Lilienthal, in Germany, in considering equilibrium, experimented with what are called gliding machines--aëroplanes which are launched from some hillside against the wind, and depend upon gravity for their motive power. In this way the art of balancing could be practised on motorless gliders. With Lilienthal commenced the age of systematic experimental flight; he made the discovery of the driving forward of arched surfaces against the wind; he made some 2,000 glides, and sometimes from a height of 30 metres he glided 300 metres. The underlying principle of maintaining equilibrium in the air has been recognised to be that the centre of pressure should at all times be on the same vertical line as the centre of gravity due to the weight of the apparatus. Lilienthal sought to keep his balance by altering the position of his centre of gravity by movements of his body. One day he was upset by a side gust and was killed. Pilcher, in England, took up his work. With his soaring machines he made some hundred glides, but he also made one too many. One day, in 1899, in attempting to soar from level ground by being towed by horses, his machine broke, and he fell to the ground. He died shortly afterwards, a British martyr of the air. Mr. Octave Chanute’s experiments in 1896–1902 formed important links in flight development. He first introduced the vital principle of making the surfaces movable instead of the aviator, and he made use of superposed surfaces. Though his work was a stage in the development of the flying machine, it was reserved to two other geniuses, the brothers Wright, to bring flight to a point of progress where prejudiced critics would be for ever silenced. The brothers Wright first carried out laboratory experiments; they then, in 1900, first began to experiment with gliding machines at Kitty Hawk, North Carolina. With the comparatively small surfaces (15.3 square metres) they used in that year, they endeavoured to raise the machine by the wind like a kite; but finding that it often blew too strongly for such a system to be practical, in 1901 they abandoned the idea and resorted to gliding flight. These machines of 1901 had two superposed surfaces, 1.73 metres apart, each being 6.7 metres from tip to tip, 2.13 metres wide, and arched 1-19th. The total supporting surface was 27 square metres. They dispensed with the tail which previous experimenters had considered necessary. Instead, they introduced into their machine two vital principles, upon which not only the success of their preliminary gliding experiments depended, but also their later ones with their motor-driven aëroplanes--(1) the hinged horizontal rudder in front for controlling the vertical movements of the machine; (2) the warping or flexing of one wing or the other for steering to right or left.