the fingers of his left hand
to what appeared to be the keyboard of a typewriter set into the
instrument board. He pressed several of the keys and pulled back his
stick. There was a whistling scream from astern and I was thrown back in
my seat with painful force. With that, the motor roared into full speed
and we had left the airport far behind.

"What on earth?" I gasped.

"Rocket propulsion," laughed Hart. "I should have warned you. Those
tubes you saw outside at the tail and along the leading edge of the
wings. Only used three of them, but that was sufficient for the
take-off."

"But I thought this rocket business was not feasible on account of the
wastage of fuel due to its low efficiency," I objected.

"We should worry about fuel," said Hart.

I looked about me and saw that there was very little space for the
storage of this essential commodity. "Why?" I inquired. "What fuel do
you use?"

"Make our own," he replied shortly. He was busy at the moment,
maneuvering the _Pioneer_ into a position above and behind the SF-22 and
her convoy.

"You make your own fuel enroute?" I asked in astonishment.

"Yes. That sphere you saw on top. It is the collecting end of an
electrical system for extracting nitrogen and other elements, from the
air. This extraction goes on constantly while we are in the atmosphere
and my fuel is an extremely powerful explosive of which nitrates are the
base. The supply is replenished continuously, so we have no fear of
running short even in the upper levels."

* * * * *

George had crawled through a small opening into some inaccessible region
in the stern of the vessel. I pondered over what Hart had just told me,
still keeping my eyes glued to the port, through which could be seen the
fleet we were following. The altimeter registered thirty-five thousand
feet. We were entering air-level six--the stratosphere! Below us the
troposphere, divided into five levels, each of seven thousand feet,
teemed with the life of the air. The regular lanes were filled with
traffic, the lights of the speeding thousands of freight and pleasure
craft moving in orderly procession along their prescribed routes.

Up here in the sixth level, which was entirely for high-speed traffic of
commercial and government vessels making transcontinental or
transoceanic voyages, we were the only adventurers in sight--we and the
convoyed liner we were following. The speed indicator showed si