in. They had been roaming about together, exploring the country, and
came in now full of excitement and enthusiasm to tell us what they had
found. We two were to accompany them. They would tell us no more than
that; and as soon as we had all eaten we started off. It would be a trip
of several hours, Mercer said, and would take us around to the other side
and partly behind the Dark City.

We followed no road, but scrambled along over the open country, picking
our way as best we could, and using the lights from the city to give us
direction. The two girls half walked, half flew, and Mercer and I, with
our ability to take huge leaps, made rapid progress.

The night was black--that unluminous blackness that seems to swallow
everything, even objects near at hand. We made our way along, using little
hand searchlights that threw a red glare a short distance before us.

We kept down in the gulleys as much as possible, avoiding the higher
places where Tao's long-range beams were constantly striking, and passed
around in front of the Dark City, keeping always at least five miles away.

We had been traveling two or three hours, and still Mercer and Anina gave
us no clew to what we were about to see. It began to snow. Huge, soft
flakes soon lay thick on the ground.

"Mercer, where are you taking us?" I exclaimed once.

"You shall see very soon now," Anina answered me. "What we have found,
Ollie and I--and our plan--you shall understand it soon."

We had to be content with that. An hour later we found ourselves well
around behind the Dark City and hardly more than four miles outside it. A
great jagged cliff-face, two hundred feet high perhaps, fronted us. We, at
its base, were on comparatively low ground here, with another low line of
cliffs shading us from the light-beams of the city.

Mercer and Anina stopped and pointed upward at the cliff. A huge seam of
the soft, chalky limestone ran laterally for five hundred feet or more
across its face. I saw embedded in this seam great irregular masses of
sulphur.

"There you are," said Mercer triumphantly. "Sulphur--stacks of it. All we
have to do is set fire to it. With the wind blowing this way--right toward
the city--" His gesture was significant.

The feasibility of the plan struck us at once. It was an enormous deposit
of free sulphur. From this point the prevailing wind blew directly across
the city. The sulphur lay in great masses sufficiently close together so
that if we