It may not be out of place here to consider what Constitutes a plane, as that term is used in aviation. It is that part of the aeroplane, the pressure of the air upon the surface of which, lifts and sustains the aeroplane aloft. The plane may take a variety of forms; it may be curved or its parts may meet in an angle; it may be uniform and unbroken in shape or divided into parts. The two wings of a bird would constitute a monoplane, when they are in a horizontal position for soaring, or when the tips are uplifted and they form an angle like a broad V, called a dihedral angle. If the aeroplane has two such planes, one back of the other, it is still called a monoplane, or, more definitely, a tandem monoplane; but if one of the planes is above the other it is called a biplane. A similar arrangement of three planes, one above the other, could be called a triplane and one of several planes a multiplane. Essentials of the Aeroplane _The planes_, as already described are, of course, a necessary part of the aeroplane. _The propeller_ supplies motive power to the aeroplane. This moves in a circle much like the blades of the electric fan or the propeller of a motor boat or modern stern ship. By driving the air backward it propels the aeroplane forward. While the blades of the propeller are of considerable length they are usually inconspicuous in photographs, and as one who has never seen an aeroplane looks at a photograph he naturally asks, “What moves it through the air?” The propeller is driven by the engine. _The engine_ is usually of the gasoline type which develops high power with light weight, frequently one horse power for every three pounds of weight and in rare instances as high as one horse power for every pound of weight. These powerful little engines are marvels of mechanism and they have had much to do in the rapid modern progress of aeronautics. _The rudder_, as its name indicates, guides the aeroplane in its flight. It consists in the main of small horizontal and vertical planes under the control of the pilot. These may be in the front of the machine, but they are usually placed in the rear. By skillful manipulation of these the aeroplane can be guided upward, downward, to right or left at will. It is also guided and controlled as we shall see, by the “warping” or “curving” of the wings or planes. The Wright Brothers and Their Problem