These experiments, which destroyed the calculations of Newton, long held to be correct, showed that mechanical flight was at least theoretically possible, indicating as it did that a weight of two hundred pounds could be moved through the air at express-train speed with the expenditure of only one horse-power of energy. Since engines could be constructed weighing less than twenty pounds to the horse-power, theoretically such an engine should support ten times its own weight in horizontal flight in an absolute calm. As a matter of fact there is no such thing as an absolute calm in nature, air-currents being constantly stirring even on the calmest day, and this introduces another element in attaining aerial flight that is an all-important one. Indeed it has long been recognized that the mechanical power for flight is not the only requisite for flying--there is, besides, the art of handling that power. EXPERIMENTS IN SOARING Those who have watched soaring birds sail for hours on rigidly extended wings will remember that while there is no flying movement, there are certain shifts of the rigid body, either to offset some unexpected gust of wind, or to produce movement in a desired direction. There is an art of balancing here that has become instinctive in the bird by long practice which could not be hoped for in the same degree in a mechanical device, and which man could hope to acquire only by practice. But in the nature of the case man has little chance to learn this art of balancing in the air, and it is for this reason that the many members of the balloonist school advocate the inflated bag in place of the aeroplane. The argument advanced by them is that since man has no chance naturally to acquire familiarity with balancing in the air, the simplest and best way for him to acquire it is by making balloon ascensions. When he has acquired sufficient skill he can gradually reduce the lifting part of his flying-machine, or gas-bag, gradually increasing the aeroplane or other means of propulsion and lifting, until the balloon part of his device can be dispensed with entirely. In short, this argument of the balloon advocates is comparable to two schools of swimming-teachers, one of whom advocates the use of sustaining floats until the knack of swimming is acquired, the other depending upon the use only of muscular movements and quickly acquired skill. In this comparison the aviators have all the best of the argument; for it is a common observation that persons who attempt to learn to swim by the use of floats of any kind acquire that art slowly if at all; while those who plunge in boldly, although they run more risks, quickly learn the art that seems ridiculously easy when once acquired. [Illustration: LEARNING HOW TO FLY.