Sentence:  Part, on the plain or in the air sublime, / Upon the wing or in swift race contend, / As at the Olympian games or Pythian fields; / Part curb their fiery steed, or shun the goal / With rapid wheel, or fronted brigads form : / As when, to warn proud cities, war appears / Waged in the trouble sky, and armies rush / To battle in the clouds; before each van / <WSD>Prick</WSD> forth the aery knights, and couch their spears / Till thickest legions close; with feats of arms / From either end of heaven the welkin burns. 
Meaning of the word marked with <WSD> tags:  To urge one's horse on; to ride quickly. (from 14th c.)