"By what thou swear'st against the thing thou swear'st."
I rather think that the second 'swear'st' should be swor'st. Possibly to or by was lost at the end of the line; but we have, "Thou swear'st thy gods in vain." Lear, i. 1.
Sc. 2.
"Some airy devil hovers in the sky."
Warburton proposed fiery, needlessly.
"Here, Hubert, keep this boy.—Philip, make up."