Milton's "Penseroso"

H.A.B. desires to understand the couplet— "And love the high embower'd roof, With antique pillars massy proof." He is puzzled whether to consider "proof" an adjective belonging to "pillars," or a substantive in apposition with it. All the commentators seem to have passed the line without observation. I am almost afraid to suggest that we should read "pillars'" in the genitive plural, "proof" being taken in the sense of *established strength*. Before dismissing this conjecture, I have taken the pains to examine every one of the twenty-four other passages in which Milton has used the word "proof…