Sentence:  Sometimes, like <WSD>turnspit</WSD> cur, he reels,/ Sweating in his mysterious wheels;/ And by his motion, tho' he fancies/ That higher up he still advances,/ Yet, press'd down with his nat'ral weight,/ After much toiling, still the "Knight"/ Is found to be in the same place/ Where first he did begin his race;/ And has done less, for all his boasting,/ Than <WSD>turnspit</WSD> dog the goose when roasting. 
Meaning of the word marked with <WSD> tags:  A short-legged, long-bodied dog, now extinct, bred to run on a wheel to turn a spit.