Second in importance only to the problem of increasing the safety and reliability of aeroplanes is the question of reducing their head resistance. If aviation was to take its place as an established means of commercial transport the first essential was obviously to make it reasonably safe and certain, and the main energies of investigators and designers immediately after the War were directed to this object. Economy of operation, of which the reduction of head resistance is one of the major factors, was, by comparison, somewhat neglected. But in the last three or four years, with the safety and reliability problems essentially solved, more attention has been devoted to the reduction of drag.

In reviewing the progress that has been made, and the possibilities of further improvement, it is helpful to inquire what is the best we can hope for. We know that the generation of the lift required to carry weight in heavier-than-air aircraft is inseparably associated with a certain drag—the induced drag.

Due proportions being established, it can be said that the present time, as regards what has been called the “conquest of the air,” is comparable to that extremely remote epoch in which Nature for her part also solved the problem of flight.

Man arrived late in the history of his life at the realisation of locomotion in the air, but Nature also reached the same realisation only in a second stage of the evolution of living forms. The early animal species were all wingless, the development of the wings being the effect of a successive adaptation.

At present the number of fliers is very large.

From calculations of the zoologist Dödering, mentioned by the zoologist Zschokke, together with a large number of interesting data and information about animal flight in a very important book, we know that among 420,000 animal species at present existing at least 260,000 are able to move through air. As is seen, flight is a faculty widespread in Nature; from the figures some 62 per cent, of the species living to-day is adapted to aerial locomotion. If from these calculations we exclude water creatures, which give a small contribution of fliers (and moreover of a very poor quality), the ratio of the flying to the nonflying species rises to 75 per cent.