bullets whizzed past them.

All at once the condors seemed to come to a decision unanimously.
Uttering their harsh, screaming cries they rushed at the aeroplane,
tearing and snapping with beak and claws. The machine yawed under
their attack till it seemed it must turn over. Still, so far, Frank
managed to keep it on an even keel.

"Bang! bang!" cracked the rifles again and again, but the loud angry
cries of the birds almost drowned the sharp sound of the artillery.

It was a battle in the clouds between a man-made bird and nature's
fliers.

Suddenly Frank gave a shout.

"The dynamite bombs!"

Swiftly and cautiously Harry got one of the deadly explosives ready.
They were provided with a cap that set them off when they encountered
any solid substance, as, for instance, when they struck the earth, but
a small, mechanical contrivance enabled them to be adjusted also so
that they could be exploded in midair.

"Isn't there danger of upsetting the aeroplane?" gasped Billy, as he
saw the preparations.

"We'll have to chance that," was Harry's brisk response, "the birds
are too much for us."