Higher it rose and higher.

Now it stood, upright, a slender towering pillar, a gigantic slim figure whose tip pointed a full hundred feet in the air.
Then slowly it inclined itself toward us; drew closer, closer to the ground; touched and lay there for an instant inert.

Abruptly it vanished.
But well I knew what I had seen.

The span over which we had passed had raised itself even as had the baby bridge of the fortress; had lifted itself across the chasm and dropping itself upon the hither verge had disintegrated into its units; was following us.
A bridge of metal that could build itself--and break itself.

A thinking, conscious metal bridge! A metal bridge with volition--with mind--that was following us.
There sighed from behind a soft, sustained wailing; rapidly it neared us.