untry. The valley had many sharp bends and
turns, as though in some distant past it had been the bed of a great river
that had eroded its tortuous course through the rock.

The Lone City lay shut in at the bottom of this valley between two of its
bends. It was a settlement of perhaps ten thousand people, the only city
in the Twilight Country, with one exception, on this hemisphere of
Mercury.

We established our field base here at the river, and I devoted the next
few days to informing myself of the exact lay of the country, and the
methods of defense of the city Tao had provided.

I found this defense the height of simplicity, and for its purpose as
effective as it well could be. A vertical barrage of light surrounded the
city, extending upward into the air with the most powerful projectors some
ten or fifteen miles, and, with those of the spreading rays, forming a
solid wall of light at the lower altitudes. There were no projectors past
the first turn in the valley toward the river--where they could have been
directed horizontally--and none of them on the cliff tops above the city.
Thus, although we could not get over this light-barrage, we could approach
it closely in many places.

Tao's tactics became immediately evident. He had thrown an almost
impregnable barrier close about him and, trusting to its protection, was
making no effort to combat us for the moment with any moves of offense.

My first endeavor was to find a position on top of the cliffs from which
the city could be reached with a projector. It was practically the only
thing to do. The city could not be approached in front from the valley
floor; its entire surface beyond the turn was swept by the light-rays.
Approach from below in the rear was likewise barred.

Had the barrage been not so high our girls might have flown over it and
dropped bombs, or we might have sent rockets over it and dropped them into
the city. Neither of these projects was practical. The girls could not fly
over that barrage. It was too cold in the higher altitudes. Nor could we
send rockets over, for rockets sent through the light were exploded before
they could reach their mark.

The projectors along the sides of the city were located for the most part
a hundred feet or more back from the base of the surrounding cliffs. This
allowed them to cut the cliff face at the top. It will be understood then
that we could approach the brink of the cliff in many places, but never
suffi