Sentence:  We near him! We near him [an ostrich]! Ah, laggard, 'tis vain / That with rapid feet casting the dusty clouds back, / In circles wide wheeling, thou <WSD>scourest</WSD> the plain, / For Ayoud, untiring, is close on thy track. 
Meaning of the word marked with <WSD> tags:  To move swiftly over; to brush along.