In this apparatus, altered and improved from time to time, Lilienthal, during the next five years, made more than two thousand successful glides. At first he used to jump off a spring-board; then he practised on some hills in the suburbs of Berlin; then, in the spring of 1894, he built a conical hill at Gross-Lichterfelde to serve him as a starting-ground. Later on, he moved to the Rhinow hills. His best glides were made against a light breeze at a gradient of about 1 in 10; and he could easily travel a hundred yards through the air. 'Regulating the centre of gravity', he says, 'becomes a second nature, like balancing on a bicycle; it is entirely a matter of practice and experience.' His most alarming experiences were from gusts of wind which would suddenly raise him many metres in the air and suspend him in a stationary position. But his skill was so great that he always succeeded in resuming his flight and alighting safely. He continued to improve and develop his machine. He made a double-surface glider, on the biplane principle, and flew on it. He experimented with engines, intended to flap the extremities of the wings--first a steam-engine of two horse-power, weighing forty-four pounds, then a simpler and lighter type, worked by compressed carbonic acid gas. But he explains that these can be safely introduced only if they do not impair the gliding efficiency of the machine, and he does not seem to have made much progress with them. His last improvement was a movable horizontal tail, or elevator, worked by a line attached to his head, to control the fore-and-aft balance of the machine. This fresh complexity was perhaps the cause of his death. On the 9th of August 1896 he started on a long glide from a hill about a hundred feet high; a sudden gust of wind caught him, and it is supposed that the involuntary movements of his head in the effort to regain his balance made matters worse; the machine plunged to the ground, and he was fatally injured. Lilienthal was a good mathematician, a careful recorder of the results of his experiments, and a disinterested student of nature. Complete success was denied to him, but his work informed and stimulated others. The Wright brothers, when they first took up the problem of flight, had the advantage of acquaintance with Professor S. P. Langley's aeronautical researches, but their gliding experiments were shaped and inspired by what they had read of Lilienthal's achievements.