from his father Vulcan, the god of fire, he had got the power of
breathing smoke and flame through his mouth and nostrils. He was a
scourge and a terror to the country round, as besides being a robber,
he killed and devoured men. But by good fortune the hero Hercules
happened to pass that way, driving before him a herd of cattle which
he had taken from another cruel monster--the three-bodied giant
Ge'ry-on, whom he had destroyed. As these cattle were grazing by the
river, Hercules having lain down on the bank to rest, Cacus stole four
bulls and four heifers, the finest of the herd. To conceal the theft he
dragged the animals backwards by the tails into his den, so that their
footprints seemed to show that they had gone from the cave instead of
into it. This trick had almost succeeded, for Hercules, after
searching in vain for the missing animals, was about to resume his
journey, when a lowing from within the cave reached his ears.

The oxen at departing fill
With noisy utterance grove and hill,
And breathe a farewell low;
When hark! a heifer from the den
Makes answer to the sound again
And mocks her wily foe.
CONINGTON, _AEneid_, BOOK VIII.

Hercules now knowing what had become of his cattle rushed to the top
of the mount where he had seen the giant, but Cacus fled into his
cave, and instantly let drop the huge stone which he kept suspended by
iron chains over the entrance. This stone even the mighty Hercules
could not move from its place, for it was held fast by great bolts on
the inside. But searching around the mount for another entrance, he
saw a rock overhanging the river, which formed a back for the cavern.