Rockies. As the now rapidly cooling greyhound of the skies
passed over the western ranges of the Bitter Roots it became apparent
that her goal was a vast, flat-topped, and conical mountain, shrouded in
livid light; a mountain whose height awed even its stupendous neighbors.

While not artificial, the Hill had been altered markedly by the
Triplanetary engineers who had built into it the headquarters of the
Secret Service. Its mile-wide top was a jointless expanse of gray armor
steel; the steep, smooth surface of the truncated cone was a
continuation of the same immensely thick sheet of metal. No known
vehicle could climb that smooth, hard, forbidding slope of steel; no
known projectile could mar that armor; no known craft could even
approach the Hill without detection. Could not approach it at all, in
fact, for it was constantly inclosed in a vast hemisphere of lambent
violet flame through which neither material substance nor destructive
ray could pass.

As the _Silver Sliver_, crawling along at a bare three-hundred miles an
hour, approached that transparent, brilliantly violet wall of
destruction, a violet light filled her control room and as suddenly went
out; flashing on and off again and again.

"Giving us the once-over, eh?" Cleveland asked. "That is something new,
isn't it, Fred?"

"Yes, it's a high-powered ultra-wave spy," Rodenbush returned. "The
light is simply a warning, which can be carried if desired. It can also
carry voice and vision...."

"Like this," Samms' voice interrupted from the powerful dynamic speaker
upon the pilots' panel and his clear-cut face appeared upon the
television screen. "I don't suppose Fred thought to mention it, but this
is one of his inventions of the last few days. We are just trying it out
on you. It doesn't mean a thing though, as far as the _Sliver_ is
concerned. Come ahead!"

A circular opening appeared in the wall of force, an opening which
disappeared as soon as the plane had darted through it; and at the same
time her landing-cradle rose into the air through a great trap-door.
Slowly and gracefully the space-plane settled downward into that
cushioned embrace. Then cradle and nestled _Sliver_ sank from view and,
turning smoothly upon mighty trunnions, the plug of armor drove solidly
back into its place in the metal pavement of the mountain's lofty
summit. The cradle-elevator dropped rapidly, coming to rest many levels
down in the heart of the Hill, and Cleveland and Ro