irregular rhythm.

And the gleaming needle pointed straight at the planet Mars!

Dan stood a long time, watching the purple ring upon the silver
towers, with the shining white needle hanging below it. He stared at
Mars, glowing like a red and sinister eye above the incredible
mechanism.

His mind was in a wild storm of wonder shot with fear. What was the
meaning of the gleaming ring and needle? What connection did this
great device have with the signal of distress from the cliff, and the
green fire that had destroyed the _Virginia_? And why did the glowing
needle point at Mars?

* * * * *

He did not know when he first began to hear the sound. For a time it
was merely part of the strange mystery of the island, only another
element in the atmosphere of fear and wonder that surrounded him. Then
it rose a little, and he became suddenly sharply conscious of it as an
additional menace. The sound was not loud, but deep and vibrant. A
whir or hum, like that of a powerful, muffled motor, but deeper than
the sound of any motor man has ever made. It came down the gorge, from
the direction of the machine on the mountain.

That deep, throbbing noise frightened Dan as none of his previous
experiences had done. Shivering from fear as much as from cold, he
crouched down beside a huge boulder in the edge of the tangle of brush
that covered the bottom of the ravine. His heart pounded wildly. He
was in the clutches of an unreasoning fear that some terrible Thing
had seen him, and was about to seek him out. For a moment he had to
use all his will to keep himself from panic flight through the brush.
The unknown is always terrible, and he had invaded the domain of a
force he could not understand.

In a moment, however, he recovered himself. He would be as safe there
in the jungle, he thought, as anywhere on the island. He thought of
starting a fire, then realized that he had no matches, and that he
would not dare to make a light if he were able. He pulled a few
handfuls of dry grass to make a sort of bed, upon which he huddled up,
thanking his lucky stars that the island was in semi-tropical
latitudes.

His mind returned again to the riddles that confronted him: the green
flash and the strange mechanism on the peak. He recalled fantastic
stories he had read, of hermit scientists conducting amazing
experiments in isolated parts of the world. Presently he decided that
something of the kind must be on