their muscles could not long aid them against the exhaustion which the
thin air was imposing on them. His thoughts spun for a moment to
Milton, in the laboratory behind, and then back to their own desperate
plight.

Abruptly shapes loomed in the misty light before them! A group of
three great Martians, reptilian shapes that had been coming toward
them and had stopped for an instant in amazement at sight of the
running pair. There was no time to halt themselves, to evade the
three, and with a mutual instinct Lanier and Randall seized together
the last expedient open to them. They ran straight forward toward the
astounded three, and when a half-score feet from them, leaped with all
their force upward and toward them, their tensed bodies flying through
the air with feet outstretched before them.

Then they had struck the group of three with feet-foremost, and with
the impetus of that great leap had knocked them sprawling to this side
and that, while with a supreme effort the two kept their balance and
leaped on. The cries of the three added to the din behind them as they
threw themselves forward.

* * * * *

They flung themselves past a last cone building to halt for an instant
in utter amazement despite the nearing pursuit. Before them were no
more streets and structures, but a huge smooth-flowing waterway! It
gleamed in the moonlight and lay at right angles across their path,
seeming to flow along the Martian city's edge.

"A canal!" cried Lanier. "It's one of the canals that meet at this
city and flow around it! We're trapped--we've reached the city's
edge!"

"Not yet!" Randall gasped. "Look!"

As he pointed to the left Lanier shot a glance there; and then both of
them were running in that direction, along the smooth metal paving
that bordered the mighty canal. They came to what Randall had seen, a
mighty metal arch that soared out over the waterway to its opposite
side. A bridge!

They were on it, were racing up the smooth incline of it. Randall
glanced back as they reached the arch's summit. From that height the
city stretched far away behind them, a lace of crimson lights in the
night. He glimpsed the gleam of the giant waterway that encircled the
city completely, one that was fed by other canals from far away that
emptied into it, the great city's vital water-supply brought thus from
this world's melting polar snows.

There were moving lights behind now, too, pouring out o