The propeller is whirled by the engine. Without the engine to whirl it the propeller is useless, for without _thrust_ we would have no _lift_. That makes the engine the governing factor in flight. _Weight_ also is a serious force in flight, and the Wrights and Curtiss found from the beginning that the four-cycle gasoline engine would give greater power for its weight than would a steam or electric engine. The principle of the airplane engine is the same as the one used in the automobile engine. However, weight always has been a problem to aircraft designers. The automobile engine always has been too heavy for use in a plane. When the Wrights built their first plane, automobile engines weighed 25 to 35 pounds per horsepower. The Wrights built one that weighed 13 pounds per horsepower and produced 12 horsepower. They used this engine in 1903 to power their first plane. Since that time all practical airplanes have been powered with gasoline engines, designed specifically for use in heavier-than-air machines. Since the first flight, engineers constantly have strived to produce engines with greater power and less weight per horsepower. How well they have succeeded is proved by the progress of the airplane. [Illustration: HOW THE FOUR-CYCLE ENGINE WORKS FOUR MOVEMENTS PRODUCE THE ENGINE’S POWER STROKE 1. Valve (1) opens for the INTAKE of gasoline and air vapor, and the piston sucks it into the cylinder. 2. Valve (1) closes and the piston pushes up, producing a COMPRESSION of the gas vapor in the cylinder. 3. Valves are closed and an electrical spark from the sparkplug explodes the gas, forcing the piston down in the POWER stroke that turns the crankshaft. This whirls the propeller. 4. Valve (2) opens and the piston rushes up, forcing out the EXHAUST gases.] It was in 1905 that the Wright Brothers had first offered to the Army a license to use their patents; but nothing came of it. Reports coming from Dayton during the next two years, concerning their flying activities, caused the newspapers to publish a number of articles about them. Theodore Roosevelt, then our President, was a diligent reader, and several articles about the Wrights attracted his attention. One day he clipped one of these articles from a newspaper and scribbled across it one word: “Investigate!” He passed it along to his Secretary of War, William Howard Taft. In a short time the almost forgotten Wright Brothers had a call from Brigadier General James Allen, U. S. Army Signal Corps. In the autumn of 1907 Wilbur Wright appeared in Washington to confer with the War Department.