These circumstances lead to the conviction that the aëronautic problem will most probably admit of a satisfactory solution only by means of dynamic flying machines. Prof. Wellner is of the opinion that, manifold as are the difficulties besetting the road to this end, it is likely to be attained before the close of our century. The inventor's starting point is the observation and analysis of the flight of insects and birds, which have of late been made possible by the graphic methods of chronometric observation and photography. It would, of course, be a mistake to attempt a close imitation of the organs of motion observed in birds or insects, as these would lose their utility through the unavoidable clumsiness of man's appliances. What must be aimed at is the most advantageous shape and construction of wings, productive of the greatest propelling power with the expenditure of the smallest possible sum of energy; the application of a light but powerful motor; a contrivance for easy and efficient steering, available alike at the time of the ascent, while flying, and in alighting; and arrangements for the prevention of accidents.
It may be regarded as an established fact that gently curved or arched surfaces, pointed toward their ends, make the fittest wings for flying machines; they must be carried against the atmosphere at small angles. The lifting power of the wings arises from the carrying quality of the air compressed and forming, we might say, an air-cushion under the aëroplanes. The air pressure increases in a duplicate ratio to the velocity of the latter; consequently very small wings are capable of great lifting power when vigorously and swiftly plied. In order to advance the airship, a power imparting horizontal motion is needed; this has to be secured by a retrograde movement, thrusting the air backward and thus causing it to propel the air-ship. The shape, position, and direction of the aëroplanes will have to be such as to combine the vertical lifting with the horizontal propulsion. The two great distinct groups of air-ships attempting to secure this end are those propelled by screws and those working on the principle of a kite.
The models constructed by Ponton d'Amécourt, Achenbach, Dieuaide, Forlanini, Philipps Popper, Jarolimik, and some others are based on the screw principle. They employ propeller screws moving horizontally on vertical axes and connected with aëroplane surfaces which are capable of inclination between the horizontal and vertical positions like those of a windmill. These aëroplanes pressing down the air act very much like the propeller screw in a ship, which drives the water backward so as to make it move the ship forward. The difficulty inherent to this construction arises from a considerable loss of motive power, caused by the difference in revolving speed of the screw at various points
