THE CENTER OF PRESSURE.--This is a term used to indicate the place on the plane where the air acts with the greatest force. It has reference to a point between the front and rear margins only of the plane. _Fig. 24. Air lines below a concaved Plane._ UTILIZING THE RAREFIED AREA.--This structure, however, has another important advantage, as it utilizes the rarefied area which is produced, and which may be understood by reference to Fig. 25. The plane B, with its upward curve, and at the same angle as the straight plane, has its lower end so curved, with relation to the forward movement, that the air, in rushing past the upper end, cannot follow the curve rapidly enough to maintain the same density along C, hence this exerts an upward pull, due to the rarefied area, which serves as a lifting force, as well as the compressed mass beneath the plane. CHANGING CENTER OF PRESSURE.--The center of pressure is not constant. It changes with the angle of the plane, but the range is considerably less on a concave surface than on a flat plane. _Fig. 25. Air lines above a convex Plane._ In a plane disposed at a small angle, A, as in Fig. 26, the center of pressure is nearer the forward end of the plane than with a greater positive angle of incidence, as in Fig. 27, and when the plane is in a normal flying angle, it is at the center, or at a point midway between the margins. PLANE MONSTROSITIES.--Growing out of the idea that the wing in nature must be faithfully copied, it is believed by many that a plane with a pronounced thickness at its forward margin is one of the secrets of bird flight. Accordingly certain inventors have designed types of wings which are shown in Figs. 28 and 29. _Fig. 28 Changing centers of Pressures._ _Fig 29. Bird-wing structures._ Both of these types have pronounced bulges, designed to "split" the air, forgetting, apparently, that in other parts of the machine every effort is made to prevent head resistance. THE BIRD WING STRUCTURE.--The advocates of such construction maintain that the forward edge of the plane must forcibly drive the air column apart, because the bird wing is so made, and that while it may not appear exactly logical, still there is something about it which seems to do the work, and for that reason it is largely adopted. WHY THE BIRD'S WING HAS A PRONOUNCED BULGE.--Let us examine this claim. The bone which supports the entire wing surface, called the (pectoral), has a heavy duty to perform. It is so constructed that it must withstand an extraordinary torsional strain, being located at the forward portion of the wing surface. Torsion has reference to a twisting motion. In some cases, as in the bat, this primary bone has an attachment to the rear of the main joint, where the rear margin of the wing is attached to the leg of the animal, thus giving it a support and the main bone is, therefore, relieved of this torsional stress.