574 FAMOUS LIVING AMERICANS faces are most efficient at one angle, others at other angles. The shape of the edge also makes a difference, so that thous- ands of oombinations are possible in so simple a thing as a wing. They studied the flights of birds. Many hours of many afternoons they spent lying flat on their backs watching the birds wheel, circle, and soar, unconscious of the spying eyes which were slowly catching their secrets and would one day successfully claim the supremacy of the air. Their results are told by Orville Wright himself. A bird is really an aero- plane. The portions of its wings near the body are used as planes of support, while the more flexible parts outside, when flapped, act as propellers. Some of the soaring birds are lit- tle more than animated sailing machines and few can rise from the ground without a nmning start. Everyone who has been outdoors has seen a buzzard or a hawk soaring ; or ev- eryone at sea has seen the gulls sailing after a steamship for miles with scarcely a movement of the wings. All these birds are doing the same thing — they are balancing on rising cur- rents of air. The buzzards and hawks find the currents blow- ing upward off the land; the gulls that follow the steamers from New York to Florida are merely sliding downhill a thousand miles on rising currents in the wake of the steamer, and on the hot air rising from her smoke-stacks. On clear, warm days the buzzards find the high, rotary, rising currents of air, and go sailing around and around in them. On damp, windy days they hang above the edge of a steep hill on the air which comes rising up its slope. From their position in the air they can glide down at will. The brothers studied the various principles of balancing, and pored over the best data obtainable, but again and again their experiments failed. Finally they cast aside as little more than guesswork the existing tables and began experi- menting for a new law of aerodynamics. Small sheets of steel of different sizes and shapes were delicately balanced in a long tube through which steady currents of air were blown. Then by changing the angles and speeds of air they noted down carefully the results. By studying the mass of figures
