The period of Ganymede's revolution round its gigantic primary is seven days, three hours, and forty-three minutes, practically a terrestrial week, and on their return to their native world both the daring navigators of Space described this as the most interesting and delightful week in their lives, excepting always the period which they spent in the Eden of the Morning Star.

Yet in one sense, it was even more interesting.
There the inhabitants had never learnt to sin; here they had learnt the lesson that sin is mere foolishness, and that no really sensible or properly educated man or woman thinks crime worth committing.
The life of the Crystal Cities, of which they visited four in different parts of the satellite, using the _Astronef_ as their vehicle, was one of peaceful industry and calm, innocent enjoyment.

It was quite plain that their first impressions of this aged world were correct.

Outside the cities spread a universal desert on which life was impossible.

There was hardly any moisture in the thin atmosphere.