In the very centre of the deck, however, was a low railing that protected the head of a staircase, and down this well the two looked.

“Shall I explain?” asked the old priest, smiling.  “This is the latest model, you know.  It has not been in use for more than a few months.”

The other nodded.

“Tell me everything, please.”

“Well, look right down there, below the second flight.  The first flight leads to the second-class deck, and the flight below to the working parts of the ship.  Now do you see that man’s head, straight in the middle, in the bright light?—­yes, immediately under.  Well, that’s the first engineer.  He’s in a glass compartment, you see, and can look down passages in every direction.  The gas arrangements are all in front of him, and the——­”

“Stop, please.  What power is it that drives the ship?  Is it lighter than air, or what?”

“Well, you see the entire framework of the ship is hollow.  Every single thing you see—­even the chairs and tables—­they’re all made of the metal aerolite (as it’s generally called).  It’s almost as thin as paper, and it’s far stronger than any steel.  Now it’s the framework of the ship that takes the place of the old balloon.  It’s infinitely safer, too, for it’s divided by automatically closing stops into tens of thousands of compartments, so a leak here and there makes practically no difference.  Well, when the ship’s at rest, as it is now, there’s simply air in all these tubes; but when it’s going to start, there is forced into these tubes, from the magazine below, the most volatile gas that has been discovered——­”

“What’s it called?”

“I forget the real name.  It’s generally called aeroline.  Well, this is forced in, until the specific gravity of the whole affair, passengers and all, is as nearly as possible the same as the specific gravity of the air.”

“I see.  Good Lord, how simple!”

“And the rest is done with planes and screws, driven by electricity.  The tail of the boat is a recent development.  (You’ll see it when we’re once started.) It’s exactly like the tail of a bird, and contracts and expands in every direction.  Then besides that there are two wings, one on each side, and these can be used, if necessary, in case the screws go wrong, as propellers.  But usually they are simply for balancing and gliding.  You see, barring collisions, there’s hardly the possibility of an accident.  If one set of things fails, there’s always something else to take its place.  At the very worst, we can but be blown about a bit.”

“But it’s exactly like a bird, then.”

“Of course, Monsignor,” said the priest, with twinkling eyes, “it isn’t likely that we could improve upon Almighty God’s design.  We’re very simple, you know. . . .  Look, he’s signalling.  We’re going to start.  Come to the prow.  We shall see better from there.”