The present Paper is a record of experimental work carried out with a view to overcoming the phenomenon of “ burbling.” As is well known, the total pressure on an aerofoil is the sum of the positive pressure on the under side and the negative suction on the upper. If this negative suction can be made to increase progressively with increasing angle of incidence to angles greater than heretofore, the maximum value of the aerofoil lift coefficient will be increased. The effect of such an increase on aeroplane design depends upon the magnitude of the increase and the extra structure weight of the device necessary to obtain it. The present method which, is now described has been evolved from experimental data, and an outline of the results is given below.

In a Paper which I read before the Royal Aeronautical Society in April, 1911—ten years ago—I attempted an analysis of the somewhat meagre results then available on the pressures on plane and curved surfaces moving through the air.

The Cairo-Cape flights of 1920 brought into prominence many peculiarities of conditions arising from the high temperature combined with high altitude ot the aerodromes on the route. The conditions were for the most part new, for flight from elevated aerodromes in Europe and the United States has been made as a rule in temperate climates, where the temperature at such altitude was low. These peculiarities, which led to great difficulties for the pilots, may be divided into two classes :—

(1) Engine peculiarities, especially defects of carburation and unequal heating of the cylinder jackets owing to the high temperature of the air.

(2) Aerodynamical peculiarities, owing to the tenuity of the atmosphere. In the former class the difficulties were not to be foreseen, and much valuable information has been obtained as a result of these flights.

The latter class of difficulty, on the other hand, was one that could have been predicted with certainty by anyone who has had experience in the testing of the performance of aeroplanes under varying conditions of the atmosphere.

In my last lecture I promised to try and show how the pilot can foresee and effectively eliminate practically all the danger which attends the art of flying. You will probably notice that most of the danger arises, at least indirectly, from some misbehaviour of the engine; so that in estimating the danger I shall be compelled to lay considerable emphasis on the possibility of engine failure, in spite of the fact that it is comparatively remote. If we look back to the infancy of any mechanical means of locomotion, we realise how unreliable it was in the initial stages; we may therefore look forward with absolute confidence to the future reliability of the aero engine and its installation.

Every day almost marks an improvement, and if the early pioneers who lost their lives could trace its development what surprise they would express ! At the same time the possibility of one unreasonable engine failure is a menace to the whole of flying, which cannot and must not be blinked at.