Everyone will remember the distance to which a skilful juggler can project an ordinary playing-card by giving it a certain inclination in throwing. It will travel upward or on a level, and continue this direction until the force of the movement of throwing is exhausted. Obviously, if this force were self-contained in the card--if it could continue rotating and moving forward--it could fly indefinitely. Henson had studied and experimented with these miniature aeroplanes, and was convinced that if the same principle that governed their flight were to be applied to larger machines, practical flying-machines could be made.
"If any light and flat, or nearly flat, article," he wrote, "be projected edgeways in a slightly inclined position, the same will rise on the air till the force exerted is expended, when the article so thrown or projected will descend; and it will readily be conceived that if the article possessed in itself a continuous power or force equal to that used in throwing or projecting it, the article would continue to ascend so long as the forward part of the surface was upward in respect to its hinder part, and that such article, when the power was stopped, or when the inclination was recovered, would descend by gravity only if the power was stopped, or by gravity, aided by the force of the power contained in the article, if the power be contained, thus imitating the flight of a bird."
But when Henson attempted to fly in his elaborately planned and constructed flying-machine, it proved a complete failure. It showed a tendency to rise, but its lifting power was insufficient for the weight of the engine driving the propellers. It was evident, however, that if the power of the engine could be sufficiently increased, or, what amounts to the same thing, its weight sufficiently lightened, a machine built on the aeroplane principle could be made to fly. But at that time the lightest type of engine was a crude, heavy machine, and for the moment nothing more was attempted in producing a mechanical flying-machine propelled by steam.
Meanwhile the possibility of producing a dirigible balloon was again brought into prominence by the suggestion of two aeronauts, Scott and Martainville, to change the shape of the envelope of the balloon. Hitherto, all balloons had been made globular or pear-shaped--shapes that offered great resisting surfaces to the atmosphere. Now it was proposed to make them in the form of long, horizontal cylinders, with pointed ends, these cigar-shaped, or boat-shaped balloons offering much less resistance. But here, as in the case of the flying-machine, engines that were sufficiently strong to work the propellers were found to be too heavy for the balloon to lift. Meanwhile the aeroplane idea was brought into prominence from an unexpected quarter.