See the Preface to the American edition of 1837.
It need hardly be explained that, in the case of a modern poet, these various readings are not like the conjectural guesses of critics and commentators as to what the original text was (as in the case of the Greek Poets, or of Dante, or even of Shakespeare). They are the actual alterations, introduced deliberately as improvements, by the hand of the poet himself.
The collection in the British Museum, and those in all the University Libraries of the country, are incomplete.