I have no access to the current standard edition of Clare's poetry (Robinson and Summerfield, 1964), which might have much of interest to say on this point; but it appears impossible to know whether Clare wrote rack or rock. The poem exists only in transcripts made by W.F. Knight, the house steward of the asylum where Clare was confined, who wrote that they were “faithfully transcribed to the best of my knowledge from the pencil originals many of which were so obliterated that without refering [sic] to the Author I could not decipher” and that “whenever I have wished him to correct a single stanza he has ever shewn the greatest disinclination to take in hand what to him seems a great task.”
On the face of it, rack would seem the obvious reading, for I find it difficult to conceive what an Evening rock might be, and the meaning Barrie England cites from OED, ‘a mass of cloud moving quickly, esp. above lower clouds’ suits very nicely with the preceding reference to the dismal storm.†
But the principle of lectio difficilior suggests we look at this more closely. Clare was deeply read in the Bible, and the allusion to Moses’ rod surely points to the story told in Numbers 20:11-12:
11 And Moses lifted up his hand, and with his rod he smote the rock twice: and the water came out abundantly, and the congregation drank, and their beasts also.
12 And the Lord spake unto Moses and Aaron, Because ye believed me not, to sanctify me in the eyes of the children of Israel, therefore ye shall not bring this congregation into the land which I have given them.
Moses’ rod is also the instrument by which Moses defeated Pharaoh’s wizards and summoned the plagues and parted the Red Sea, freeing the Children of Israel from bondage.
It is hard not to think that in this transparently autobiographical poem Clare sees Moses as himself and the rod as his poetry: the source at once of his liberating vision and of the madness which excluded him from society and from free enjoyment of God’s creation.
But I’m afraid that gets us no closer to resolving the rock/rack crux. Perhaps we are not called upon to choose—perhaps Clare is balancing on a tightrope between them. He may even have a third sense in mind: the rack as an instrument of torture, employed to compel speech ...
†OED 1, incidentally, defines it as ‘Clouds, or a mass of cloud, driven before the wind in the upper air’, which is even more appropriate. And it gives, too, an obsolete sense ‘A rush of wind; a gale, a storm’.