ITS ADVANTAGES.--Then the vertical height of the machine is much less than in the biplane. As a result the weight, which is farther below the supporting surface than in the biplane, aids in maintaining the lateral stability, particularly since the supporting frame is higher. Usually, for the same wing spread, the monoplane is narrower, laterally, which is a further aid to prevent tilting. ITS DISADVANTAGES.--But it also has disadvantages which must be apparent from its structure. As all the supporting surface is concentrated in half the number of planes, they must be made of greater width fore and aft, and this, as we shall see, later on, proves to be a disadvantage. It is also doubted whether the monoplane can be made as strong structurally as the other form, owing to the lack of the truss formation which is the strong point with the superposed frame. A truss is a form of construction where braces can be used from one member to the next, so as to brace and stiffen the whole. THE BIPLANE.--Nature does not furnish a type of creature which has superposed wings. In this particular the inventor surely did not follow nature. The reasons which led man to employ this type may be summarized as follows: In experimenting with planes it is found that a broad fore and aft surface will not lift as much as a narrow plane. This subject is fully explained in the chapter on The Lifting Surfaces of Planes. In view of that the technical descriptions of the operation will not be touched upon at this place, except so far as it may be necessary to set forth the present subject. This peculiarity is due to the accumulation of a mass of moving air at the rear end of the plane, which detracts from its lifting power. As it would be a point of structural weakness to make the wings narrow and very long, Wenham many years ago suggested the idea of placing one plane above the other, and later on Chanute, an engineer, used that form almost exclusively, in experimenting with his gliders. It was due to his influence that the Wrights adopted that form in their gliding experiments, and later on constructed their successful flyers in that manner. Originally the monoplane was the type generally employed by experimenters, such as Lilienthal, and others. STABILITY IN BIPLANES.--Biplanes are not naturally as stable laterally as the monoplane. The reason is, that a downward tilt has the benefit of only a narrow surface, comparable with the monoplane, which has broadness of wing. To illustrate this, let us assume that we have a biplane with planes five feet from front to rear, and thirty-six feet in length. This would give two planes with a sustaining surface of 360 square feet. The monoplane would, probably, divide this area into one plane eight and a half feet from front to rear, and 42 feet in length.