AIR LINES ON THE UPPER SIDE OF THE PLANE.-- Now, another factor must be considered, namely, the effect produced on the upper side of the plane, over which a rarefied area is formed at certain points, and, in practice, this also produces, or should be utilized to effect a lift. RAREFIED AREA.--What is called a rarefied area, has reference to a state or condition of the atmosphere which has less than the normal pressure or quantity of air. Thus, the pressure at sea level, is about 14 3/4 per square inch As we ascend the pressure grows less, and the air is thus rarer, or, there is less of it. This is a condition which is normally found in the atmosphere. Several things tend to make a rarefied condition. One is altitude, to which we have just referred. Then heat will expand air, making it less dense, or lighter, so that it will move upwardly, to be replaced by a colder body of air. In aeronautics neither of these conditions is of any importance in considering the lifting power of aeroplane surfaces. RAREFACTION PRODUCED BY MOTION.--The third rarefied condition is produced by motion, and generally the area is very limited when brought about by this means. If, for instance, a plane is held horizontally and allowed to fall toward the earth, it will be retarded by two forces, namely, compression and rarefaction, the former acting on the under side of the plane, and the latter on the upper side. Of the two rarefaction is the most effectual, and produces a greater effect than compression. This may be proven by compressing air in a long pipe, and noting the difference in gauge pressure between the ends, and then using a suction pump on the same pipe. When a plane is forced through the air at any angle, a rarefied area is formed on the side which is opposite the one having the positive angle of incidence. If the plane can be so formed as to make a large and effective area it will add greatly to the value of the sustaining surface. Unfortunately, the long fiat plane does not lend any aid in this particular, as the stream line flows down along the top, as shown in Fig. 23, without being of any service. _Fig. 23. Air lines on the upper side of a Plane._ THE CONCAVED PLANE.--These considerations led to the adoption of the concaved plane formation, and for purposes of comparison the diagram, Fig. 24, shows the plane B of the same length and angle as the straight planes. In examining the successive stream lines it will be found that while the 1st, 2d and 3d lines have a little less angle of impact than the corresponding lines in the straight plane, the last lines, 5, 6 and 7, have much greater angles, so that only line 4 strikes the plane at the same angle. Such a plane structure would, therefore, have its center of pressure somewhere between the lines 3 and 4, and the lift being thus, practically, uniform over the surface, would be more effective.