oming to rest in little
more than one second.

The question of navigation solved, the two next devoted themselves to
perfecting the "X-plosive bullet," as Seaton called it. From his notes
and equations Seaton calculated the weight of copper necessary to exert
the explosive force of one pound of nitro-glycerin, and weighed out, on
the most delicate assay-balance made, various fractions and multiples of
this amount of the treated copper, while Crane fitted up the bullets of
automatic-pistol cartridges to receive the charges and to explode them
on impact.

They placed their blueprints and working notes in the safe, as usual,
taking with them only those notes dealing with the object-compass and
the X-plosive bullet, upon which they were still working. No one except
Shiro knew that the original tracings, from which the blue-prints had
been made, and their final, classified notes were always kept in the
vault. They cautioned him and the three guards to keep a close watch
until they returned. Then they set out in the biplane, to try out the
new weapon in a lonely place where the exploding shells could do no
damage.

* * * * *

They found that the X-plosive came fully up to expectations. The
smallest charge they had prepared, fired by Crane at a great stump a
full hundred yards away from the bare, flat-topped knoll that had
afforded them a landing-place, tore it bodily from the ground and
reduced it to splinters, while the force of the explosion made the two
men stagger.

"She sure is big medicine!" laughed Seaton. "Wonder what a real one will
do?" and drawing his pistol, he inserted a cartridge carrying a much
heavier charge.

"Better be careful with the big ones," cautioned Crane. "What are you
going to shoot at?"

"That rock over there," pointing to a huge boulder half a mile away
across the small valley. "Want to bet me a dinner I can't hit it?"

"No. You forget that I saw you win the pistol trophy of the District."

The pistol cracked, and when the bullet reached its destination the
great stone was obliterated in a vast ball of flame. After a moment
there was a deafening report--a crash as though the world were falling
to pieces. Both men were hurled violently backward, stumbling and
falling flat. Picking themselves up, they looked across the valley at
the place where the boulder had stood, to see only an immense cloud of
dust, which slowly blew away, revealing a huge hole in the