John Calvin on Isaiah 21:4

4. My heart was shaken. Others render it not amiss, “my heart wandered;” for excessive terror moves the heart, as it were, out of its place. He declares how sudden and unlooked for will be the destruction of Babylon, for a sudden calamity makes us tremble more than one which has been long foreseen and expected. Daniel relates, that what Isaiah here foretells was accomplished, and that he was an eye-witness. Belshazzar had that night prepared a magnificent banquet, when the Persians suddenly rushed upon him, and nothing was farther from his expectation than that he would be slain. High delight was thus suddenly changed into terror. (Daniel 5:30.)