him now? He had no idea of the principle on which it operated;
he could only hope.

Then before him he saw the ghostly gray of light and squirmed with
renewed vigor--to be faced then by a grille, beyond which was the open
world. Once more his knife came into use as he pried and dug at the
barrier. He worked for long moments until the grille splashed out into
the sluggish current a foot or so below, and then he made ready to
lower himself into the same flood.

It was only because he was a trained hunter that he avoided death in
that moment. Some instinct made him dodge even as he slipped through,
and the hurtling black box did not strike true at the base of his
brain but raked along his scalp, tearing the flesh and sending him
tumbling unconscious into the brown water.

14

THE PRISONER

Raf was two streets away from the circling box but still able to keep
it in sight when its easy glide stopped, and, in a straight line, it
swooped toward a roof emitting a shrill, rising whistle. It rose again
a few seconds later as if baffled, but it continued to hover at that
point, keening forth its warning. The pilot reached the next
building, but a street still kept him away from the conical structure
above which the box now hung.

Undecided, he stayed where he was. Should he go down to street level
and investigate? Before he had quite made up his mind he saw the
foremost of the alien scouting party round into the thoroughfare below
and move purposefully at the cone tower, weapons to the fore. Judging
by their attitude, the box had run to earth there the prey they had
been searching for.

But it wasn't to be so easy. With another eerie howl the machine
soared once more and bobbed completely over the cone to the street
which must lie beyond it. Raf knew that he could not miss the end of
the chase and started on a detour along the roof tops which should
bring him to a vantage point. By the time he had made that journey he
found himself on a warehouse roof which projected over the edge of the
river.

From a point farther downstream a small boat was putting out. Two of
the aliens paddled while a third crouched in the bow. A second party
was picking its way along the bank some distance away, both groups
seemingly heading toward a point a building or two to the left of the
one where Raf had taken cover.

He heard the shrilling of the box, saw it bobbing along a line toward
the river. But in that direction there was on