dwellers. For it had been discovered that the Moon
craters were extraordinarily rich in gold, and gold was still the
medium of exchange on Earth.

To supplement the vestigial atmosphere, huge stations had been set up,
which extracted the oxygen from the subterranean waters five miles
below the Moon's crust, and recombined it with the nitrogen with
which the surface layer was impregnated, thus creating an atmosphere
which was pumped to the workers.

Then a curious discovery had been made. It was impossible for human
beings to exist without the addition of those elements existing in the
air in minute quantities--neon, krypton, and argon. And the ships that
brought the gold bars back from the Moon had conveyed these gaseous
elements there.

* * * * *

The droning of the sixteen atomic motors grew louder, and mingled with
the hum of gyroscopes. The ladder was drawn up and the port hole
sealed. On the enclosed bridge Nat threw the switch of durobronze that
released the non-conducting shutter which gave play to the sixteen
great magnets. Swiftly the great ship shot forward into the air. The
droning of the motors became a shrill whine, and then, growing too
shrill for human ears to follow it, gave place to silence.

Nat set the speed lever to five hundred miles an hour, the utmost that
had been found possible in passing through the earth's atmosphere,
owing to the resistance, which tended to heat the vessel and damage
the delicate atomic engines. As soon as the ether was reached, the
speed would be increased to ten or twelve thousand. That meant a
twenty-two hour run to the Moon Colony--about the time usually taken.

He pressed a lever, which set bells ringing in all parts of the ship.
By means of a complicated mechanism, the air was exhausted from each
compartment in turn, and then replaced, and as the bells rang, the men
at work trooped out of these compartments consecutively. This had been
originated for the purpose of destroying any life dangerous to man
that might unwittingly have been imported from the Moon, but on one
occasion it had resulted in the discovery of a stowaway.

Then Nat descended the bridge to the upper deck. Here, on a platform,
were the two batteries of three ray-guns apiece, mounted on swivels,
and firing in any direction on the port and starboard sides
respectively. The guns were enclosed in a thin sheath of osmium,
through which the lethal rays penetrated unchang