Explain the image of the fading coal on page 449; "The greatest poet even cannot say it: for the mind in creation is as a fading coal which some invisible influence, like an inconstant wind, awakens to transitory brightness, this power arises from within, like the colour of a flower which fades and changes as it is developed, and the conscious portions of our natures are unprophetic either of its approach or its departure." (The Norton Anthology of English Literature. 10th edition. "Defense of Poetry" by Percy Shelley)

"A Defence of Poetry" is an essay written by English dramatist and poet Percy Bysshe Shelley. Although he wrote the essay in 1821, it was published posthumously in 1840 as a part of a collection titled Essays, Letters from Abroad, Translations and Fragments.
Essentially, "A Defense of Poetry" concerns exactly what the title suggests; Shelley defends the production of poetry from critics who consider it an inessential art. Shelley wrote the essay as a response to a piece published in 1820, titled "The Four Ages of Poetry," written by his friend Thomas Love Peacock, an English poet and novelist. He wrote to Peacock explaining how his analysis and criticism of poetry was inaccurate. Shelley also wrote to Peacock's publishers, who were—amusingly—his own publishers, concerning the essay. Therefore, he was inspired to write an essay of his own—taking the opposite stance.
Interestingly enough, however, Shelley didn’t write a full and thorough defense of poetic form; instead, he focused on the philosophical aspect of it. He explains the admirable complexity of the poet's thought processes, the role and relevance that the poet has in society, and the impact he or she can have on literature.
Thus, at one point, Shelley compares the mind of the poet to a "fading coal," writing:

The greatest poet even cannot say it; for the mind in creation is as a fading coal, which some invisible influence, like an inconstant wind, awakens to transitory brightness; this power arises from within, like the color of a flower which fades and changes as it is developed, and the conscious portions of our natures are unprophetic either of its approach or its departure.

With this quote, Shelley alludes to the power of poetic inspiration. He believed that all poets write poetry in order to reach out to the deepest parts of the human soul, that poetry stimulated the reader's emotions, thoughts, and senses. He doesn’t say that the poet's work is underappreciated, but he suggests that the poets themselves are not aware of the beauty that they create nor the power that they hold when writing poetry.
By comparing the mind of the poet to the image of a fading coal, Shelley argues that poets do not rely on "will power" and "determination," as is the case with logic or reason; instead, they focus on the beauty of everything and everyone in nature. Thus, they have no control over when or how inspiration will strike. Shelley argues that poetry is human, and its composition is basically uncontrollable. In fact, the poet’s most inspiring and creative moments never strike when they begin writing:

the most glorious poetry that has ever been communicated to the world is probably a feeble shadow of the original conceptions of the poet.

This is why poets cannot determine or guarantee the effect or the success their work will have when they publish it. Shelley, however, believes that this is what makes them the "unacknowledged legislators of the world." For him, poetry is divine and holds the answer to the meaning of the universal truth, and the language of the poets is the only reliable way of understanding it.
You can find Shelley's full essay here.