kpit of the
plane. The tank driver gave a grunt of satisfaction and turned the
nose of his vehicle in that direction. The second tank followed.

Hardly had they turned in the new direction before the ground began to
get soft under their tracks and the heavy vehicles began to sink. The
driver of the Doctor's tank forced it ahead, but the tank sank deeper
in the mire until water flowed in around the feet of the occupants.

"I reckon we'll have to get out and walk pretty soon, Doctor," said
the driver.

* * * * *

Dr. Bird grunted in acquiescence. The tank made its way forward a few
yards before the engine sputtered and died. The second tank stopped
when the first one did, fifty yards behind it. Donning vitrilene
helmets and taking vitrilene shields in their hands, the crews of both
tanks climbed out into the waist-deep water and gathered around the
Doctor for orders.

"Form a skirmish line at ten-pace intervals and cross the swamp," he
directed. "We may meet with no opposition, but if there is, the more
scattered we are, the safer we will be. You all have hand grenades as
well as your rifles?"

A murmur of assent answered him and the line formed and started across
the swamp. They had gone perhaps a hundred yards when three red lights
came from one of the planes circling overhead.

"Down!" cried the doctor, dropping to his knees in the muck.

Four hundred yards ahead of them a concrete platform emerged from the
marsh and rose slowly into the air. It was roofed with a dome of what
looked like plate glass, but which the doctor shrewdly suspected was
vitrilene. When the base of the platform was two-feet above the level
of the water the dome slid silently aside disclosing two men bending
over a tiny gun. Dr. Bird leveled his binoculars.

"That's the Breslau gun model that was stolen as sure as I'm a foot
high!" he cried. "They must have made some miniature shells and be
planning to fire it."

Slowly a pall of intense blackness rose from the marsh and enveloped
the platform and hid it from view. A whining noise came from overhead,
and then a crash like a thunderbolt. The blast of the explosion threw
the attackers face down in the swamp, and when they arose and looked
back there was merely a gaping hole where the leading tank had been.
The second tank suddenly seemed to rise in the air and fly into
millions of tiny fragments, and a second thunderous blast sent them
again to their knees