"Trepidation talk'd" (Vol. iii., p. 450.).
—The words attributed to Milton are—
"That crystalline sphere whose balance weighs
The trepidation talk'd, and that first moved."
Paterson's comment, quoted by your correspondent, is exquisite: he evidently thinks there were two trepidations, one talked, the other first moved.
The trepidation (not a tremulous, but a turning or oscillating motion) is a well-known hypothesis added by the Arab astronomers to Ptolemy, in explanation of the precession of the equinoxes. This precession they imagined would continue retrograde for a long period, after which it would be direct for another long period, then retrograde again, and so on. They, or their European followers, I forget which, invented the crystal heaven, an apparatus outside of the starry heaven (these cast-off phrases of astronomy have entered into the service of poetry, and the empyreal heaven with them), to cause this slow turning, or trepidation, in the starry heaven. Some used two crystal heavens, and I suspect that Paterson, having some confused idea of this, fancied he found them both in Milton's text. I need not say that your correspondent is quite right in referring the words first moved to the primum mobile.
Again, balance in Milton never weighs. Scale is his word (iv. 997. x. 676.) for a weighing apparatus. Where he says of Satan's army (i. 349.),
"In even balance down they light
On the firm brimstone,"
he appears to mean that they were in regular order, with a right wing to balance the left wing. The direct motion of the crystal heaven, following and compensating the retrograde one, is the "balance" which "was the trepidation called;" and this I suspect to be the true reading. The past tense would be quite accurate, for all the Ptolemaists of Milton's time had abandoned the trepidation. As the text stands it is nonsense; even if Milton did dictate it, we know that he never saw it; and there are several passages of which the obscurity may be due to his having had to rely on others. Witness the lines in book iv. 995-1002.