210 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1914. 
30 per cent and 50 per cent back of the front edge of the surface. The 
location of the center of pressure on any given surface is definitely 
fixed by the angle of incidence at which the surface is exposed to 
the air. 
The placing of the center of gravity of the machine below its cen- 
ter of support appears, at first glance, to be a solution of the problem 
of equilibrium. This is the method used in maintaining equilibrium 
in marine vessels and in balloons and airships, but in flying machines 
it has the opposite of the desired effect. If a flying machine con- 
sisting of a supporting surface, without elevator or other means of 
balancing, were descending vertically as a parachute, the center of 
eravity vertically beneath the center of support would maintain its 
equilibrium. But as soon as the machine begins to move forward 
the center of pressure, instead of remaining at the center of the sur- 
faces, as was the case when descending vertically, moves toward that 
edge of the surface which is in advance. The center of gravity being 
located at the center of the surface and the center of pressure in ad- 
vance of the center of the surface, a turning moment is created which 
tends to lift the front of the machine, thus exposing the surfaces at 
a larger angle of incidence and at the same time to a greater resist- 
ance to forward movement. The momentum of the machine, acting 
through its center of gravity below the center of forward resistance, 
combines with the forward center of pressure in causing the surface to 
be rotated about its lateral axis. The machine will take an upward 
course until it finally comes to a standstill. The rear edge of the 
surface will now be below that of the front edge and the machine 
will begin to slide backward. The center of pressure immediately 
reverses and travels toward the rear edge of the surface, which now 
in the backward movement has become the front edge. The center 
of gravity again being back of the center of pressure, the advancing 
edge of the surface will be lifted as before, and the pendulum effect 
of the low weight will be repeated. A flying machine with a low cen- 
ter of gravity, without rudders or other means to maintain its equi- 
librium, will oscillate back and forth in this manner until it finally 
falls to the ground. 
It will have been observed from the foregoing that the equilibrium 
in the horizontal plane was disturbed by two turning moments acting 
about the lateral horizontal axis of the machine; one produced by the 
force of gravity and the lift of the surface acting in different vertical 
lines, and the other by the center of momentum and the center of 
resistance acting in different horizontal lines. 
It is evident that a low center of gravity is a disturbing instead of a 
correcting agent. The ideal form of flying machine would be one in 
which the center of gravity lies in the line of the center of resistance 
to forward movement and in the line of thrust. In practice this is not 
