a huddled group of birds nests, Grantline's domes clung and
gazed down upon the inner valley.

Intricate task, the building of these glassite shelters! There were
three. The main one stood close at the brink of the ledge. A quadrangle
of glassite walls, a hundred feet in length by half as wide, and a scant
ten feet high to its flat-arched dome roof. Built for this purpose in
Great-New York, Grantline had brought his aluminite girders and braces
and the glassite panels in sections.

* * * * *

The air here on the Moon surface was negligible--a scant one
five-thousandth of the atmospheric pressure at the sea-level on Earth.
But within the glassite shelter, a normal Earth-pressure must be
maintained. Rigidly braced double walls to withstand the explosive
tendency, with no external pressure to counteract it. A tremendous
necessity for mechanical equipment had burdened Grantline's small
ship to its capacity. The chemistry of manufactured air, the
pressure equalizers, renewers, respirators, the lighting and
temperature-maintenance systems--all the mechanics of a space-flyer were
here.

And within the glassite double walls, there was necessity for a constant
circulation of the Erentz temperature insulating system.[D]

There was this main Grantline building, stretching low and rectangular
along the front edge of the ledge. Within it were living rooms, messroom
and kitchen. Fifty feet behind it, connected by a narrow passage of
glassite, was a similar, though smaller structure. The mechanical
control rooms, with their humming, vibrating mechanisms were here. And
an instrument room with signaling apparatus, senders, receivers,
mirror-grids and audiphones of several varieties; and an
electro-telescope, small but modern, with dome overhead like a little
Earth observatory.

From this instrument building, beside the connecting pedestrian passage,
wire cables for light, and air-tubes and strings and bundles of
instrument wires ran to the main structure--gray snakes upon the
porous, gray Lunar rock.

The third building seemed a lean-to banked against the cliff-wall, a
slanting shed-wall of glassite fifty feet high and two hundred in
length. Under it, for months Grantline's borers had dug into the cliff.
Braced tunnels were here, penetrating back and downward into this vein
of radio-active rock.

[D] An intricate system of insulation against extremes of temperature,
developed by the Erentz