s if we fly into them I'm sure I don't know. I
don't think they'll kill our motor. If whoever or whatever controls the
light column decides to us prisoners.... Well, we'll hope to have better
luck combating them than Kress had."

* * * * *

And so begin that hours-long vigil of quartering the stratosphere over
the unmarked area which Jeter had set as a limit. Now and again Hadley
spoke to Jeter. Yes, the demolitions were still continuing in Manhattan.
Could all telescopes on the ground pick out their space ship? Yes, said
Hadley, and a young scientist in New Jersey was constantly watching
them. Were they, since sunrise, ever out of his sight? Only when clouds
at comparatively low altitudes intervened. However, the sky was
unusually clear and it was hoped to keep their plane in sight during the
entire day.

"Hadley," Jeter almost whispered, "I'm satisfied we're above the area of
force, else we'd have flown into the anti-gravitation field. Get in
touch with that Jersey chap by direct personal wire or radiophone if he
is equipped with it. See that his watch is set with yours, which is
synchronised with ours. Got that?"

"Yes."

"When you've done that give him these instructions: He is never to take
his eyes of us for more than a split second at a time--unless someone
else takes his place. I doubt if, at this distance, this will work, but
it may help us a little. If we become invisible for even the briefest of
moments, he is to look at his watch and observe the exact time, even to
split seconds. We shall try to follow a certain plan hereafter in
quartering the stratosphere, and I shall mark our location on the
navigational charts every minute until we hear from this chap, or until
we decide nothing is to be accomplished by this trick. Understand?"

"You're hoping that the enemy, while invisible to all eyes, yet has
substance...."

"Shut up!" snapped Jeter, but he was glad that Hadley had grasped the
idea. It was a slim chance, but such as it was it was worth trying. If
the plane were invisible for a time, then it would be proof of some
opaque obstruction between the plane and the eye of the beholder on the
surface of the Earth. Refraction had to be figured, perhaps. Oh, there
were many arguments against it.

The fliers followed the very outer edge of the area above the world they
had mapped out as their limit of exploration. This circuit completed,
they banked inward, shortening thei