The helix advances in the direction of its axis.

Is the axis vertical?
Then it moves vertically.

Is the axis horizontal?
Then it moves horizontally.
The whole of Robur's flying apparatus depended on these two movements, as will be seen from the following detailed description, which can be divided under three heads--the platform, the engines of suspension and propulsion, and the machinery.
Platform .-- This was a framework a hundred feet long and twelve wide, a ship's deck in fact, with a projecting prow.

Beneath was a hull solidly built, enclosing the engines, stores, and provisions of all sorts, including the watertanks.

Round the deck a few light uprights supported a wire trellis that did duty for bulwarks.