sometimes the entire machine will suddenly drop
for many feet, while in flight.

ELASTICITY OF THE AIR.--Air is the most elastic
substance known. The particles constituting it
are constantly in motion. When heat or cold penetrate
the mass it does so, in a general way, so as
to permeate the entire body, but the conductivity
of the atmospheric gases is such that the heat
does not reach all parts at the same time.

AIR HOLES.--The result is that varying strata
of heat and cold seem to be superposed, and also
distributed along the route taken by a machine,
causing air currents which vary in direction and
intensity. When, therefore, a rapidly-moving
machine passes through an atmosphere so disturbed,
the surfaces of the planes strike a mass of
air moving, we may say, first toward the plane,
and the next instant the current is reversed, and
the machine drops, because its support is temporarily
gone, and the aviator experiences the sensation
of going into a "hole."

RESPONSIBILITY FOR ACCIDENTS.--These so-called
"holes" are responsible for many accidents. The
outstretched wings, many of them over forty feet
from tip to tip, offer opportunities for a tilt at one
end or the other, which has sent so many machines
to destruction.