square, rectangular
proportions. Between them moved countless frog-hordes, swirling
throngs in streets and squares, and over the roofs darted thick swarms
of flying-boats. And at the city's center, in a great, circular, clear
space, lay a wide, round, green pool--the opening of the water-tunnel
up through which they had come.

Norman pointed down toward it. "That's your answer!" he cried. "The
only entrance to this frog-city is from the sea, up through that
water-tunnel!"

"Good God, an amphibian city!" Hackett was shaken, white-faced.

The two boats were driving quickly over the city, through the swarming
craft. Norman glimpsed towering buildings that might have been
palaces, temples, laboratories. They slowed and dipped toward one
block-like building not far from the water-tunnel's opening. Armed
frog-guards were on its roof, and other boats rested there. The two
came to rest and the two captives were jerked out, the guards seizing
them.

Half-dragged and half-floating they were led toward an opening in the
roof from which a stair led downward. They passed down thus into the
building's interior, lit by many windows. Norman glimpsed long halls
ending in barred doors, guards here and there. Tube-lines ran along
the walls and somewhere machines were throbbing dully. They came at
last to a barred door whose guard opened it at the croaking order of
the frog-men who held the two, and they were thrust inside, as the
door clanged. They turned, and exclaimed in amazement. The room held
fully a half-hundred men!

They were men such as the two fliers had never seen before, like
humans except that their skins were a light green instead of the
normal white and pink. They were dressed in dark short tunics, and
kept talking to each other in a tongue quite unintelligible to Norman
and Hackett. They came closer, flocking curiously around the two men,
with a babel of voices quite meaningless to the two. Then one of the
men uttered an exclamation, and all turned.

* * * * *

The barred door had swung open and a half-dozen frog-guards entered,
followed by two frog-men carrying a square little mechanism from which
tubing led back out through the door.

"Norman--these men--" Hackett was whispering rapidly. "If there are
men in this world too, it may be that--"

"Quiet, Hackett--look at what they're doing."

The two frog-men had set their mechanism in place and then croaked out
a brief word or o