and it is realised only too well by thoseactually engaged in such investigations, the more that time passes andexperience is gained.

Last year I was fortunate enough to visit Germany and America at the kind invitations of Dr. Eckener and Admiral Moffett, and I hope that a short description of the development abroad will be of interest.

I propose to devote the short time at my disposal to these two countries, as, although France is still building non-rigid and semi-rigid airships for naval work and Russia has embarked on a most extensive airship programme, I think that the recent developments and improvements of the rigid type airship will be of more interest.

An occasional glance into the past is often of interest. The following notes refer to an event which was one of the principal landmarks in the history of aeronautics, and which attracted much interest abroad, especially in France. It is hardly necessary to remind the reader that the Aeronautical Society of Great Britain subsequently referred to, is now the Royal Aeronautical Society.

The year 1868 was a year of mark in aeronautics, for it was then that the Aeronautical Society of Great Britain held the first Aero Exhibition in the Crystal Palace.

When this instrument is in its observing position the visible horizon is reflected at three points vertically downwards into a telescope. The portions of the horizon observed are facing the observer and lie 60° to his right and left. The images of the visible horizon appear as three straight lines in the field of view intersecting at about 60°. The observation consists in bringing the image of the celestial object into a certain point defined by the configuration of the horizon-lines. It is of no consequence whether all the rays after the downward reflection are again reflected into a horizontal direction, and whether this occurs before or after the rays enter the object-glass. It will be shown below how the observer is enabled to keep the error of inclination so small that its effect on the result, of second order, can be neglected.