Thus he spake; and towards him the aged sire opened his sightless eyes and lifted them up and replied with these words:
“Be silent, store not up such thoughts in thy heart, my child. Let the son of Leto be my witness, he who of his gracious will taught me the lore of prophecy, and be witness the ill-starred doom which possesses me, and this dark cloud upon my eyes, and the gods of the underworld —and may their curse be upon me if I die perjured thus—no wrath of heaven will fall upon you two for your help to me.”
Then were those two eager to help him because of the oath. And quickly the younger heroes prepared a feast for the aged man, a last prey for the Harpies; and both stood near him, to smite with the sword those pests when they swooped down. Scarcely had the aged man touched the food when they forthwith, like bitter blasts or flashes of lightning, suddenly darted from the clouds, and swooped down with a yell, fiercely craving for food; and the heroes beheld them and shouted in the midst of their onrush. But they, at the cry, devoured everything and sped away over the sea afar, and an intolerable stench remained. And behind them the two sons of Boreas, raising their swords, rushed in pursuit.
For Zeus imparted to them tireless strength; but without Zeus they could not have followed, for the Harpies used ever to outstrip the blasts of the west wind when they came to Phineus, and when they left him. And, as when, upon the mountain-side, hounds, cunning in the chase, run in the track of horned goats or deer, and as they strain a little behind, gnash their teeth upon the edge of their teeth in vain; so Zetes and Calias rushing very near, just grazed the Harpies in vain with their fingertips.
And assuredly they would have torn them to pieces despite heaven’s will when they had overtaken them far off at the Floating Islands, had not swift Iris seen them and leaped down from the sky from heaven above and checked them with these words: “It is not lawful, O sons of Boreas, to strike with your swords the Harpies, the hounds of mighty Zeus; but I myself will give you a pledge, that hereafter they shall not draw near to Phineus.”
With these words she took an oath by the water of Styx, which to all the gods is most dread and most awful, that the Harpies would never thereafter again approach the home of Phineus, son of Agenor, for so it was fated. And the heroes, yielding to the oath, turned back their flight to the ship. And, on account of this, men called them the Islands of Turning, though aforetime they had called them the Floating Islands. And the Harpies and Iris parted. They entered their den in Minoan Crete; but she sped up to Olympus, soaring aloft on her swift wings.
Meantime the chiefs carefully cleansed the old man’s squalid skin, and, with due selection, sacrificed sheep which they had borne away from the spoil of Amycus. And when they had laid a huge supper in the hall, they sat down and feasted, and with them feasted Phineus ravenously, delighting his soul as in a dream. And there, when they had taken their fill of food and drink, they kept awake all night, waiting for the sons of Boreas. And the aged sire himself sat in the midst, near the hearth, telling of the end of their voyage and the completion of their journey.
For the rest of the story you can visit link Phineus and the Harpies.