recesses down there.
He found nothing, not even a speck of dust.

Back in his chair again he began to doubt his first conviction. Perhaps
there really had been some kind of an invisible force or entity outside
the bubble. Both Horne and Silverman had said that "it" had tried to get
in to kill them.

They had been very definite about that part.

* * * * *

There were six windows around the bubble's walls, set there to enable
the attendant to see all the outside-mounted instruments and dials. He
went to them to look out, one by one, and from all of them he saw the
same vast emptiness that surrounded him. The galaxy--his galaxy--was so
far away that its stars were like dust. In the other directions the
empty gulf was so wide that galaxies and clusters of galaxies were tiny,
feeble specks of light shining across it.

All around him was a void so huge that galaxies were only specks in
it....

Who could know what forces or dangers might be waiting out there?

A light blinked, reminding him it was time to attend to his duties. The
job required an hour and he was nervous and not yet hungry when he had
finished. He went to the exerciser springs on the wall and performed a
work-out that left him tired and sweating but which, at least, gave him
a small appetite.

The day passed, and the next. He made another search of the bubble's
interior with the same results as before. He felt almost sure, then,
that there was nothing in the bubble with him. He established a routine
of work, pastime and sleep that made the first week pass fairly
comfortably but for the gnawing worry in his mind that something
invisible was lurking just outside the windows.

Then one day he accidentally kicked the wall with his metal shoe tip.

It made a sound like that from kicking a tight-stretched section of tin
and it seemed to him it gave a little from the impact, as tin would do.
He realized for the first time how thin it was--how deadly, dangerously
thin.

According to the specifications he had read it was only one-sixteenth of
an inch thick. It was as thin as cardboard.

He sat down with pencil and paper and began calculating. The bubble had
a surface area of 146,500 square inches and the internal air pressure
was fourteen pounds to the square inch. Which meant that the thin metal
skin contained a total pressure of 2,051,000 pounds.

Two million pounds.

The bubble in which he sat was a bomb, waiting to e