peated.

"Solved?" exclaimed the other from his place at the controls. "Man, it
is only begun!" He depressed a lever, and a muffled roar marked their
passage to a distant shaft of blue, where he turned the ship on end
and shot like a giant shell for the higher air.

There was northbound travel at thirty-five, and northward Harkness
would go, but he shot straight up. At forty thousand he motioned the
master-pilot to take over the helm.

"Clear through," he ordered; "up into the liner lanes; then north for
our own shop." Nor did he satisfy the curiosity in Chet Bullard's eyes
by so much as a word until some hours later when they floated down.

* * * * *

An icy waste was beneath them, where the sub-polar regions were
wrapped in the mantle of their endless winter. Here ships never
passed. Northward, toward the Pole, were liner lanes in the higher
levels, but here was a deserted sector. And here Walter Harkness had
elected to carry on his experiments.

A rise of land showed gaunt and black, and the pilot was guiding the
ship in a long slant upon it. He landed softly beside a building in a
sheltered, snow-filled valley.

Harkness shivered as he stepped from the warmth of their insulated
cabin, and he fumbled with shaking fingers to touch the combination
upon the locked door. It swung open, to close behind the men as they
stood in the warm, brightly-lighted room.

Nitro illuminators were hung from the ceiling, their diffused
brilliance shining down to reflect in sparkling curves and ribbons of
light from a silvery shape. It stood upon the floor, a metal cylinder
a hundred feet in length, whose blunt ends showed dark openings of
gaping ports. There were other open ports above and below and in
regular spacing about the rounded sides. No helicopters swung their
blades above; there were only the bulge of a conning tower and the
heavy inset glasses of the lookouts. Nor were there wings of any kind.
It might have been a projectile for some mammoth gun.

Harkness stood in silence before it, until he turned to smile at the
still-wondering pilot.

"Chet," he said, "it's about finished and ready--just in time. We've
built it, you and I; freighted in the parts ourselves and assembled
every piece. We've even built the shop: lucky the big steeloid plates
are so easily handled. And you and I are the only ones that know.

"Every ship in the airlanes of the world is driven by detonite--and we
have