the machine forward at a high velocity; that very large aëroplanes, if well made and placed at a proper angle, will lift as much as 2½ pounds per square foot at a velocity not greater than 40 miles an hour; also that it is possible for a machine to be made so light and at the same time so powerful that it will lift not only its own weight but a considerable amount besides, with no other energy except that derived from its own engines. Therefore there can be no question but that a flying machine is now possible without the aid of a balloon in any form."
In 1891, Professor Langley published his now famous memoir entitled Experiments in Aëodynamics, and in 1893 his equally celebrated book on The Internal Work of the Wind. The experiments upon which many of the statements in these books were based were begun in 1887, and from 1891 to 1896 Professor Langley was more or less constantly at work perfecting a model flying machine, which finally culminated in his aërodrome. This machine made a flight of three-quarters of a mile on November 28, 1896, and is described as follows in the Aëronautical Annual, 1897:
"The weight, with fuel and water sufficient for the flights described, is about 30 pounds. The weight of the engine and boiler together is about 7 pounds. The power of the engine under full steam is rather more than one horse-power. There are two cylinders, each having a diameter of 1¼ inches. The piston stroke is 2 inches. The two screws are 39 inches from tip to tip, and are made to revolve in opposite directions; the pitch is 1¼: they are connected to the engines by bevel gears most carefully made; the shafts and gears are so arranged that the synchronous movement of the two screws is secured. The boiler is a coil of copper tubing; the diameter of the coil externally is 3 inches: the diameter of the tubing externally is ⅜ inch: the pressure of steam when the aerodrome is in flight varies from 110 to 150 pounds to the square inch. The flame is produced by the æoli-pile, which is a modification of the naphtha "blow-torch"' used by plumbers; the heat of this flame is about 2000° F. Four pounds of water are carried at starting, and about ten ounces of naphtha. In action the boiler evaporates about one pound of water per minute."
The two most valuable sets of experiments conducted with large aërocurves capable of carrying one man are those of Herr Otto Lilienthal of Germany and Mr. Octave Chanute. a well known American engineer. Practically the same methods of carrying out their experiments were employed by both of these gentlemen, although the machines experimented with were quite different in form; and they are briefly described by Mr. Chanute, as follows:
"The method of carrying on these adventures is for the operator to place himself within and under the apparatus, which should preferably be light enough to be easily carried on the shoulders or by the hands, and to face the wind on a hillside. The operator should in no wise be attached to the machine. He may be suspended by his arms, or sit upon a seat, or stand on a dependent running board, but he must be able to disengage himself instantly from the machine should anything go wrong, and be able to come down upon his legs in landing.
"Facing dead into the wind, and keeping the front edge of the supporting surfaces depressed, so that the wind shall blow upon their backs and press them downward, the operator first adjusts his apparatus and himself to the veering wind. He has to struggle to obtain a poise, and in a moment of relative steadiness he runs forward a few steps as fast as he may, and launches himself upon the breeze, by raising up the front edge of the sustaining surfaces, so as to receive the wind from beneath at a very small angle (2 to 4 degrees) of incidence. If the surfaces and wind be adequate, he finds himself thoroughly sustained, and then sails forward on a descending or undulating course, under the combined effects of gravity and of the opposing wind. By shifting either his body or his wings, or both, he can direct his descent, either sideways or up or down, within certain limits; he can cause the apparatus to sweep upward so as to clear an obstacle, and he is not infrequently lifted up several feet by a swelling of the wind. The course of the glide eventually brings the apparatus within a few feet of the ground (6 to 10 feet), when the operator, by throwing his weight backward, or his wings forward, if they be movable, causes the front of the supporting surfaces to tilt up to a greater angle of incidence, thus increasing the wind resistance, slowing the forward motion, and enabling him, by a slight oscillation, to drop to the ground as gently as if he had fallen only one or two feet."
With the machine shown in the illustration Herr Lilienthal, starting from a height, was able to sail several hundred feet — the flight in some instances being against a wind of 24 miles per hour — and to make turns to the right or left with considerable certainty. Mr. Chanute's experiments were conducted first with a machine like Herr Lilienthal's but with one pair of wings only; second, with a machine having five pairs of wings, one above the other, and a sixth pair forming a tail; third, with a machine consisting of two wings, one above the other, and without any break in the middle, as shown in the first of the two illustrations of his apparatus: and finally with a large bird-like structure of the form shown in the illustration. The greatest success, perhaps, must be credited to the double-winged machine, which made numerous flights, some of them against winds of from 10 to 31 miles per hour. The longest flight made was 359 feet, from a starting point 62 feet higher than the point of landing.
Bibliography. As most of those who have carried on aëronautical experiments and have made systematic balloon ascents have been either scientific or military men, there is a valuable literature. Consult Hattan Turnor, Astra Castra: Experiments and Adventures in the Atmosphere (London, 1865): T. Glashier, Voyages aériens (London, 1871); Tissandier, Les ballons dirigibles (Paris, 1872); Coxwell, My Life and Balloon Experiences (London, 1888); Pettigrew, Animal Locomotion (New York, 1872); S. P. Langley, Aërodynamics and Internal Work of the Wind, Smithsonian Institution (Washington, 1891); O. Chanute, Progress in Flyinq Machines; Proceedings of the International Conference on Aërial Navigation; The Aëronautical Annual (London, 1895-97); the Proceedings of the Aëronautical Society of Great Britain; the Balloon Society of Great Britain; Academie d'Aérostation of France and the German Aëronautical Society. Among the periodicals devoted to aëro-
