making preliminary measurements on a great number of different shaped surfaces, to secure a general understanding of the subject, they began systematic measurement of standard surfaces, so varied in design as to bring out the underlying causes of differences noted in their pressures. Measurements were tabulated on nearly fifty of these at all angles to the wind from zero to 45 degrees, at intervals of 21⁄2 degrees. Measurements were also secured showing the effects on each other when surfaces are superimposed or when they follow one another.

With this monumental work accomplished, they built a new glider embodying their most valuable results, and returned to Kill Devil Hill the next season.

In September and October 1902, nearly 1000 gliding flights were made, several of which covered distances of over 600 feet. Some made in perfect safety against a wind of 36 miles an hour, gave proof of the soundness of their controlling devices. In 1903 in strong winds they made a number of flights in which they remained in the air for over a minute, often soaring for a considerable time in one spot, without any descent at all. Little wonder that their unscientific assistant should think the only thing needed to keep the glider indefinitely in the air would be a coat of feathers to make it light.

With accurate data for making calculations, and a

system of balance effective in winds as well as in calms, they thought they were now in a position to build a successful powered machine. The first designs provided for a total weight of 600 lbs., including the operator and an eight horse-power motor. But, upon completion the motor gave more power than had been estimated for, and this allowed 150 lbs. to be added for strengthening the wings and other parts.

The carefully compiled air pressure tables made the designing of the wings an easy matter. But matters. were very much harder when it came to the airscrew or propeller. The Wrights thought they would get help from the marine engineers who had been using the screw-propeller for nearly a century, but they had only rule of thumb methods, which could not be applied to the air propeller. No real information of any sort was available, and the Wrights had neither the time nor the money to make lengthy experiments. With the machine moving forward, the air flying backward, the propellers turning sidewise, and nothing standing still, it seemed impossible to find a starting point for the analysis of the problem. Nowhere in their careers did the Wrights show a greater ability to think than at this stage of their work. After several months had passed, and every phase of the problem had been thrashed over and over, they developed a theory, based on the fact that the propeller blades

were themselves wings traveling in a spiral course; and their first propellers built entirely from calculation gave in useful work 66 percent of the power delivered by the motor. Their very first effort secured one-third more efficiency than had been obtained by Maxim or Langley, who had had infinitely greater resources at their command.

With the solution of the propeller problem, their main difficulties were over. The first powered machine the Wrights built was a modification of the glider. It had two main supporting surfaces. Each wing was forty feet in spread and six and a half feet wide, giving a total wing area of about five hundred and forty square feet. The elevator used to steer the machine up and down was placed ahead of the machine, instead of the better position now used at the tail end of the airplane. But the aviators had acquired an almost uncanny skill in feeling gusts that were about to strike the machine, and they were aided in this with the forward elevator. The steering vertical rudder was well behind the wings so as to give a large purchase. The curved wings were carefully braced with wooden uprights or struts, and steel wires. The motor had its four cylinders of 4 inch bore and 4 inch stroke placed horizontally, and developed some twelve horse-power, or about one-third the power of a Ford motor. The pilot was to lie flat on his

stomach so as to offer the least possible resistance to the air. Foreseeing accidents, they placed the motor on one side of the man, so that in case of plunge head first, the motor could not fall upon him. To provide against the machine rolling over forward in landing, skids were designed like sled runners, extending out in front of the main wings. The wings themselves were slightly curved or cambered, so as to give as smooth an air flow as possible. Two propellers were used, placed out on either side and driven by chains from the motor. They were driven in opposite directions to avoid gyroscopic effects. The whole machine, fully loaded with operator and fuel, weighed 745 to 750 pounds. Its maximum speed was about 31 miles per hour. They had no wheels to help in making a get-away, but used a species of catapault comprising a monorail, a towline and a falling weight to gain initial momentum.

The construction of the early Wright biplanes looks crude and homemade to a modern engineer. The power was tiny compared with the war planes carrying huge 400 horse-power Liberty motors-so famous in the great war-and the speed seems very small when we think of present day racing planes. But all the essentials were there. The Wrights had solved all the fundamental problems, all subsequent work has

been mainly a matter of development and refinement, not further invention.

Wilbur and Orville left Dayton on September 23, 1903, and arrived at their camp at Kill Devil Hill on Friday, the 25th. They found there provisions and tools, which had been shipped by freight several weeks in advance. Their building, erected in 1901 and enlarged in 1902, was found to have been blown by a storm from its foundation posts a few months previously. Their very first job was to put the old building in repair and erect a new one to serve as a workshop and for assembling and housing the machine.

As if the problem of flying the first airplane was not difficult enough in itself, the weather and everything else seemed to conspire against them. Some of the worst storms that had visited Kitty Hawk made matters all the harder. One night a wind of 75 miles an hour threatened to blow the roof off. When Orville, trying to nail the tar paper roof down, ascended a ladder and reached the edge of the roof, the wind caught under his large coat, blew it round his head and bound his arms till he was perfectly helpless. After Wilbur had rescued him, the wind was so strong that he could not guide his hammer and he succeeded in striking his fingers as often as the nails. They met all sorts of mechanical difficulties. Using their bicycle experience, they had to use tire cement