“The balancing of a gliding or flying machine is very simple in theory; it merely consists in causing the centre of pressure to coincide with the centre of gravity; but in actual practice there seems to be an almost boundless incompatibility of temperament between the two, which prevents their remaining peaceably together for a single instant.” [Illustration: FIG. 24.--Centres of gravity and pressure.] Here, in a sentence, is the problem. As a cambered plane is moved in flight, the air-pressure upon it is not disposed equally over the surface, but tends to locate itself at a spot to the front of the middle line of the plane. When a plane is at a normal inclination to the air, indeed, this centre of pressure, as it is called, is at a point upon the surface about one-third of the distance between the front and rear edges. As to the centre of gravity, the second factor in the problem, this may be explained best, perhaps, by a practical illustration. Take a small sheet of cartridge paper, cut to represent the plane of a flying machine, and lay this along the blade of a knife. By moving it to and fro and adjusting its equilibrium, you will be able to make it rest upon the knife-edge without falling forward or backward; this point at which it balances itself represents its centre of gravity. Here, then, are the forces: the centre of pressure, which is the thrust of the air, and the centre of gravity, which is the pull of the earth seeking to drag down a machine when in flight. These two forces must, as we have been told, be made to coincide. In the next illustration (Fig. 24) the problem is made clearer. In diagram A is seen a plane A.B. which is moving through the air in the direction indicated by the arrow. The two forces--that is to say, the centre of pressure (C.P.) and the centre of gravity (C.G.)--coincide with each other: therefore the plane is in equilibrium. But now suppose a gust of wind strikes the plane. This tends to tilt it upward; and the result is that the centres of pressure and gravity show that “boundless incompatibility of temperament” of which Wilbur Wright complained. The impact of the gust, making the plane rear up, throws the centre of pressure farther back along its surface, as is shown in diagram B. The plane is at once out of balance. Or it may be argued that, as it passes through the air, the wind pressure under the plane is suddenly lessened. This would cause its front edge to drop; whereupon the centre of pressure would, as is seen in diagram C, move immediately forward upon the plane--and so throw it out of balance again.