2d and 3d stream lines. These stream lines are
taken by imagining that the air approaches and
contacts with the plane only along the lines indicated
in the sketch, although they also in practice
are active against every part of the plane.

THE CENTER OF PRESSURE.--In such a plane the
center of pressure is near its upper end, probably
near the line 3, so that the greater portion of the
lift is exerted by that part of the plane above
line 3.

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