him to a rock of Caucasus, and causing a vulture to devour his still-renewed heart.  There was a prophecy afloat in heaven portending the fall of Jove, the secret of averting which was known only to Prometheus; and the god offered freedom from torture on condition of its being communicated to him.  According to the mythological story, this referred to the offspring of Thetis, who was destined to be greater than his father.  Prometheus at last bought pardon for his crime of enriching mankind with his gifts, by revealing the prophecy.  Hercules killed the vulture, and set him free; and Thetis was married to Peleus, the father of Achilles.

Shelley adapted the catastrophe of this story to his peculiar views.  The son greater than his father, born of the nuptials of Jupiter and Thetis, was to dethrone Evil, and bring back a happier reign than that of Saturn.  Prometheus defies the power of his enemy, and endures centuries of torture; till the hour arrives when Jove, blind to the real event, but darkly guessing that some great good to himself will flow, espouses Thetis.  At the moment, the Primal Power of the world drives him from his usurped throne, and Strength, in the person of Hercules, liberates Humanity, typified in Prometheus, from the tortures generated by evil done or suffered.  Asia, one of the Oceanides, is the wife of Prometheus—­she was, according to other mythological interpretations, the same as Venus and Nature.  When the benefactor of mankind is liberated, Nature resumes the beauty of her prime, and is united to her husband, the emblem of the human race, in perfect and happy union.  In the Fourth Act, the Poet gives further scope to his imagination, and idealizes the forms of creation—­such as we know them, instead of such as they appeared to the Greeks.  Maternal Earth, the mighty parent, is superseded by the Spirit of the Earth, the guide of our planet through the realms of sky; while his fair and weaker companion and attendant, the Spirit of the Moon, receives bliss from the annihilation of Evil in the superior sphere.

Shelley develops, more particularly in the lyrics of this drama, his abstruse and imaginative theories with regard to the Creation.  It requires a mind as subtle and penetrating as his own to understand the mystic meanings scattered throughout the poem.  They elude the ordinary reader by their abstraction and delicacy of distinction, but they are far from vague.  It was his design to write prose metaphysical essays on the nature of Man, which would have served to explain much of what is obscure in his poetry; a few scattered fragments of observations and remarks alone remain.  He considered these philosophical views of Mind and Nature to be instinct with the intensest spirit of poetry.

More popular poets clothe the ideal with familiar and sensible imagery.  Shelley loved to idealize the real—­to gift the mechanism of the material universe with a soul and a voice, and to bestow such also on the most delicate and abstract emotions and thoughts of the mind.  Sophocles was his great master in this species of imagery.