This sensation is much greater in a balloon than in an
aeroplane, for a balloon, unless there is a strong wind blowing,
goes straight up, while an aeroplane ascends on a long slant, and
always into the teeth of the wind, to take advantage of its
lifting power on the underside of the planes. The reason for this
sensation--that of the earth's dropping down, instead of one's
feeling, what really happens, that one is ascending--is because
there are no objects by which comparison can be made. If one
starts off on the earth's surface at slow, or at great speed, one
passes stationary objects--houses, posts, trees, and the like--
and judges the speed by the rapidity with which these are left
behind.

Going up is unlike this. There is nothing to pass. One simply
cleaves the air, and only as it rushes past can one be sure of
movement. And as the air is void of color and form, there is no
sensation of passing anything.

So Mary Nestor, as she shot into the air with Tom Swift, had a
sensation as though the earth were dropping from beneath her. For
a moment she felt as though she were in some vast void--floating
in space--and she had a great fear. Then she calmed herself. She
looked at Tom sitting in front of her. Of course, all she could
see was his back, but it looked to be a very sturdy back, indeed,
and he sat there in the aircraft as calmly as though in a chair
on the ground. Then Mary took courage, and ceased to grasp the
sides of the cockpit with a grip that stiffened all her muscles.
She was beginning to "find herself."