216 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1914, 
lift and cause the machine to rise. The machine will now be turned 
upward, with the surfaces meeting the air at an angle of 5°. On the 
contrary, if the power of the motor be reduced or entirely turned 
off, the machine will immediately begin to decrease in speed, requiring 
a larger angle of incidence for support. But as soon as the angle 
begins to increase the air will strike the regulating vane on the under- 
side and the elevator will be turned, pointing the machine downward 
till the component of gravity in the direction of travel becomes 
sufficient to maintain the normal speed. In this case the planes 
will be inclined downward with reference to the horizontal. It is 
evident that a machine controlled by regulating the angle of the 
machine with reference to the impinging air is not lable to the dangers 
of ‘‘stalling”’ already described. 
Several other methods of maintaining fore-and-aft equilibrium 
automatically have been proposed. One utilizes the force of gravity 
acting on a pendulum or a tube of mercury; the other, the gyroscopic 
force of a rapidly revolving wheel. In both of these systems the angle 
of the machine is regulated with reference to the horizontal, or 
some other determined plane, instead of with the angle of the imping- 
ing air. 
In the case just referred to, in which the power of the motor was 
suddenly turned off while traveling on a level course, with these 
systems, the planes would be maintained at their original angle with 
the horizontal without any regard to the angle of incidence. The 
machine would continue forward till, through the loss of momentum, 
its speed would become so reduced and its angle of incidence so great 
that it would be exposed to the dangers of diving. 
The pendulum and mercury tube have other serious faults which 
render them useless for regulating fore-and-aft equilibrium. If the 
machine suddenly meet with a greater resistance to forward travel, 
either as a result of change in direction or of meeting a stronger gust 
of wind from in front, and its speed be ever so slightly checked, the 
pendulum will swing forward and instead of turning the machine 
downward, so as to maintain the normal speed, will cause the machine 
to be inclined upward in front and thus further increase its forward 
resistance. 
The pendulum has proved itself an exceedingly useful device, 
however, in regulating the lateral stability of aeroplanes. In this 
case the effects of momentum and centrifugal force act on the pendu- 
lum in the proper direction to produce desired results. 
I believe the day is near at hand when the flier will be almost 
entirely relieved of the work of maintaining the equilibrium of his 
machine, and that his attention will be required only to keeping it 
on its proper course and in bringing it safely in contact with the ground 
when landing. 
