OEVILLB WBIGHT 577 culty, they designed Buitable propellers^ with proper diameter, pitch, and area of blade. High efficiency in a screw-propeller is not dependent npon any particular or peculiar shape, and there is no such thing as a ^^best" screw. Every propeller must be designed to meet the particular conditions of the machine to which it is to be applied. This use of the screw-propeller appears to some aeronautical authorities to be the greatest weakness of the Wright designs because of the great difficulty of getting two wooden blades of the same resistance and pow- er. The severest injury which Orville Wright has sustained was caused by a fall from his machine when a propeller-blade snapped. The first flights with the power machine were made on De- cember 17, 1903. Five persons besides the inventors witness- ed the four flights. The first attempt lasted only twelve sec- onds, but in the last the machine sustained itself in the air for fifty-nine seconds and covered eight hundred and fifty-two feet of ground against a twenty-mile wind. In 1904, they made another machine with which they made the successful flights of 1904 and 1905 — over one hundred and fifty in all, averaging one mile each. They had not been flying long in 1904 before they found that the problem of equilibrium had not been fully solved. Sometimes, in making a circle, the machine would turn over sidewise in spite of anything the operator could do. When the causes of these troubles were finally overcome late the next year, the flights rapidly increased in length until the experiments were discontinued on account of the number of people attracted to the field. In May, 1907, experiments were resumed at Kitty Hawk. These flights were made to test the machine's ability to meet the requirements of the United States government which asked for a flyer capable of carry- ing two men and sufficient supplies for a flight of one hundred and twenty-five miles, with a speed of forty miles an hour. The machine used in these tests was the same one with which the flights were made near Dayton in 1905, though several changes had been made to meet present requirements. The
