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.