human purposes and
for irrigating the land within the cities. Still the total quantity was
steadily diminishing, for it was not only evaporating from the surface,
but, as the orb cooled more and more rapidly towards its centre, it
descended deeper and deeper below the surface, and could now only be
reached by means of marvellously constructed borings and pumping
machinery which extended several miles below the surface.

The fast-failing store of heat in the centre of the little world, which
had now cooled through more than half its bulk, was utilised for warming
the air of the cities, and to drive the machinery which propelled it
through the streets and squares. All work was done by electric energy
developed directly from this source, which also actuated the repulsive
engines which had prevented the _Astronef_ from descending.

In short, the inhabitants of Ganymede were engaged in a steady,
ceaseless struggle to utilise the expiring natural forces of their world
to prolong their own lives and the exquisitely refined civilisation to
which they had attained to the latest possible date. They were, indeed,
in exactly the same position in which the distant descendants of the
human race may one day be expected to find themselves.

Their domestic life, as Zaidie and Redgrave saw it while they were the
guests of their host, was the perfection of simplicity and comfort, and
their public life was characterised by a quiet but intense
intellectuality which, as Zaidie had said, made them feel very much like
children who had only just learnt to speak.

As they possessed magnificent telescopes, far surpassing any on Earth,
their guests were able to survey, not only the Solar System, but the
other systems far beyond its limits as no others of their kind had ever
been able to do before. They did not look through or into the
telescopes. The lens was turned upon the object, and this was thrown,
enormously magnified, upon screens of what looked something like ground
glass some fifty feet square. It was thus that they saw, not only the
whole visible surface of Jupiter as he revolved above them and they
about him, but also their native Earth, sometimes a pale silver disc or
crescent close to the edge of the Sun, visible only in the morning and
the evening of Jupiter, and at other times like a little black spot
crossing the glowing surface.

But there was another development of the science of the Crystal Cities
which interested them far