John Calvin on Isaiah 21:8

8. And he cried, A lion. “Having hearkened diligently with much heed,” at length he observes a lion. This is supposed to mean Darius who conquered and pillaged Babylon, as we learn from Daniel. (Daniel 5:28, 31.)

I stand continually. When the watchman says that he is continually on his watchtower by day and by night, this tends to confirm the prediction, as if he had said that nothing can be more certain than this vision; for they whom God has appointed to keep watch are neither drowsy nor dim-sighted. Meanwhile, by this example, he exhorts and stimulates believers to the same kind of attention, that by the help of the lamp of the word, they may obtain a distant view of the power of God.