The Scofield Bible Commentary, by Cyrus Ingerson Scofield, , at sacred-texts.com
(1) A sign of Jeremiah's faith in his own predictions of the restoration of Judah (Jer 32:15) for the field was then occupied by the Babylonian army; and
(2) a sign to Judah of that coming restoration.
Or, engines of shot.
(See Scofield) - (Psa 19:9).