have been able to see them.

But look as he would there was not a soul in sight. All about him
stretched the barren frowning mountains sleeping under the moon.

But the sound that he had heard?

There was no mistaking it. It had been too like the low humming of
a human voice for him to have been misled. Perhaps he had been
dreaming?

But as if to give the lie to any such supposition the strange sound
that had so alarmed him at that moment made itself manifest once
more:

"A-hooo-A-AH-HOOO-00-a-ho-ho-ho-o-!"

It started softly and gradually ran up the scale till it reached a
crescendo shout and then died out in a soft sound like a woman's
wail. Heard anywhere the sound would have been alarming enough, but
coming as it did in the midst of these unknown, mysterious Mountains
of the Moon it struck a chill to the boy's heart and caused his
scalp to tighten in a manner that even the bravest man or boy in the
world would have had no reason to feel shame over.

A human enemy, a foe he could see, Frank would have faced with iron
nerve; but this strange wailing noise coming from what quarter of
the compass he could not judge--was so uncanny that he was really
disturbed. He bounded into the chassis and roused Ben and Harry.
He had hardly whispered to them the extraordinary intelligence when