closed in about the aeroplane.

"They mean to attack us," cried Frank, suddenly.

[Illustration: "They Mean to Attack Us."]

As he spoke one immense condor drove full at him, its evil head
outstretched as if it meant to tear him with its hooked beak. The boy
struck at it with one arm while he controlled the aeroplane with the
other and the monstrous bird seemed nonplussed for a moment. With a
scream of rage it rejoined its mates and they continued to circle
about the aeroplane, every minute growing, it seemed, more numerous
and bold.

"We shall have to fire at them," cried Frank at last. "If they keep on
increasing in numbers they may attack us all at once and wreck our
airship."

Hastily Harry and Billy unslung their heavy "Express" rifles and began
firing. Ordinarily it is no easy task to hit a bird on the wing with a
rifle, but so large a target did the huge bodies present that four
fell at the first volley. As they dropped some of their cannibal
companions fell on them and tore them to ribbons in midair. It was a
horrible sight, but the boys had little time to observe it. Their
attention was now fully occupied with beating off the infuriated mates
of the dead birds, who beat the air about the aeroplane with their
huge wings until the air-storm created threatened to overbalance it.

Again and again the boys fired, but failed to hit any more of the
birds, although feathers flew from some of the great bodies as the