ping at the base of the gleaming machine.

"We'll have to work fast," said MacGregor. "I must know, I must learn."
He drew himself up and into the shattered shell.

It was of metal, some forty feet across, its framework a maze of
latticed struts. The central part was clear. Here in a wide, shallow pan
the monster had rested. Below this was tubing, intricate coils, massive,
heavy and strong. MacGregor lowered himself upon it, Thurston was
beside him. They went down into the dim bowels of the deadly instrument.

"Hydrogen," the physicist was stating. "Hydrogen--there's our starting
point. A generator, obviously, forming the gas--from what? They couldn't
compress it! They couldn't carry it or make it, not the volume that they
evolved. But they did it, they did it!"

* * * * *

Close to the coils a dim light was glowing. It was a pin-point of
radiance in the half-darkness about them. The two men bent closer.

"See," directed MacGregor, "it strikes on this mirror--bright metal and
parabolic. It disperses the light, doesn't concentrate it! Ah! Here is
another, and another. This one is bent--broken. They are adjustable. Hm!
Micrometer accuracy for reducing the light. The last one could reflect
through this slot. It's light that does it, Thurston, it's light that
does it!"

"Does what?" Thurston had followed the other's analysis of the diffusion
process. "The light that would finally reach that slot would be hardly
perceptible."

"It's the agent," said MacGregor, "the activator--the catalyst! What
does it strike upon? I must know--I must!"

The waves were splashing outside the shell. Thurston turned in a
feverish search of the unexplored depths. There was a surprising
simplicity, an absence of complicated mechanism. The generator, with its
tremendous braces to carry its thrust to the framework itself, filled
most of the space. Some of the ribs were thicker, he noticed. Solid
metal, as if they might carry great weights. Resting upon them were
ranged numbers of objects. They were like eggs, slender, and inches in
length. On some were propellers. They worked through the shells on long
slender rods. Each was threaded finely--an adjustable arm engaged the
thread. Thurston called excitedly to the other.

"Here they are," he said. "Look! Here are the shells. Here's what blew
us up!"

* * * * *

He pointed to the slim shafts with their little propellerlike fans