时间: 2019年12月11日 09:39

Borelli鈥檚 study is divided into a series of propositions in which he traces the principles of flight, and the mechanical actions of the wings of birds. The most interesting of these are the propositions in which he sets forth the method in which birds move their wings during flight and the manner in which the air offers resistance to the stroke of the wing. With regard to the first of these two points he says: 鈥榃hen birds in repose rest on the earth their wings are folded up close against their flanks, but when wishing to start on their flight they first bend their legs and leap into the air. Whereupon the joints of their wings are straightened out to form a straight line at right angles to the lateral surface of the breast, so that the two wings, outstretched, are placed, as it were, like the arms of a cross to the body of the bird. Next, since the wings with their feathers attached form almost a plane surface, they are raised slightly above the horizontal, and with a most quick impulse beat down in a direction almost perpendicular to the wing-plane, upon the underlying air; and to so intense a beat the air, notwithstanding it to be fluid,23 offers resistance, partly by reason of its natural inertia, which seeks to retain it at rest, and partly because the particles of the air, compressed by the swiftness of the stroke, resist this compression by their elasticity, just like the hard ground. Hence the whole mass of the bird rebounds, making a fresh leap through the air; whence it follows that flight is simply a motion composed of successive leaps accomplished through the air. And I remark that a wing can easily beat the air in a direction almost perpendicular to its plane surface, although only a single one of the corners of the humerus bone is attached to the scapula, the whole extent of its base remaining free and loose, while the greater transverse feathers are joined to the lateral skin of the thorax. Nevertheless the wing can easily revolve about its base like unto a fan. Nor are there lacking tendon ligaments which restrain the feathers and prevent them from opening farther, in the same fashion that sheets hold in the sails of ships. No less admirable is nature鈥檚 cunning in unfolding and folding the wings upwards, for she folds them not laterally, but by moving upwards edgewise the osseous parts wherein the roots of the feathers are inserted; for thus, without encountering the air鈥檚 resistance the upward motion of the wing surface is made as with a sword, hence they can be uplifted with but small force. But thereafter when the wings are twisted by being drawn transversely and by the resistance of the air, they are flattened as has been declared and will be made manifest hereafter.鈥? PART I LIFE IN ENGLAND With all Charlotte鈥檚 gaiety and merriment, her delight in dancing and acting, and her love of games, there was a stern side, even in those early days, to her girlish nature; and in this respect she and Robert were well suited the one to the other. She was, as one says who knew her well,[28] 鈥榓 born heroine鈥? indeed, both she and Robert were of the stuff of which in former centuries martyrs have been made. An idea of the state of development arrived at about this time may be gained from the fact that the Commandant of the Military Wing of the Royal Flying Corps in a lecture before the Royal Aeronautical Society read in February, 1913, asked for single-seater scout aeroplanes with a speed of 90 miles an hour and a landing speed of 45 miles an hour鈥攁 performance which even two years later would have been considered modest in the extreme. It serves to show that, although higher performances were put up by individual machines on occasion, the general development had not yet reached the stage when such performances could be obtained in machines suitable for military purposes. So far as seaplanes were concerned, up to the beginning of 1913 little attempt had been made to study the novel problems involved, and the bulk of the machines at the Monaco Meeting in April, 1913, for instance, consisted of land301 machines fitted with floats, in many cases of a most primitive nature, without other alterations. Most of those which succeeded in leaving the water did so through sheer pull of engine power; while practically all were incapable of getting off except in a fair sea, which enabled the pilot to jump the machine into the air across the trough between two waves. Stability problems had not yet been considered, and in only one or two cases was fin area added at the rear high up, to counterbalance the effect of the floats low down in front. Both twin and single-float machines were used, while the flying boat was only just beginning to come into being from the workshops of Sopwith in Great Britain, Borel-Denhaut in France, and Curtiss in America. In view of the approaching importance of amphibious seaplanes, mention should be made of the flying boat (or 鈥榖at boat鈥?as it was called, following Rudyard Kipling) which was built by Sopwith in 1913 with a wheeled landing-carriage which could be wound up above the bottom surface of the boat so as to be out of the way when alighting on water. The final development of this three-cylinder radial was the 鈥榊鈥?type of engine, in which the cylinders were regularly disposed at 120 degrees apart; the bore was 4鈥?, stroke 4鈥? inches, and the power developed was 30 brake horse-power at 1,300 revolutions per minute. Well, I don't say they're all ugly. Some of 'em are handsome enough, and there's plenty of thorough-breds among 'em, but there's a sight of plain-headed ones. There's quite as much beauty in your spear as there is among the county folks, Miss Susan. I'll answer for that. 好看AV中文字幕在线观看,日本一本道a不卡免费,a片大全-知乎 I am 'driving' at nothing more than the simple sense of my words implies, she answered, with a marked shade of surprise in her countenance. "I mean just what I say. Is your work going pretty smoothly? Have you any complaints? Does your clerk do well?" So does Cleopatra, said Mrs. Kenyon, who had fallen into the habit of addressing her room-mate by the name she assumed. "Do you forward her letters to Mark Antony?" Five flights on the American continent up to the end of 1919 are worthy of note. On December 13th, 1918, Lieut. D. Godoy of the Chilian army left Santiago, Chili, crossed the Andes at a height of 19,700 feet and landed at Mendoza, the capital of the wine-growing province of Argentina. On April 19th, 1919, Captain E. F. White made the first non-stop flight between New York and Chicago in 6 hours 50 minutes on a D.H.4 machine driven by a twelve-cylinder Liberty engine. Early in August Major Schroeder, piloting a French Lepere machine flying at a height of 18,400 feet, reached a speed of 137 miles per hour with a Liberty motor fitted with a super-charger. Toward the end of August, Rex Marshall, on a Thomas-Morse biplane, starting from a height of 17,000 feet, made a glide of 35 miles with his engine cut off, restarting it when at a height of 600 feet above the ground. About a month later R. Rohlfe, piloting a Curtiss triplane, broke the height record by reaching 34,610 feet. No; I am waiting for you. For Heaven's sake, hush! exclaimed Kenyon, looking round him nervously.