It is not possible to go fully into all the aspects of this problem in the time at my disposal. I am accordingly confining myself to a discussion of the feasibility of operating heavier-than-air craft in fog with reference to the instrumental equipment required. The meteorological aspects were dealt with in a paper by Captain Entwistle two years ago.

In order to obtain a correct perspective on the problems of flying in fog it is necessary to realise that many cases of impaired visibility, which would not normally be regarded as fogs, may seriously increase the risks of flying. This reduction of visibility may be caused by fog proper or by haze, drizzle, rain, snow or dust. The problem I wish to consider then is that of flying, navigating and landing when the visibility is such, that flight would be impossible or unduly dangerous without special equipment.

Before beginning this lecture I have first of all to thank the Air Ministry for permission to give it and to explain that any opinions expressed are my own and do not necessarily reflect the opinion of the Air Ministry.

Secondly I must express my appreciation of the honour conferred on me by the Society in asking me to deliver a lecture before them; though I wish they had chosen a less difficult subject.

As regards the title of this lecture it sounds very circumscribed, but you can't test anything without having a fair idea of what its complete function is or without some standard of reference; and in trying to discuss these parts of the problem I am afraid you will find that I have been, if anything, too. general.

The co-ordinate system used is shown in Fig. i.

The axis coincides with the axis of rotation and the plane is chosen parallel to the plane of rotation.

The plane serves as a longitudinal datum plane for the airscrew blade, and is perpendicular to the planes of the cross-sections of the blade. The axis OX is drawn in the direction of the radius from root to tip.

As the subject of the reduction of astronomical observations in the air had been the subject of several conversations with the late Squadron Leader Johnston, who was one of our best exponents of aerial navigation, Mr. A. J. Hughes, O.B.E., suggested to the writer that a short note based on Squadron Leader Johnston's methods would be useful in order that his valuable experience should not be entirely lost, and that some systematic survey of the methods available would be useful in connection with a subject that is becoming of increasing importance now that the range and reliability of aircraft have been so largely extended.