The Cup That Never Empties

From D5.11, Iskandar Tests the Sage, the Physician, and the Cup. It illustrates D5.11.84–94:

D5.11.84
At his command they brought the cup of gold,Filled with cold water, splendid to behold.
D5.11.85
All drank from dawn until the close of day,Yet still the water had not ebbed away.
D5.11.86
He turned and asked the learned sage to tell:“Explain this art to me; you know it well.
D5.11.87
What fills this cup? Is it the stars’ device,Or Indian craft that makes its stores suffice?”
D5.11.88
The sage replied: “Despise it not, O king;Long years of labor shaped this wondrous thing.
D5.11.89
Wise astrologers came from every land,The foremost masters, skilled to understand.
D5.11.90
They gathered round King Kayd to make it right,Through daylight and the lapis-colored night.
D5.11.91
They watched the workings of the stars on high,And spent long days in study of the sky.
D5.11.92
Observe how loadstone draws the iron near;So take that force as your example here.
D5.11.93
By nature, this draws water in its turn,Sweet water from the heavens, as we learn.
D5.11.94
It gathers water when its stores grow slight,Its filling hidden from all human sight.”