Above it stood the seraphims; rather, above him were standing seraphim. The "seraphim" are introduced, not as well known, with the article, but without it, as unknown. The word means "fiery ones," and is supposed to denote the burning love of the blessed spirits spoken of.
They appeared to the prophet as standing above the King as he sat upon his throne—"standing" to show their readiness to minister; but why "above him" is not so clear. Perhaps, simply, as those that stand are "above" those that sit; perhaps as ready to fly through infinite space at the bidding of him who was seated in his palace, as it were upon the ground.
Their form, as seen by the prophet, appears to have been human, and only distinguished from ordinary humanity by the wings. Thus, though in name they resembled those other "fiery ones," which had punished the Jews in the wilderness (Numbers 21:6-9), there is nothing to show that Isaiah in any way connected the two.
Each one had six wings. Gesenius is mistaken in saying that there are at Persepolis any six-winged figures. The Persians not infrequently represented their genii with four wings; but no six-winged figures have been found, so far as I know, among the Persian remains.
With twain he covered his face, etc. The general idea of the six wings was probably rapid flight, the carrying out of God's behests "with speed swiftly." But, in the Divine presence, the wings were applied to a different use.
One pair veiled the seraph's head from the intolerable effulgence of the Divine glory; another concealed the feet, soiled in their various ministrations, and unmeet for the all-pure presence; the third pair alone sustained the seraph in mid-air, as he hovered in readiness to depart on any errand on which Jehovah aright send him.