As was to be expected in the very nature of the case, Lilienthal found that part of the secret of success lay in maintaining his equilibrium and in acquiring the faculty of doing this instinctively, as a bird does. But he found, like the person learning to ride a bicycle, that this was developed by repeated efforts. The action of the machine itself was carefully studied, and various changes were made in his apparatus from time to time as experience suggested them. Among other things, feather-like sails, worked by a small motor, were attached to the edge of the wings; and two smaller frames placed one above the other were tried in place of one large frame. And still the operator continued to make successful flights in all kinds of winds, sometimes narrowly escaping disaster, but for three years always coming to the ground safely. His confidence increased day by day, and as his remarkable performances multiplied it seemed as if it would only be a matter of time until he would be able to imitate the soaring bird and sail almost as he pleased. In writing of his experiences when, as it sometimes happened, he found himself practically motionless in the air at a point higher than that from which he started, he says: "I feel very certain that if I leaned a little to one side, and so described a circle, and further partook of the motion of the lifting air around me, I should sustain my position. The wind itself tends to direct this motion; but then it must be remembered that my chief object in the air is to overcome the tendency of turning to the right or left, because I know that behind or under me lies the hill from which I started, and with which I would come in rough contact if I allowed myself to attempt this circle-sailing. I have, however, made up my mind, by means of either stronger wind or by flapping the wings, to get higher up and farther away from the hills, so that sailing round in circles, I can follow the strong, uplifting current, and have sufficient air-space under and around me to complete with safety a circle, and lastly to come up against the wind again to land." Before he was ready to make this attempt, however, Lilienthal was killed by a fall caused by a treacherous gust of wind which tilted his machine beyond his control and hurled him to the ground.