The result aimed at in the helicopter is the ability to rise vertically from the starting point, instead of first running along the ground for from 100 to 300 feet before sufficient speed to rise is attained, as the aeroplanes do. The device employed to accomplish this result is a propeller, or propellers, revolving horizontally above the machine. After the desired altitude is gained it is proposed to travel in any direction by changing the plane in which the propellers revolve to one having a small angle with the horizon. [Illustration: The force necessary to keep the aeroplane moving in its horizontal path is the same as that required to move the automobile of equal weight up the same gradient--much less than its total weight.] The great difficulty encountered with this type of machine is that the propellers must lift the entire weight. In the case of the aeroplane, the power of the engine is used to slide the plane up an incline of air, and for this much less power is required. For instance, the weight of a Curtiss biplane with the pilot on board is about 700 pounds, and this weight is easily slid up an inclined plane of air with a propeller thrust of about 240 pounds. Another difficulty is that the helicopter screws, in running at the start before they can attain speed sufficient to lift their load, have established downward currents of air with great velocity, in which the screws must run with much less efficiency. With the aeroplanes, on the contrary, their running gear enables them to run forward on the ground almost with the first revolution of the propeller, and as they increase their speed the currents--technically called the “slip”--become less and less as the engine speed increases. In the Cornu helicopter, which perhaps has come nearer to successful flight than any other, these downward currents are checked by interposing planes below, set at an angle determined by the operator. The glancing of the currents of air from the planes is expected to drive the helicopter horizontally through the air. At the same time these planes offer a large degree of resistance, and the engine power must be still further increased to overcome this, while preserving the lift of the entire weight. With an 8-cylinder Antoinette motor, said to be but 24 horse-power, turning two 20-foot propellers, the machine is reported as lifting itself and two persons--a total weight of 723 pounds--to a height of 5 feet, and sustaining itself for 1 minute. Upon the interposing of the planes to produce the horizontal motion the machine came immediately to the ground. [Illustration: Diagram showing principle of the Cornu helicopter. _P_, _P_, propelling planes. The arrow shows direction of travel with planes at angle shown.]