but the machine-gun kept up a steady rattle of fire and the soldiers
kept their heads almost at the level of the water. There came the roar
of an airplane motor, and one of the planes swept over the platform, a
hundred yards in the air, with two machine-guns spraying streams of
bullets onto the platform. Two men abandoned their machine-gun and
crouched under the partially folded-back dome as the second plane
swept over, and Dr. Bird took advantage of the lull to advance his
party a few yards nearer. Again the defenders of the platform rushed
to their gun, but the first plane had turned and swooped down with
both guns going, and again they were forced to take shelter while the
Doctor and his force made another advance.

The second plane had turned and followed the first, but the defenders
had had enough. The transparent dome closed over them and the platform
sank into the marsh. With a shout, Dr. Bird led the way forward again.

The attackers were within a hundred yards of the platform when it
again rose above the surface of the water. The guns had disappeared,
but in their place stood an airship. It was a small affair with stubby
wings above which were two helicopter blades revolving at high speed.
No sound of a motor could be heard.

The transparent dome rolled back and like a bullet the little craft
shot into the air, followed by a futile volley from the soldiers.
Hardly had it appeared than the two airplanes bore down on it with
machine-guns going. The helicopter paid no attention to them for a
moment, and then came a puff of smoke from its side. The leading plane
swerved sharply and the helicopter fired again. The leading plane
maneuvered about, trying to get a machine-gun to bear, while the
second plane climbed swiftly to get above the helicopter and pour a
deadly stream of fire down into it. It gained position and swooped
down to the attack, but another puff of smoke came from the side of
the helicopter and there was a thunderous report and a blinding flash
in the sky. As the smoke cleared away, no trace of the ill-fated plane
could be seen. The helicopter hung motionless in the air as though
daring the remaining plane to attack.

* * * * *

The plane accepted the challenge and bore down at full speed on the
stranger. Again came a puff of smoke, but the plane swerved and an
answering shot came from its side. It was above the helicopter, and
the shell which missed its mark plung