Locke fainted while the Garden died, and God took the spinning-jenny out of his side. The speaker got this truth from a medium’s mouth, dark like the crowns of Nineveh.
The very title of this poem is a play on its contents, which are fragmental. The first stanza is humorous. Locke is, of course, John Locke, who was an Enlightenment thinker who advocated for reason. The idea of him swooning is therefore comical. The Garden is the Garden of Eden, the implication being that with the flowering of reason comes the withering of the Bible and traditional faith. God taking a spinning-jenny out of his side reenacts the act of Adam taking out his rib to fashion Eve, but rather than a new type of person, an industry is created.
The second stanza is slightly darker, suggesting that greater truth is not available through reason or the Bible. The supreme authority on both is a medium, or a reader of spirits’ thoughts. These are as powerful and ancient as the Assyrian crowns Yeats compares them with.