on from the others, so as to avoid any tendency to
gyration. Hence the screws as they rose on the vertical column of air
retained their equilibrium by their horizontal resistance.
Consequently the apparatus was furnished with seventy-four suspensory
screws, whose three branches were connected by a metallic circle
which economized their motive force. In front and behind, mounted on
horizontal axes, were two propelling screws, each with four arms.
These screws were of much larger diameter than the suspensory ones,
but could be worked at quite their speed. In fact, the vessel
combined the systems of Cossus, La Landelle, and Ponton d'Amecourt, as
perfected by Robur. But it was in the choice and application of his
motive force that he could claim to be an inventor.

Machinery.--Robur had not availed himself of the vapor of water or
other liquids, nor compressed air and other mechanical motion. He
employed electricity, that agent which one day will be the soul of
the industrial world. But he required no electro-motor to produce it.
All he trusted to was piles and accumulators. What were the elements
of these piles, and what were the acids he used, Robur only knew. And
the construction of the accumulators was kept equally secret. Of what
were their positive and negative plates? None can say. The engineer
took good care--and not unreasonably--to keep his secret
unpatented. One thing was unmistakable, and that was that the piles
were of extraordinary strength; and the accumulators left those of
Faure-Sellon-Volckmar very far behind in yielding currents whose
amperes ran into figures up to then unknown. Thus there was obtained
a power to drive the screws and communicate a suspending and
propelling force in excess of all his requirements under any
circumstances.

But--it is as well to repeat it--this belonged entirely to Robur.
He kept it a close secret. And, if the president and secretary of the
Weldon Institute did not happen to discover it, it would probably be
lost to humanity.

It need not be shown that the apparatus possessed sufficient
stability. Its center of gravity proved that at once. There was no
danger of its making alarming angles with the horizontal, still less
of its capsizing.

And now for the metal used by Robur in the construction of his
aeronef--a name which can be exactly applied to the "Albatross."
What was this material, so hard that the bowie-knife of Phil Evans
could not scratch it, and Uncle Prudent