For a little while he could not banish the thought of possible accidents from his mind.

Throb, throb, throb--beat; suppose some trivial screw went wrong in that supporting engine! Suppose--! He made a grim effort to dismiss all such suppositions.

After a while they did at least abandon the foreground of his thoughts.

And up he went steadily, higher and higher into the clear air.
Once the mental shock of moving unsupported through the air was over, his sensations ceased to be unpleasant, became very speedily pleasurable.

He had been warned of air sickness.